stat

This command causes Syd to output sandbox state on standard error.

reset

This command causes Syd to reset sandboxing to the default state. Allowlists, denylists and filters are going to be cleared. The state of the sandbox lock is not affected by reset. This ensures an unintended reset cannot open window for a sandbox bypass. In addition, the state of Crypt sandboxing is not affected by reset too. This ensures concurrent or near-concurrent encryption operations continue uninterrupted.

panic

This command causes Syd to exit immediately with code 127.

Due to security reasons, this command is only available via the virtual stat call, it may not be used with the -m command line switch or in a configuration file.

Due to safety reasons, panic may not be called when Crypt sandboxing is on. In this case the virtual stat(2) returns -1 and sets errno to "EBUSY". This ensures concurrent or near-concurrent encryption operations continue uninterrupted.

ghost

This command initiates Ghost mode. Ghost mode is irreversible so you can call this command only once during Syd runtime. See Ghost mode section in syd(7) manual page for more information. In case of successful initiation, the virtual stat(2) call returns -1 and sets the errno to "EOWNERDEAD".

Due to security reasons, this command implies "reset", ie. the sandbox state is reset before Ghost mode initiation to ensure there're no run-away exec processes after the invocation of the "ghost" command.

Due to security reasons, this command is only available via the virtual stat call, it may not be used with the -m command line switch or in a configuration file.

config/expand

Given zero as timeout in seconds, which is the default, enables environment variable and tilde expansion using the "shellexpand" crate. This runs much faster as it does not require confinement, however it does not support command substitution and recursive environment variable expansion like wordexp(3) does. Notably, unset environment variables are not expanded to empty strings. On environment variable lookup errors and UTF-8 decoding errors Syd stops parsing and exits with error. This is done for safety as an unintended empty-string expansion can potentially cause the resulting sandboxing rule to allowlist unintended paths without the user easily noticing it. The user is recommended to set default values for environment variables using the familiar "${HOME:-/var/empty}" notation. If you really want empty-string expansion on unset environment variables, you can get this effect using the notation "${HOME:-}" but this is not recommended and should be used with care.

Given a positive integer as timeout in seconds, enables environment variable expansion and command substitutiton for configuration using wordexp(3). The fork process which calls "/bin/sh" for expansion is executed in a confined environment and it is terminated if its runtime exceeds the given timeout. Confinement is done using Landlock, namespaces and seccomp.

Note, this is a static, ie startup-only, setting: For safety reasons, no expansion is performed for runtime configuration.

lock

Set the state of the sandbox lock. Possible values are on, off and exec. If the sandbox lock is on no sandbox commands are allowed. If exec is specified, the sandbox lock is set to on for all processes except the initial process, aka Syd exec child. Note, the sandbox lock used to default to exec but as a hardening measure and to ensure security by default, as of version 3.17.0, this has been changed such that the default is unset and if no lock clause has been specified by the time Syd executes the initial sandbox process, then the sandbox lock is automatically set to on. This means if no "lock" clause is specified in any of the profiles, configuration files or "-m" CLI arguments, the lock will be on by default. Setting lock to on at any point during configuration parsing prevents further commands from being emitted by the sandbox. This feature may be used to lock site-wide defaults for a Syd login shell by adding a "lock:on" clause at the end of the site-wide configuration file which prevents Syd from subsequently parsing the user configuration file, practically enforcing the site-wide defaults.

Note, setting lock to off or exec at startup makes Syd skip preventing execve(2) and execveat(2) system calls as part of the "Execution Control (EEC)" feature. This is done to allow "cmd/exec" command to execute commands outside the sandbox.

log/level

Set the log level. Available log levels are "emerg", "alert", "crit", "error", "warn", "notice", "info", and "debug". Defaults to "warn" unless "SYD_LOG" environment variable is set at startup. Notably, Syd logs access violations with the "warn" log level. You may also use an integer in the closed range [0,7] as an argument to set the level where "0" corresponds to "emerg" and "7" corresponds to "debug".

log/syslog

Set host syslogging. If this variable is set to on, Syd sends log messages with log level "warn" and higher to syslog(3). Defaults to on unless "SYD_NO_SYSLOG" environment variable is set at startup.

log/verbose

Set verbose logging. In verbose mode, Syd acquires various details about the current seccomp(2) request and adds this information to the JSON payload under the "req" key. When disabled, Syd only logs the process ID of the current seccomp(2) request. This option is on by default, turning it off can help reduce log size.

sandbox/stat

Turn Stat sandboxing on or off.

sandbox/read

Turn Read sandboxing on or off.

sandbox/write

Turn Write sandboxing on or off.

sandbox/exec

Turn Exec sandboxing on or off.

sandbox/ioctl

Turn Ioctl sandboxing on or off.

For performance reasons, this only works at startup. If not given at startup, Syd will just allow ioctl(2) system calls at seccomp-bpf level, and all ioctl(2) requests except those in the denylist will be allowed regardless of path. Turning this sandboxing off during runtime is still possible, in this case the ioctl(2) system call handler will only check the ioctl(2) request against the allowlist and the denylist but will not check the path to the ioctl(2) file descriptor for access.

sandbox/create

Turn Create sandboxing on or off.

sandbox/delete

Turn Delete sandboxing on or off.

sandbox/rename

Turn Rename sandboxing on or off.

sandbox/symlink

Turn Symlink sandboxing on or off.

sandbox/truncate

Turn Truncate sandboxing on or off.

sandbox/chdir

Turn Chdir sandboxing on or off.

sandbox/readdir

Turn Readdir sandboxing on or off.

sandbox/mkdir

Turn Readdir sandboxing on or off.

sandbox/chown

Turn Chown sandboxing on or off.

sandbox/chgrp

Turn Chgrp sandboxing on or off.

sandbox/chmod

Turn Chmod sandboxing on or off.

sandbox/chattr

Turn Chattr sandboxing on or off.

sandbox/chroot

Turn Chroot sandboxing on or off.

sandbox/utime

Turn Utime sandboxing on or off.

sandbox/mkdev

Turn Mkdev sandboxing on or off.

sandbox/mkfifo

Turn Mkfifo sandboxing on or off.

sandbox/mktemp

Turn Mktemp sandboxing on or off.

sandbox/net

Turn Network sandboxing on or off.

sandbox/lock

Turn Landlock sandboxing on or off.

sandbox/force

Turn Force sandboxing on or off.

sandbox/tpe

Turn Trusted Path Execution (TPE) sandboxing on or off.

sandbox/crypt

Turn Crypt sandboxing on or off.

To set this option on, a key must have already been specified with "crypt/key".

Note, setting this sandboxing type to on implies "trace/allow_safe_kcapi:1" to allow cryptographic operations using the Kernel Cryptography API (KCAPI).

Note, setting this sandboxing type to on implies "trace/exit_wait_all:1" so as not to leave any ongoing encryption processes behind on sandbox process exit.

sandbox/proxy

Turn Proxy sandboxing on or off.

Defaults to proxying through TOR. See the options "proxy/addr", "proxy/port", "proxy/ext/host", and "proxy/ext/port" to configure a different proxy.

Implies unshare/net:1.

sandbox/mem

Turn Memory sandboxing on or off.

