aa_change_profile, aa_change_onexec - change a tasks profile

#include <sys/apparmor.h>

int aa_change_profile(const char *profile);

int aa_change_onexec(const char *profile);

Link with -lapparmor when compiling.

An AppArmor profile applies to an executable program; if a portion of the program needs different access permissions than other portions, the program can "change profile" to a different profile. To change into a new profile, it can use the aa_change_profile() function to do so. It passes in a pointer to the profile to transition to. Confined programs wanting to use aa_change_profile() need to have rules permitting changing to the named profile. See apparmor.d(8) for details.

If a program wants to return out of the current profile to the original profile, it may use aa_change_hat(2). Otherwise, the two profiles must have rules permitting changing between the two profiles.

Open file descriptors may not be remediated after a call to aa_change_profile() so the calling program must close(2) open file descriptors to ensure they are not available after calling aa_change_profile(). As aa_change_profile() is typically used just before execve(2), you may want to use open(2) or fcntl(2) with close-on-exec.

The aa_change_onexec() function is like the aa_change_profile() function except it specifies that the profile transition should take place on the next exec instead of immediately. The delayed profile change takes precedence over any exec transition rules within the confining profile. Delaying the profile boundary has a couple of advantages, it removes the need for stub transition profiles and the exec boundary is a natural security layer where potentially sensitive memory is unmapped.

On success zero is returned. On error, -1 is returned, and errno(3) is set appropriately.

The apparmor kernel module is not loaded, neither a profile nor a namespace was specified, or the communication via the /proc/*/attr/current file did not conform to protocol.
Insufficient kernel memory was available.
The calling application is confined by apparmor and the no_new_privs bit is set.
The task does not have sufficient permissions to change its domain.
The specified profile does not exist, or is not visible from the current Namespace.

The following example shows a simple, if contrived, use of aa_change_profile(); a typical use of aa_change_profile() will aa_change_profile() just before an execve(2) so that the new child process is permanently confined.

#include <stdlib.h>
#include <string.h>
#include <sys/apparmor.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
int main(int argc, char * argv[])
        int fd;
        char buf[10];
        char *execve_args[4];
        printf("Before aa_change_profile():\n");
        if ((fd=open("/etc/passwd", O_RDONLY)) < 0) {
               perror("Failure opening /etc/passwd");
               return 1;
        /* Confirm for ourselves that we can really read /etc/passwd */
        memset(&buf, 0, 10);
        if (read(fd, &buf, 10) == -1) {
                perror("Failure reading /etc/passwd");
                return 1;
        buf[9] = '\0';
        printf("/etc/passwd: %s\n", buf);
        printf("After aa_change_profile():\n");
        /* change profile to the "i_cant_be_trusted_anymore" profile, which
         * should not have read access to /etc/passwd. */
        if (aa_change_profile("i_cant_be_trusted_anymore") < 0) {
            perror("Failure changing profile -- aborting");
        /* confirm that we cannot read /etc/passwd */
        execve_args[0] = "/usr/bin/head";
        execve_args[1] = "-1";
        execve_args[2] = "/etc/passwd";
        execve_args[3] = NULL;
        execve("/usr/bin/head", execve_args, NULL);

This code example requires a profile similar to the following to be loaded with apparmor_parser(8):

profile i_cant_be_trusted_anymore {
    /etc/ld.so.cache      mr,
    /lib/ld-*.so*         mrix,
    /lib/libc*.so*        mr,
    /usr/bin/head ix,

The output when run:

$ /tmp/change_p
Before aa_change_profile():
/etc/passwd: root:x:0:
After aa_change_profile():
/usr/bin/head: cannot open `/etc/passwd' for reading: Permission denied

If /tmp/change_p is to be confined as well, then the following profile can be used (in addition to the one for 'i_cant_be_trusted_anymore', above):

# Confine change_p to be able to read /etc/passwd and aa_change_profile()
# to the 'i_cant_be_trusted_anymore' profile.
/tmp/change_p {
    /etc/ld.so.cache          mr,
    /lib/ld-*.so*             mrix,
    /lib/libc*.so*            mr,
    /etc/passwd               r,
    # Needed for aa_change_profile()
    /usr/lib/libapparmor*.so* mr,
    /proc/[0-9]*/attr/current w,
    change_profile -> i_cant_be_trusted_anymore,

None known. If you find any, please report them at https://gitlab.com/apparmor/apparmor/-/issues. Note that using aa_change_profile(2) without execve(2) provides no memory barriers between different areas of a program; if address space separation is required, then separate processes should be used.

apparmor(7), apparmor.d(5), apparmor_parser(8), aa_change_hat(2) and https://wiki.apparmor.net.

2024-02-07 AppArmor 3.1.7