For performance reasons, this only works at startup. If not given at startup, Syd will just allow brk(2), mmap(2), mmap2(2), and mremap(2) system calls at seccomp-bpf level. Turning this sandboxing off during runtime is still possible, in this case the respective system calls handlers will do nothing and just continue the calls.

sandbox/pid

Turn PID sandboxing on or off.

default/stat

Specify the default action for Stat sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/read

Specify the default action for Read sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/write

Specify the default action for Write sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/exec

Specify the default action for Exec sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/ioctl

Specify the default action for Ioctl sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/create

Specify the default action for Create sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/delete

Specify the default action for Delete sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "stop", "kill", "panic", or "exit", where the default is "deny".

default/rename

Specify the default action for Rename sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "stop", "kill", "panic", or "exit", where the default is "deny".

default/symlink

Specify the default action for Symlink sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "stop", "kill", "panic", or "exit", where the default is "deny".

default/truncate

Specify the default action for Truncate sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/chdir

Specify the default action for Chdir sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/readdir

Specify the default action for Readdir sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/mkdir

Specify the default action for Mkdir sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/chown

Specify the default action for Chown sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/chgrp

Specify the default action for Chgrp sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/chmod

Specify the default action for Chmod sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/chattr

Specify the default action for Chattr sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/chroot

Specify the default action for Chattr sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/utime

Specify the default action for Utime sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/mkdev

Specify the default action for Mkdev sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/mkfifo

Specify the default action for Mkfifo sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/mktemp

Specify the default action for Mktemp sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/net

Specify the default action for Network sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/block

Specify the action for IP blocklist violations.

The value must be exactly one of "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/force

For force sandboxing, define the default action to take when the path of a binary is not in the Integrity Force map.

The value must be either one of "warn", "filter", "stop", "deny", "panic", "stop", "kill", "exit", where the default is "deny".

default/segvguard

Specify the action for SegvGuard access violations.

The value must be exactly one of "warn", "filter", "deny", "panic", "stop", "stop", "kill", or "exit", where the default is "deny".

default/tpe

Specify the action for TPE sandboxing access violations.

The value must be exactly one of "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/mem

Specify the action for Memory sandboxing access violations.

The value must be exactly one of "allow", "warn", "filter", "deny", "panic", "stop", "kill", or "exit", where the default is "deny".

default/pid

Specify the action for PID sandboxing access violations.

The value must be either one of "warn", "filter", "stop", "kill", "exit", where the default is "kill".

unshare/mount

Create Mount namespace on startup.

unshare/uts

Create UTS namespace on startup.

unshare/ipc

Create IPC namespace on startup.

unshare/user

Create User namespace on startup.

unshare/pid

Create Pid namespace on startup.

unshare/net

Create Net namespace on startup.

unshare/cgroup

Create CGroup namespace on startup.

unshare/time

Create Time namespace on startup. Syd resets the boot-time clock such that uptime(1) will report container uptime rather than host uptime.

root

Change directory and chroot to root before starting the process. The directory root/proc must exist to mount private proc filesystem.

As of version 3.23.14, symbolic links are not followed in any part of the root directory and path traversal using ".." is not permitted. In addition, root directory must be an absolute path, relative paths are not permitted.

root/map

Map current user to root in the sandbox on startup.

This option does nothing without "unshare/user:1".

root/fake

In fakeroot mode, the system will return a user/group id of 0, mimicking the root user. This allows users to execute commands with apparent root privileges, without actual superuser rights. It's useful for tasks like package building where root-like environment is needed, but not actual root permissions.

name/host

Set host name in the sandbox. Only useful when combined with unshare/uts:1.

name/domain

Set NIS/YP domain name in the sandbox. Only useful when combined with unshare/uts:1.

time

Set clock monotonic and boottime offset (seconds) in Time Namespace.

ioctl/allow

Add to or remove a request from the ioctl(2) request allowlist. Accepts an unsigned 64-bit integer as argument. Prefix with "0x" for hexadecimal and "0o" for octal input. Use "ioctl/allow+<request>" to add to, and "ioctl/allow-<request>" to remove from the allowlist. By default the list contains the ioctl(2) requests FIOCLEX, FIONCLEX, FIONBIO, FIOASYNC, FIOQSIZE, FIFREEZE, FITHAW, FS_IOC_FIEMAP, FIGETBSZ, FICLONE, FICLONERANGE, FIDEDUPERANGE, FS_IOC_GETFSUUID, and FS_IOC_GETFSSYSFSPATH.

Note, for rules added at startup deny rules have precedence over allow rules because the denylist is checked at kernel-space, whereas the allowlist is checked at user-space. For rules added after startup, the last matching rule wins.

ioctl/deny

Add to or remove a request from the ioctl(2) request denylist. Accepts an unsigned 64-bit integer as argument. Prefix with "0x" for hexadecimal and "0o" for octal input. Use "ioctl/deny+<request>" to add to, and "ioctl/deny-<request>" to remove from the allowlist. By default the list of denylisted ioctl(2) requests are FIBMAP, FS_IOC_FSGETXATTR, FS_IOC_FSSETXATTR, FS_IOC_SETFLAGS, KDSETKEYCODE, KDSIGACCEPT, TIOCCONS, TIOCLINUX, TIOCSETD, and TIOCSTI.

Note, for security reasons, the ioctl(2) denylist is applied at the parent seccomp-bpf filter at startup. This means the Syd process is included in this restriction as well. This also means, removing elements from this list after startup has no effect. However, if Ioctl sandboxing was enabled at startup, adding new elements to the ioctl(2) denylist will further restrict the ioctl(2) request space.

Note, for rules added at startup, deny rules have precedence over allow rules because the denylist is checked at kernel-space, whereas the allowlist is checked at user-space. For rules added after startup, the last matching rule wins.

Further reading about denylisted ioctl(2) requests:

1.

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-1523

2.

https://a13xp0p0v.github.io/2017/03/24/CVE-2017-2636.html

3.

http://phrack.org/issues/52/6.html#article

4.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=83efeeeb3d04b22aaed1df99bc70a48fe9d22c4d

5.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8d1b43f6a6df7bcea20982ad376a000d90906b42

6.

https://seclists.org/oss-sec/2024/q1/13

7.

https://seclists.org/oss-sec/2024/q1/14

8.

https://forums.grsecurity.net/viewtopic.php?f=7&t=2522

9.

http://lkml.indiana.edu/hypermail/linux/kernel/9907.0/0132.html

10.

http://linux.derkeiler.com/Mailing-Lists/Kernel/2007-11/msg07723.html

mem/max

This setting specifies the limit on per-process memory usage. Setting this value to 0 disables testing for this type of memory usage. Note, the value is parsed using the parse-size crate. Refer to their documentation for information on formatting.

mem/vm_max

This setting specifies the limit on per-process virtual memory usage. Setting this value to 0 disables testing for this type of memory usage. Note, the value is parsed using the parse-size crate. Refer to their documentation for information on formatting.

pid/max

This setting specifies the limit on the number of running tasks for pid sandboxing. Setting this value to 0 is functionally equivalent to setting sandbox/pid to off.

bind

This command causes Syd to bind mount a directory on startup. The format is source-dir:target-dir:mount-options,... where the source and target directories may be equal. Mount options are a comma-separated list of a combination of the following options:

• "ro" to mount the filesystem read-only.
• "nodev" to not interpret character or block special devices on the filesystem.
• "noexec" to not permit direct execution of any binaries on the mounted filesystem.
• "nosuid" to not honour set-user-ID and set-group-ID bits or file capabilities when executing programs from this filesystem. In addition, SELinux domain transitions require permission "nosuid_transition", which in turn needs also policy capability "nnp_nosuid_transition".
• "nosymfollow" to not follow symbolic links when resolving paths. Symbolic links can still be created, and readlink(1), readlink(2), realpath(1), and realpath(3) all still work properly.
• "noatime" to not update inode access times on this filesystem (e.g. for faster access on the news spool to speed up news servers). This works for all inode types (directories too), so it implies "nodiratime".
• "nodiratime" to not update directory inode access times on this filesystem. (This option is implied when "noatime" is set.)
• "relatime" to update inode access times relative to modify or change time.

Mount options may be omitted. If the source directory does not start with a slash, it is interpreted as the filesystem type rather than the source directory. This may be used to mount special filesystems such as cgroupfs, overlayfs or tmpfs(5) into the mount namespace. In this case, any mount options supported by this filesystem type may be submitted in options argument not just the ones listed above. You may find some examples below:

• bind+/:/:ro
• bind+tmpfs:/tmp:noexec,size=16M
• bind+cgroup2:/sys/fs/cgroup:nodev,noexec,nosuid
• bind+overlay:/tmp/target:lowerdir=/tmp/lower,upperdir=/tmp/upper,workdir=/tmp/work,nosuid

This command may be used to create immutable containers. For example, the command "bind+/:/:ro" is functionally equivalent to "deny/write+/***" except the restriction happens at kernel VFS layer rather than at user level using seccomp-notify. Alternatively this can also be achieved at the kernel level using Landlock.

Note, if the source or target directory does not exist, Syd logs an "info" level entry and continues processing the rest of the mounts. Any other error during bind mounting is fatal.

As of version 3.23.14, symbolic links are not followed in any part of the source or target directory paths and path traversal using ".." is not permitted. In addition, target directory must be an absolute path, relative paths are not permitted.

As of version 3.23.14, mounting the special "proc" filesystem under a custom path is not permitted. Syd handles this mount itself specially after all bind mounts are processed.

crypt

Specifies a list of glob(3p) patterns to encrypt for Crypt sandboxing.

crypt/kdf/salt

Specify salt used in key derivation function for Crypt sandboxing.

There are two usage options for the salt:

• Empty or static for domain separation in a private setting
• Guaranteed to be uniformly-distributed and unique in a public setting

crypt/kdf/info/enc

Specify informational context string for encryption key used in key derivation function for Crypt sandboxing.

This information acts as a label and is not private.

crypt/kdf/info/mac

Specify informational context string for authentication key used in key derivation function for Crypt sandboxing.

This information acts as a label and is not private.

crypt/key

Specify 256-bit AES-CTR key for Crypt sandboxing. The key must be encoded in hexadecimal and be exactly 64 characters.

crypt/tmp

Specify temporary backing directory for transparent file decryption. The argument must be an absolute path or the special value "mem". The user must ensure this directory is secure as decrypted contents will be written to temporary files under this directory. Specify the special value "mem" to use anonymous files which live in RAM with a volatile backing storage created with memfd_create(2). This is the default. The user is encouraged to specify this option for efficient handling of large files for Crypt sandboxing.

force

Add or remove an integrity force rule for Force Sandboxing. The format is force+/path:hashhex:action for addition and force-/path for removal. Use force^ to clear the Integrity Force map. Available actions are the following:

• kill: Send the offending process SIGKILL signal, default if ":action" is omitted.
• stop: Send the offending process SIGSTOP signal.
• warn: Allow execution.

In all cases a warning to the syslog is written. hashhex is either a 8-character CRC32 checksum, 16-character CRC64 checksum, 32-character MD5 checksum, a 40-character SHA1 checksum, a 64-character SHA3-256 checksum, a 96-character SHA3-384 checksum or a 128-character SHA3-512 checksum.

• syd-sha(1) is a helper tool to calculate checksums of files.
• syd-path(1) is a helper tool to write integrity force rules for binaries under PATH.

proxy/addr

Set internal address for Proxy sandboxing. This must be an IPv4 or an IPv6 address. Defaults to 127.0.0.1.

proxy/port

Set internal port for Proxy sandboxing. Defaults to 9050.

proxy/ext/host

Set external address for Proxy sandboxing. This must either be an IPv4 address or an IPv6 address or a hostname. If the argument does not parse as an IP address, Syd resolves the name using the system DNS resolver and selects a response IP randomly.

Defaults to "127.0.0.1", which may be be overriden with the environment variable SYD_PROXY_HOST at startup.

proxy/ext/port

Set external port for Proxy sandboxing.

Defaults to 9050, which may be overriden with the environment variable SYD_PROXY_PORT at startup.

segvguard/expiry

Specify SegvGuard expiry timeout in seconds. Set to 0 to disable SegvGuard.

segvguard/suspension

Specify SegvGuard suspension timeout in seconds.

segvguard/maxcrashes

Specify SegvGuard max crashes.

tpe/gid

Specify untrusted GID for Trusted Path Execution (TPE). By default, TPE is applied to users of all groups including root and this setting can be used to limit it to a certain group. To unset a previously set GID and return to the default state set "none" as the value.

tpe/negate

Negate GID logic for Trusted Path Execution (TPE). This turns "tpe/gid" from untrusted into trusted such that users belonging to this group will be exempt from TPE.

tpe/root_owned

Ensure directory is root-owned for Trusted Path Execution (TPE).

Note, this option will misbehave with "unshare/user:1" if the real root user is not mapped inside the container.

tpe/user_owned

Ensure directory is user-owned or root for Trusted Path Execution (TPE).

Note, this option will misbehave with "unshare/user:1" if the real root is not mapped inside the container.

allow/stat

Specifies a list of glob(3p) patterns to allow for Stat sandboxing.

allow/read

Specifies a list of glob(3p) patterns to allow for Read sandboxing.

allow/write

Specifies a list of glob(3p) patterns to allow for Write sandboxing.

allow/exec

Specifies a list of glob(3p) patterns to allow for Exec sandboxing.

allow/ioctl

Specifies a list of glob(3p) patterns to allow for Ioctl sandboxing.

allow/create

Specifies a list of glob(3p) patterns to allow for Create sandboxing.

allow/delete

Specifies a list of glob(3p) patterns to allow for Delete sandboxing.

allow/rename

Specifies a list of glob(3p) patterns to allow for Rename sandboxing.

allow/symlink

Specifies a list of glob(3p) patterns to allow for Symlink sandboxing.

allow/truncate

Specifies a list of glob(3p) patterns to allow for Truncate sandboxing.

allow/chdir

Specifies a list of glob(3p) patterns to allow for Chdir sandboxing.

allow/readdir

Specifies a list of glob(3p) patterns to allow for Readdir sandboxing.

allow/mkdir

Specifies a list of glob(3p) patterns to allow for Mkdir sandboxing.

allow/chown

Specifies a list of glob(3p) patterns to allow for Chown sandboxing.

allow/chgrp

Specifies a list of glob(3p) patterns to allow for Chgrp sandboxing.

allow/chmod

Specifies a list of glob(3p) patterns to allow for Chmod sandboxing.

allow/chattr

Specifies a list of glob(3p) patterns to allow for Chattr sandboxing.

allow/chroot

Specifies a list of glob(3p) patterns to allow for Chroot sandboxing.

allow/utime

Specifies a list of glob(3p) patterns to allow for Utime sandboxing.

allow/mkdev

Specifies a list of glob(3p) patterns to allow for Mkdev sandboxing.

allow/mkfifo

Specifies a list of glob(3p) patterns to allow for Mkfifo sandboxing.

allow/mktemp

Specifies a list of glob(3p) patterns to allow for Mktemp sandboxing.

allow/net/bind

Specifies a list of network address patterns to allow for Bind network sandboxing.

allow/net/connect

Specifies a list of network address patterns to allow for Connect network sandboxing.

allow/net/sendfd

Specifies a list of network address patterns to allow for SendFd network sandboxing.

allow/net/link

Specifies a list of netlink families to allow for Link network sandboxing.

Accepts a comma-delimited list of the following items: "route", "usersock", "firewall", "sock_diag", "nflog", "xfrm", "selinux", "iscsi", "audit", "fib_lookup", "connector", "netfilter", "ip6_fw", "dnrtmsg", "kobject_uevent", "generic", "scsitransport", "ecryptfs", "rdma", "crypto", and "smc".

allow/lock/read

Specifies a list of beneath paths to allow as read-only for Landlock sandboxing. In this context, read-only includes execute operations.

As of version 3.21.0, this list includes the path "/proc" by default as Syd is included in the Landlock sandbox and Syd requires access to this directory to function correctly.

allow/lock/write

Specifies a list of beneath paths to allow as read-write for Landlock sandboxing. In this context, read-write includes execute and ioctl(2) operations.

Note, Landlock ioctl(2) support requires ABI 5 or later. Fifth Landlock ABI was introduced with Linux 6.10. On older kernels, this command is not going to confine ioctl(2) operations.

As of version 3.21.0, this list includes the path "/dev/null" by default as Syd is included in the Landlock sandbox and Syd requires access to this file to function correctly.

allow/lock/bind

Specifies a list of allowed bind(2) ports for Landlock sandboxing.

Argument is either a single port or a closed range in format port1-port2.

Note, Landlock network support requires ABI 4 or later. Fourth Landlock ABI was introduced with Linux 6.7. On older kernels, this command is a no-op.

allow/lock/connect

Specifies a list of allowed connect(2) ports for Landlock sandboxing.

Argument is either a single port or a closed range in format port1-port2.

Note, Landlock network support requires ABI 4 or later. Fourth Landlock ABI was introduced with Linux 6.7. On older kernels, this command is a no-op.

warn/stat

Specifies a list of glob(3p) patterns to warn for Stat sandboxing.

warn/read

Specifies a list of glob(3p) patterns to warn for Read sandboxing.

warn/write

Specifies a list of glob(3p) patterns to warn for Write sandboxing.

warn/exec

Specifies a list of glob(3p) patterns to warn for Exec sandboxing.

warn/ioctl

Specifies a list of glob(3p) patterns to warn for Ioctl sandboxing.

warn/create

Specifies a list of glob(3p) patterns to warn for Create sandboxing.

warn/delete

Specifies a list of glob(3p) patterns to warn for Delete sandboxing.

warn/rename

Specifies a list of glob(3p) patterns to warn for Rename sandboxing.

warn/symlink

Specifies a list of glob(3p) patterns to warn for Symlink sandboxing.

warn/truncate

Specifies a list of glob(3p) patterns to warn for Truncate sandboxing.

warn/chdir

Specifies a list of glob(3p) patterns to warn for Chdir sandboxing.

warn/readdir

Specifies a list of glob(3p) patterns to warn for Readdir sandboxing.

warn/mkdir

Specifies a list of glob(3p) patterns to warn for Mkdir sandboxing.

warn/chown

Specifies a list of glob(3p) patterns to warn for Chown sandboxing.

warn/chgrp

Specifies a list of glob(3p) patterns to warn for Chgrp sandboxing.

warn/chmod

Specifies a list of glob(3p) patterns to warn for Chmod sandboxing.

warn/chattr

Specifies a list of glob(3p) patterns to warn for Chattr sandboxing.

warn/chroot

Specifies a list of glob(3p) patterns to warn for Chroot sandboxing.

warn/utime

Specifies a list of glob(3p) patterns to warn for Utime sandboxing.

warn/mkdev

Specifies a list of glob(3p) patterns to warn for Mkdev sandboxing.

warn/mkfifo

Specifies a list of glob(3p) patterns to warn for Mkfifo sandboxing.

warn/mktemp

Specifies a list of glob(3p) patterns to warn for Mktemp sandboxing.

warn/net/bind

Specifies a list of network address patterns to warn for Bind network sandboxing.

warn/net/connect

Specifies a list of network address patterns to warn for Connect network sandboxing.

warn/net/sendfd

Specifies a list of network address patterns to warn for SendFd network sandboxing.

deny/stat

Specifies a list of glob(3p) patterns to deny for Stat sandboxing.

deny/read

Specifies a list of glob(3p) patterns to deny for Read sandboxing.

deny/write

Specifies a list of glob(3p) patterns to deny for Write sandboxing.

deny/exec

Specifies a list of glob(3p) patterns to deny for Exec sandboxing.

deny/ioctl

Specifies a list of glob(3p) patterns to deny for Ioctl sandboxing.

deny/create

Specifies a list of glob(3p) patterns to deny for Create sandboxing.

deny/delete

Specifies a list of glob(3p) patterns to deny for Delete sandboxing.

deny/rename

Specifies a list of glob(3p) patterns to deny for Rename sandboxing.

deny/symlink

Specifies a list of glob(3p) patterns to deny for Symlink sandboxing.

deny/truncate

Specifies a list of glob(3p) patterns to deny for Truncate sandboxing.

deny/chdir

Specifies a list of glob(3p) patterns to deny for Chdir sandboxing.

deny/readdir

Specifies a list of glob(3p) patterns to deny for Readdir sandboxing.

deny/mkdir

Specifies a list of glob(3p) patterns to deny for Mkdir sandboxing.

deny/chown

Specifies a list of glob(3p) patterns to deny for Chown sandboxing.

deny/chgrp

Specifies a list of glob(3p) patterns to deny for Chgrp sandboxing.

deny/chmod

Specifies a list of glob(3p) patterns to deny for Chmod sandboxing.

deny/chattr

Specifies a list of glob(3p) patterns to deny for Chattr sandboxing.

deny/chroot

Specifies a list of glob(3p) patterns to deny for Chroot sandboxing.

deny/utime

Specifies a list of glob(3p) patterns to deny for Utime sandboxing.

deny/mkdev

Specifies a list of glob(3p) patterns to deny for Mkdev sandboxing.

deny/mkfifo

Specifies a list of glob(3p) patterns to deny for Mkfifo sandboxing.

deny/mktemp

Specifies a list of glob(3p) patterns to deny for Mktemp sandboxing.

deny/net/bind

Specifies a list of network address patterns to deny for Bind network sandboxing.

deny/net/connect

Specifies a list of network address patterns to deny for Connect network sandboxing.

deny/net/sendfd

Specifies a list of network address patterns to deny for SendFd network sandboxing.

panic/stat

Specifies a list of glob(3p) patterns to panic for Stat sandboxing.

panic/read

Specifies a list of glob(3p) patterns to panic for Read sandboxing.

panic/write

Specifies a list of glob(3p) patterns to panic for Write sandboxing.

panic/exec

Specifies a list of glob(3p) patterns to panic for Exec sandboxing.

panic/ioctl

Specifies a list of glob(3p) patterns to panic for Ioctl sandboxing.

panic/create

Specifies a list of glob(3p) patterns to panic for Create sandboxing.

panic/delete

Specifies a list of glob(3p) patterns to panic for Delete sandboxing.

panic/rename

Specifies a list of glob(3p) patterns to panic for Rename sandboxing.

panic/symlink

Specifies a list of glob(3p) patterns to panic for Symlink sandboxing.

panic/truncate

Specifies a list of glob(3p) patterns to panic for Truncate sandboxing.

panic/chdir

Specifies a list of glob(3p) patterns to panic for Chdir sandboxing.

panic/readdir

Specifies a list of glob(3p) patterns to panic for Readdir sandboxing.

panic/mkdir

Specifies a list of glob(3p) patterns to panic for Mkdir sandboxing.

panic/chown

Specifies a list of glob(3p) patterns to panic for Chown sandboxing.

panic/chgrp

Specifies a list of glob(3p) patterns to panic for Chgrp sandboxing.

panic/chmod

Specifies a list of glob(3p) patterns to panic for Chmod sandboxing.

panic/chattr

Specifies a list of glob(3p) patterns to panic for Chattr sandboxing.

panic/chroot

Specifies a list of glob(3p) patterns to panic for Chroot sandboxing.

panic/utime

Specifies a list of glob(3p) patterns to panic for Utime sandboxing.

panic/mkdev

Specifies a list of glob(3p) patterns to panic for Mkdev sandboxing.

panic/mkfifo

Specifies a list of glob(3p) patterns to panic for Mkfifo sandboxing.

panic/mktemp

Specifies a list of glob(3p) patterns to panic for Mktemp sandboxing.

panic/net/bind

Specifies a list of network address patterns to panic for Bind network sandboxing.

panic/net/connect

Specifies a list of network address patterns to panic for Connect network sandboxing.

panic/net/sendfd

Specifies a list of network address patterns to panic for SendFd network sandboxing.

stop/stat

Specifies a list of glob(3p) patterns to stop for Stat sandboxing.

stop/read

Specifies a list of glob(3p) patterns to stop for Read sandboxing.

stop/write

Specifies a list of glob(3p) patterns to stop for Write sandboxing.

stop/exec

Specifies a list of glob(3p) patterns to stop for Exec sandboxing.

stop/ioctl

Specifies a list of glob(3p) patterns to stop for Ioctl sandboxing.

stop/create

Specifies a list of glob(3p) patterns to stop for Create sandboxing.

stop/delete

Specifies a list of glob(3p) patterns to stop for Delete sandboxing.

stop/rename

Specifies a list of glob(3p) patterns to stop for Rename sandboxing.

stop/symlink

Specifies a list of glob(3p) patterns to stop for Symlink sandboxing.

stop/truncate

Specifies a list of glob(3p) patterns to stop for Truncate sandboxing.

stop/chdir

Specifies a list of glob(3p) patterns to stop for Chdir sandboxing.

stop/readdir

Specifies a list of glob(3p) patterns to stop for Readdir sandboxing.

stop/mkdir

Specifies a list of glob(3p) patterns to stop for Mkdir sandboxing.

stop/chown

Specifies a list of glob(3p) patterns to stop for Chown sandboxing.

stop/chgrp

Specifies a list of glob(3p) patterns to stop for Chgrp sandboxing.

stop/chmod

Specifies a list of glob(3p) patterns to stop for Chmod sandboxing.

stop/chattr

Specifies a list of glob(3p) patterns to stop for Chattr sandboxing.

stop/chroot

Specifies a list of glob(3p) patterns to stop for Chroot sandboxing.

stop/utime

Specifies a list of glob(3p) patterns to stop for Utime sandboxing.

stop/mkdev

Specifies a list of glob(3p) patterns to stop for Mkdev sandboxing.

stop/mkfifo

Specifies a list of glob(3p) patterns to stop for Mkfifo sandboxing.

stop/mktemp

Specifies a list of glob(3p) patterns to stop for Mktemp sandboxing.

stop/net/bind

Specifies a list of network address patterns to stop for Bind network sandboxing.

stop/net/connect

Specifies a list of network address patterns to stop for Connect network sandboxing.

stop/net/sendfd

Specifies a list of network address patterns to stop for SendFd network sandboxing.

kill/stat

Specifies a list of glob(3p) patterns to kill for Stat sandboxing.

kill/read

Specifies a list of glob(3p) patterns to kill for Read sandboxing.

kill/write

Specifies a list of glob(3p) patterns to kill for Write sandboxing.

kill/exec

Specifies a list of glob(3p) patterns to kill for Exec sandboxing.

kill/ioctl

Specifies a list of glob(3p) patterns to kill for Ioctl sandboxing.

kill/create

Specifies a list of glob(3p) patterns to kill for Create sandboxing.

kill/delete

Specifies a list of glob(3p) patterns to kill for Delete sandboxing.

kill/rename

Specifies a list of glob(3p) patterns to kill for Rename sandboxing.

kill/symlink

Specifies a list of glob(3p) patterns to kill for Symlink sandboxing.

kill/truncate

Specifies a list of glob(3p) patterns to kill for Truncate sandboxing.

kill/chdir

Specifies a list of glob(3p) patterns to kill for Chdir sandboxing.

kill/readdir

Specifies a list of glob(3p) patterns to kill for Readdir sandboxing.

kill/mkdir

Specifies a list of glob(3p) patterns to kill for Mkdir sandboxing.

kill/chown

Specifies a list of glob(3p) patterns to kill for Chown sandboxing.

kill/chgrp

Specifies a list of glob(3p) patterns to kill for Chgrp sandboxing.

kill/chmod

Specifies a list of glob(3p) patterns to kill for Chmod sandboxing.

kill/chattr

Specifies a list of glob(3p) patterns to kill for Chattr sandboxing.

kill/chroot

Specifies a list of glob(3p) patterns to kill for Chroot sandboxing.

kill/utime

Specifies a list of glob(3p) patterns to kill for Utime sandboxing.

kill/mkdev

Specifies a list of glob(3p) patterns to kill for Mkdev sandboxing.

kill/mkfifo

Specifies a list of glob(3p) patterns to kill for Mkfifo sandboxing.

kill/mktemp

Specifies a list of glob(3p) patterns to kill for Mktemp sandboxing.

kill/net/bind

Specifies a list of network address patterns to kill for Bind network sandboxing.

kill/net/connect

Specifies a list of network address patterns to kill for Connect network sandboxing.

kill/net/sendfd

Specifies a list of network address patterns to kill for SendFd network sandboxing.

exit/stat

Specifies a list of glob(3p) patterns to exit for Stat sandboxing.

exit/read

Specifies a list of glob(3p) patterns to exit for Read sandboxing.

exit/write

Specifies a list of glob(3p) patterns to exit for Write sandboxing.

exit/exec

Specifies a list of glob(3p) patterns to exit for Exec sandboxing.

exit/ioctl

Specifies a list of glob(3p) patterns to exit for Ioctl sandboxing.

exit/create

Specifies a list of glob(3p) patterns to exit for Create sandboxing.

exit/delete

Specifies a list of glob(3p) patterns to exit for Delete sandboxing.

exit/rename

Specifies a list of glob(3p) patterns to exit for Rename sandboxing.

exit/symlink

Specifies a list of glob(3p) patterns to exit for Symlink sandboxing.

exit/truncate

Specifies a list of glob(3p) patterns to exit for Truncate sandboxing.

exit/chdir

Specifies a list of glob(3p) patterns to exit for Chdir sandboxing.

exit/readdir

Specifies a list of glob(3p) patterns to exit for Readdir sandboxing.

exit/mkdir

Specifies a list of glob(3p) patterns to exit for Mkdir sandboxing.

exit/chown

Specifies a list of glob(3p) patterns to exit for Chown sandboxing.

exit/chgrp

Specifies a list of glob(3p) patterns to exit for Chgrp sandboxing.

exit/chmod

Specifies a list of glob(3p) patterns to exit for Chmod sandboxing.

exit/chattr

Specifies a list of glob(3p) patterns to exit for Chattr sandboxing.

exit/chroot

Specifies a list of glob(3p) patterns to exit for Chroot sandboxing.

exit/utime

Specifies a list of glob(3p) patterns to exit for Utime sandboxing.

exit/mkdev

Specifies a list of glob(3p) patterns to exit for Mkdev sandboxing.

exit/mkfifo

Specifies a list of glob(3p) patterns to exit for Mkfifo sandboxing.

exit/mktemp

Specifies a list of glob(3p) patterns to exit for Mktemp sandboxing.

exit/net/bind

Specifies a list of network address patterns to exit for Bind network sandboxing.

exit/net/connect

Specifies a list of network address patterns to exit for Connect network sandboxing.

exit/net/sendfd

Specifies a list of network address patterns to exit for SendFd network sandboxing.

append

Specifies a list of glob(3p) patterns to files that should be made append-only for Write sandboxing.

If a path is append-only, Syd adds "O_APPEND" and removes "O_TRUNC" from flags on any sandbox granted attempt to open(2) this path. Unsetting the "O_APPEND" flag using fcntl(2) "F_SETFL" command is prevented. Similarly, any attempt to rename(2), truncate(2) and unlink(2) the file is prevented. This is typically useful for history and log files.

mask

Specifies a list of glob(3p) patterns to mask for Read & Write sandboxing.

If a path is masked, Syd returns a file descriptor to "/dev/null" on any sandbox granted attempt to open(2) this path. Masking can effectively be used to hide the contents of a file in a more relaxed and compatible way than denying read/write access to it. stat(2) calls on a masked file returns the original file metadata and a masked file may be executed.

block

Specifies a range of IP networks to be blocked by Connect network sandboxing. Use "block+<net>" and "block-<net>" to add and remove ip networks from the range. Alternatively the range can also be populated by including "ipset" and "netset" files from within Syd configuration. Use "block^" to clear the list and "block!" to simplify the ip range by aggregating networks together. "block!" is useful to call after importing big IP blocklists, it helps reduce memory consumption and improve matching performance.

cmd/exec

Makes Syd execute an external command without sandboxing. The process is executed in a new process group with its standard input closed. The program name and arguments must be separated with the US (unit separator, octal: 037) character. To ease usage, the syd-exec(1) helper utility is provided to construct a sandbox command of this type:

;  syd -puser -- bash -c 'test -c $(syd-exec echo hello world)'
hello world
{"cfg":"cmd/exec!echou001fhellou001fworld","cmd":"bash ...}
;

load

Read configuration from the given file descriptor, the file must be open for reading. Syd uses pidfd_getfd(2) to acquire the file descriptor and reads sandbox configuration from it. This command is useful to load a set of sandbox commands into Syd in a single step and is typically used with reset, e.g:

int fd = open("/tmp", O_RDWR | O_TMPFILE | O_CLOEXEC, 0);
if (fd == -1) errx(1, "Failed to open temporary file");
const char *syd = "sandbox:stat/on\nallow/stat+/***\ndeny/stat+/\nlock:on\n";
errx(write(fd, syd, strlen(syd)) == -1, "Failed to write config");
errx(lseek(fd, 0, SEEK_SET) == -1, "Failed to seek in file");
char load[64];
sprintf(load, "/dev/syd/load/%d", fd);
errx(stat("/dev/syd/reset", NULL) == -1, "Failed to reset syd");
errx(stat(load, NULL) == -1, "Failed to load syd profile");
errx(execvp("/bin/sh", (char *[]){"/bin/sh", "-l", NULL}) == -1, "execvp failed");

Due to security reasons, this command is only available via the virtual stat(2) call, it may not be used with the -m command line switch or in a configuration file.

As of version 3.30.0, this command may be used to load builtin profiles, when Syd falls back to parsing the "load" argument as a profile name if parsing the argument as a file descriptor fails.

trace/allow_safe_setuid

Specify whether the Linux capability "CAP_SETUID" should be retained. This option in combination with SafeSetID allows the sandbox process to change UID. Note, Syd will change its UID with the sandbox process.

Note, because NPTL uses real-time signals to ensure all threads share the same UID/GID, setting this option disables the SROP mitigator. See the "Enhanced Execution Control (EEC)" section of the syd(7) manual page for more information.

trace/allow_safe_setgid

Specify whether the Linux capability "CAP_SETGID" should be retained. This option in combination with SafeSetID allows the sandbox process to change GID. Note, Syd will change its GID with the sandbox process.

Note, because NPTL uses real-time signals to ensure all threads share the same UID/GID, setting this option disables the SROP mitigator. See the "Enhanced Execution Control (EEC)" section of the syd(7) manual page for more information.

setuid

Add, remove a UID transition or reset UID transitions. Only a single transition from a source UID can be defined. Transitions to root are not allowed.

Usage:

setuid+0:65534      # Define a UID transition from root to nobody.
setuid+root:nobody  # Same as above but using user names.
setuid-0:65534      # Remove a previously defined UID transition.
setuid^0            # Remove all UID transitions matching source UID.
setuid^             # Remove all UID transitions.

setgid

Add, remove a GID transition or reset GID transitions. Only a single transition from a source GID can be defined. Transitions to root are not allowed.

Usage:

setgid+0:65534      # Define a GID transition from root to nogroup.
setgid+root:nogroup # Same as above but using group names.
setgid-0:65534      # Remove a previously defined GID transition.
setgid^0            # Remove all GID transitions matching source GID.
setgid^             # Remove all GID transitions.

trace/allow_unsafe_cbpf

A boolean specifying whether Syd should allow additional seccomp(2) cbpf filters to be installed by sandbox processes. By default, this is denied to mitigate confused deputy problems and errno(3) is set to "EINVAL", aka "Invalid argument" for compatibility reasons. On the one hand, stacked seccomp(2) cbpf filters allow for incremental confinement and therefore added hardening, on the other hand they may be abused to install system call filters with more precedent actions than user-notify thereby bypassing Syd's own seccomp(2) cbpf filters. To quote the seccomp_unotify(2): "... a user-space notifier can be bypassed if the existing filters allow the use of seccomp(2) or prctl(2) to install a filter that returns an action value with a higher precedence than "SECCOMP_RET_USER_NOTIF" (see seccomp(2))." Setting the option "trace/allow_unsafe_prctl:1" overrides this option and allows the "PR_SET_SECCOMP" prctl(2) operation inside the sandbox. This may be changed in the future for clearer separation of mitigations.

trace/allow_unsafe_ebpf

Allows direct eBPF use inside the Syd sandbox using the bpf(2) system call, whose unprivileged use is permitted since Linux-4.4. On the one hand, eBPF programs can be used for additional hardening, on the other hand eBPF is a frequent source of vulnerabilities due to churn, complexity, improper validation and complexity of validation. eBPF may also be abused to implement efficient and portable rootkits.

trace/allow_unsafe_dumpable

A boolean specifying whether Syd should skip from setting its process dumpable attribute to false. This allows core dumps for the Syd process, and allows debugging/profiling/tracing the Syd process. You should not set this option unless you're developing Syd.

trace/allow_unsafe_exec

A boolean specifying whether exec calls with NULL argument and environment pointers should be allowed.

trace/allow_unsafe_ptrace

A boolean specifying whether ptrace(2) should be used to secure the exec handler. Setting this option to true effectively removes the ptrace(2) dependency from the sandbox. This is necessary to trace syd together with its children, e.g. with "strace -f". Warning, this option makes syd(1) keep the "CAP_SYS_PTRACE" capability and disables Force Sandboxing, SegvGuard and the exec-TOCTOU mitigator. It allows the sandbox process to trivially break out of the sandbox by e.g. attaching to the syd(1) main thread with ptrace(2) and getting a handle to the seccomp(2) notify file descriptor. Therefore, this option should only be used in trusted environments.

trace/allow_unsafe_perf

A boolean specifying whether perf calls should be allowed within the sandbox.

trace/allow_unsafe_filename

A boolean specifying whether the restrictions on file names should be lifted. By default, file names with control characters, forbidden characters or invalid UTF-8 are denied with "EINVAL" as necessary. Read "Enhanced Path Integrity Measures" of the syd(7) manual page for more information.

trace/allow_unsafe_libc

A boolean specifying whether turning on secure mode for libc should be skipped. Read "AT_SECURE" section of the getauxval(3) manual page about the implications of secure mode.

trace/allow_unsafe_magiclinks

A boolean specifying whether /proc magic links should be followed even when per-process directory id differs from the caller process id. Magic links are symbolic link-like objects that are most notably found in proc(5); examples include "/proc/pid/exe" and "/proc/pid/fd/*". See symlink(7) for more details. Unknowingly opening magic links can be risky for some applications. Examples of such risks include the following:

• If the process opening a pathname is a controlling process that currently has no controlling terminal (see credentials(7)), then opening a magic link inside "/proc/pid/fd" that happens to refer to a terminal would cause the process to acquire a controlling terminal.
• In a containerized environment, a magic link inside "/proc" may refer to an object outside the container, and thus may provide a means to escape from the container.

Because of such risks, Syd denies access to magic links which do not belong to the current process by default.

trace/allow_unsafe_nice

A boolean specifying whether process and I/O priority changes are allowed for the sandbox. See the "Process Priority and Resource Management" section of the syd(7) manual page for more information.

trace/allow_unsafe_nopie

A boolean specifying whether execution of non-PIE binaries should be allowed. This is generally not recommended but may be necessary on some systems.

trace/allow_unsafe_chown

Makes Syd keep the capability "CAP_CHOWN" and sandbox process will inherit the capability from Syd.

trace/allow_unsafe_open_path

A boolean specifying whether the mitigation to turn "O_PATH" file descriptors into "O_RDONLY" file descriptors for safe emulation should be disabled. With this option, syd continues the open(2) system calls with the "O_PATH" in the sandbox process which opens a TOCTOU vector.

trace/allow_unsafe_open_cdev

A boolean specifying whether open(2) calls to character devices should be continued in the sandbox process rather than opening them in the Syd emulator thread and sending the file descriptor. Some character devices, e.g. "/dev/kfd" for AMD GPUs, require per-application access to the GPU device, therefore opening the device in the Syd emulator thread and then continuing the subsequent ioctl(2) system calls in the sandbox process is going to return "EBADF", aka "Bad file number". Until Syd has a way to fully emulate the ioctl(2) request space and is able to call the ioctl(2) system call directly from Syd emulator threads, this option may be used to access such character devices. Note, setting this option opens a TOCTOU attack vector, whereby the sandbox process can open an arbitrary file instead of the character device in question! Syd applies the following mitigations to limit the scope of the attack vector:

• Syd continues the system call if and only if "O_RDWR" is set in the flags argument.
• Syd does not continue the system call if at least one of the flags "O_CREAT", "O_TRUNC" or "O_TMPFILE" is set in the flags argument.
• Syd returns "ENOSYS", aka "Function not implemented", for the openat2(2) system call rather than continuing it in the sandbox process to prevent the "struct open_how" pointer indirection to bypass the restrictions applied to the flags argument. Refer to the openat2(2) manual page for more information.
• This option may be changed at runtime, and it is highly recommended to unset this option using the syd(2) virtual system call API right after the character device is opened.

trace/allow_unsafe_cpu

Specify whether CPU emulation system calls should be allowed. By default, as of version 3.22.1, Syd denies the modify_ldt(2), subpage_prot(2), switch_endian(2), vm86(2), and vm86old(2) system calls, which are associated with CPU emulation functionalities. Enabling this option (trace/allow_unsafe_cpu:1) permits these calls, thus relaxing the restriction. This option should be used with caution, as allowing these system calls can introduce potential vulnerabilities by enabling processes to modify CPU state or memory protections. Use this setting only in trusted environments where the execution of these system calls is necessary.

trace/allow_unsafe_keyring

Specify whether the add_key(2), keyctl(2), and request_key(2) system calls should be allowed. Enabling this setting permits key management within the sandbox, which can introduce security risks by allowing keyring manipulations. Use only in trusted environments.

trace/allow_unsafe_pkey

Specifies whether the pkey_alloc(2), pkey_free(2), and pkey_mprotect(2) system calls should be allowed. By default, these calls are denied to enhance security. Setting this option to true enables these system calls, allowing the use of memory protection keys. This option should be used with caution and only in trusted environments where the use of these system calls is necessary.

trace/allow_unsafe_msgsnd

Specifies whether the msgsnd(2) system call should be allowed. By default, this call is denied to enhance security as the ability of this system call to allocate large, contiguous blocks of memory in the kernel heap is often used to orchestrate kernel heap spraying attacks. See the "Mitigation Against Heap Spraying" section of the syd(7) manual page for more information.

trace/allow_unsafe_time

A boolean specifying whether system calls which adjust the system time are allowed. Note, this also causes Syd to keep the CAP_SYS_TIME capability. Use syd-ls time to see the list of system calls allowed by this setting.

trace/allow_unsafe_uring

A boolean specifying whether system calls of the io_uring(7) interface are allowed. Normally, these are denied because they may be used to bypass path sandboxing. Use syd-ls uring to see the list of system calls allowed by this setting.

trace/allow_unsafe_caps

A boolean specifying whether Syd should skip dropping Linux capabilities at startup. This setting can be used to construct privileged containers and should be used with extreme care.

Note, syd-oci(1) sets this option to honour the list of capabilities specified by the container engine. You may unset it using the container configuration file. See CONFIGURATION section in syd-oci(1) manual page for more information.

trace/allow_unsafe_env

Specify whether unsafe environment variables should be allowed into the environment of the sandbox process. See syd-ls env for the list of unsafe environment variables.

trace/allow_safe_kcapi

Specify whether access to the Linux kernel cryptography API (aka: "KCAPI") should be allowed when network sandboxing is on. This option has no effect when network sandboxing is off.

As most things in life, cryptography has good and evil uses: KCAPI is convenient as it may be used to implement cryptography without depending on user-space libraries such as OpenSSL but it may also enable malicious code to efficiently turn itself into ransomware. Adhering to the goal to be secure by default Syd disallows this access by default.

Note, Syd does not hook into setsockopt(2) and the "ALG_SET_KEY" operation to set the encryption key is directly handled by the host kernel therefore the encryption key is not copied into Syd's address space.

Note again, Syd hooks into bind(2), sendto(2), sendmsg(2), and sendmmsg(2) but not read(2), write(2), recv(2), or splice(2). To reduce syscall overhead, user is recommended to use the unhooked system calls when they can to interact with KCAPI.

trace/allow_safe_syslog

Specify whether unprivileged sandbox processes can access Syd's syslog(2) emulation using dmesg(8). Unprivileged processes include the set of all sandbox processes with the sandbox lock "off", and all but the initial sandbox process with the sandbox lock set to "exec". Note, this option has nothing to do with access to the host syslog which is never allowed.

trace/allow_safe_bind

Specify whether the socket address arguments of successful bind(2) calls should be allowed for connect(2), sendto(2), sendmsg(2), and sendmmsg(2) system calls.

Note, these addresses are allowed globally and not per-process for usability reasons. Thus, for example, a process which forks to call bind(2) will have its address allowed for their parent as well.

trace/allow_unsafe_bind

Specify whether the Linux capability "CAP_NET_BIND_SERVICE", which allows a process to bind(2) to ports lower than 1024, should be retained.

trace/allow_unsafe_socket

Specify whether unsafe socket families should be allowed. When set, Syd does not drop the capability CAP_NET_RAW on startup. This allows:

1.

use of RAW and PACKET sockets;

2.

bind to any address for transparent proxying.

trace/allow_unsupp_socket

Specify whether unsupported socket families such as netlink sockets should be allowed access when network sandboxing is on. By default Syd allows sandboxed access to unix, ipv4 and ipv6 sockets. This option has no effect when network sandboxing is off.

As of version 3.16.6 Syd allows access to alg sockets with the "trace/allow_safe_kcapi" option rather than with this option. Alg sockets are used to interact with the Linux kernel cryptography API.

Note, on architectures with a multiplexed socketcall(2) system call, enabling this option is insecure because it is vulnerable to TOCTOU. You may use syd-sys(1) utility to check if this system call is available on your architecture using e.g: "syd-sys socketcall && echo vulnerable".

trace/allow_unsafe_prctl

Specify whether prctl(2) restrictions should be lifted. See syd-ls prctl for the list of allowed prctl requests.

trace/allow_unsafe_prlimit

Specify whether prlimit(2) restrictions should be lifted.

trace/allow_unsafe_syslog

Specify whether the Linux capability "CAP_SYSLOG" should be retained. This allows the process to perform privileged syslog(2) operations. This is useful when sandboxing a service such as syslogd.

trace/allow_unsafe_sync

Specify whether the sync(2) and syncfs(2) system calls should be allowed inside the sandbox. By default these system calls are turned into no-ops to prevent potential local DoS, however it may be useful to disable this restriction in scenarios where sync is actually expected to work such as when sandboxing databases.

trace/allow_unsafe_memfd

A boolean specifying whether secret memory file descriptors and executable memory file descriptors should be enabled. By default Syd strips the "MFD_EXEC" and adds the "MFD_NOEXEC_SEAL" flag to memfd_create(2) flags argument. This ensures the memory file descriptor can never be made executable. The "MFD_NOEXEC_SEAL" flag requires Linux-6.3 or newer therefore on older kernels this option must be enabled to make memory file descriptors work. However, the user should be aware that allowing encrypted memory file descriptors does allow an attacker to bypass Exec, Force and TPE sandboxing and execute denylisted code.

trace/allow_unsafe_memory

Specify whether the Memory-Deny-Write-Execute (MDWE) protections should be bypassed. See Memory-Deny-Write-Execute Protections section of the syd(7) manual page for more information.

trace/deny_dotdot

Specify whether ".." components should be denied during path resolution for chdir(2) and open(2) family system calls. This is useful in mitigating path traversal attacks. See "Path Resolution Restriction For Chdir and Open Calls" of the syd(7) manual page for more information.

trace/deny_elf32

Deny the execution of 32-bit ELF binaries.

trace/deny_elf_dynamic

Deny the execution of dynamically linked ELF binaries.

trace/deny_elf_static

Deny the execution of statically linked ELF binaries.

trace/deny_script

Deny the execution of scripts (files with #!<interpreter> on first line).

Note, the execve(2) TOCTOU mitigations do not cover this option which means the functionality is vulnerable to TOCTOU. This allows an attacker to execute a script whose path is denylisted. This TOCTOU is limited to scripts and requires the interpreter binary to be allowlisted for exec. Hence this vulnerability does not allow an attacker to execute denylisted binaries. This is why the user is recommended to deny the respective interpreter binaries for execution instead for a safe and secure approach.

trace/deny_tsc

Specify whether reading the timestamp counter should be denied. Without an accurate timer, many timing attacks are going to be harder to perform.

• This works on x86 only.
• This breaks time related calls in the vDSO, which can be trivially worked around by writing a LD_PRELOAD library to call the respective system calls directly. See libsydtime, https://lib.rs/libsydtime, for a reference implementation.
• This has a negative performance impact on programs that rely on gettimeofday(2) being a vDSO call.

trace/lock_personality

If set, locks down the personality(2) system call so that the kernel execution domain may not be changed from the default. This may be useful to improve security, because odd personality emulations may be poorly tested and source of vulnerabilities.

trace/exit_wait_all

Specify whether Syd should wait for all processes to exit before exiting. By default, Syd exits with the eldest process and any leftover processes in the background are automatically killed.

trace/force_umask

Specify an umask mode to force. To unset a previously configured force umask use -1 as the value. As of version 3.15.6, chmod(2) family system calls also honour force umask for added hardening. As of version 3.22.1, this setting does not apply to directory creation for mkdir(2) and mkdirat(2) system calls. As of version 3.26.2, this setting does not apply to UNIX domain socket creation for bind(2) system calls, and non-regular file creation for mknod(2) and mknodat(2) system calls.

trace/memory_access

Set mode on cross memory attach and /proc/pid/mem usage. Cross memory attach requires a Linux kernel with the CONFIG_CROSS_MEMORY_ATTACH option enabled. Supported modes are:

• 0: Use cross memory attach if available, use /proc/pid/mem otherwise.
• 1: Use /proc/pid/mem unconditionally.

;  strace -q -eprocess_vm_readv -fc -- syd -m trace/memory_access:1 true
[pid 1100565] ????( <detached ...>
;  strace -q -eprocess_vm_readv -fc -- syd -m trace/memory_access:0 true
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- -----------------
100,00    0,000155           6        24           process_vm_readv
------ ----------- ----------- --------- --------- -----------------
100,00    0,000155           6        24           total
;

trace/sync_seccomp

Use synchronous mode for seccomp-notify so each Syd syscall handler thread wakes up on the same CPU as the respective sandbox thread that executed the system call. This option makes no functional difference and it may or may not help with performance; it defaults to false unless SYD_SYNC_SCMP environment variable is set at startup. This is because in our benchmarks setting this option typically hurts performance which may be attributed to Syd's multithreaded nature. Use perf(1) to benchmark seccomp synchronous mode:

; perf bench sched seccomp-notify
# Running 'sched/seccomp-notify' benchmark:
# Executed 1000000 system calls
Total time: 6.736 [sec]
6.736395 usecs/op
148447 ops/sec
; perf bench sched seccomp-notify --sync-mode
# Running 'sched/seccomp-notify' benchmark:
# Executed 1000000 system calls
Total time: 4.188 [sec]
4.188846 usecs/op
238729 ops/sec

cache/capacity/path

Estimated items capacity of the security policy cache for paths.

cache/capacity/addr

Estimated items capacity of the security policy cache for IP addresses.

cache/capacity/fd

Estimated items capacity of the file descriptor cache for path canonicalization.

If this value is set to 0, which is the default, Syd determines the cache capacity automatically taking into account the file resource limits and the system-wide limit set in the file "/proc/sys/fs/file-max".