AA_STACK_PROFILE(2)

NAME
aa_stack_profile, aa_stack_onexec - combine multiple profiles to confine a task

SYNOPSIS
#include <sys/apparmor.h>

int aa_stack_profile(const char *profile);

int aa_stack_onexec(const char *profile);

Link with -lapparmor when compiling.

DESCRIPTION
AppArmor supports stacking two or more profiles when confining a task. The result is an intersection of all profiles which are stacked. Stacking profiles together is desirable when wanting to ensure that confinement will never become more permissive. When changing between two profiles, as performed with aa_change_profile(2), there is always the possibility that the new profile is more permissive than the old profile but that possibility is eliminated when using aa_stack_profile().

To stack a profile with the current confinement context, a task can use the aa_stack_profile() function. The profile parameter is a NUL-terminated string indicating a profile name that should be stacked with the current confinement.

Calling aa_stack_profile("profile_a") while unconfined is equivalent to calling aa_change_profile("profile_a") since the intersection of unconfined and "profile_a" is "profile_a". Calling aa_stack_profile("profile_b") while confined by "profile_a" results in the task's confinement to be the intersection of "profile_a" and "profile_b". The resulting confinement context will be represented as "profile_a//&profile_b" in audit log messages, the return value of aa_getcon(2), etc.

Confined programs wanting to use aa_stack_profile() need to have rules permitting stacking the named profile. See apparmor.d(8) for details.

Open file descriptors may not be remediated after a call to aa_stack_profile() so the calling program must close(2) open file descriptors to ensure they are not available after calling aa_stack_profile().

The aa_stack_onexec() function is like the aa_stack_profile() function except it specifies that the stacking 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 stacking 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.

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

ERRORS

EINVAL
    AppArmor 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.

ENOMEM
    Insufficient kernel memory was available.

ENOENT
    The specified profile does not exist, or is not visible from the current namespace.

NOTES
Using aa_stack_profile() and related libapparmor functions are the only way to ensure compatibility between varying kernel versions. However, there may be some situations where libapparmor is not available and directly interacting with the AppArmor filesystem is required to stack a profile.

To immediately stack a profile named "profile_a", as performed with aa_stack_profile("profile_a"), the equivalent of this shell command can be used:

 $ echo -n "stack profile_a" > /proc/self/attr/current

To stack a profile named "profile_a" at the next exec, as performed with aa_stack_onexec("profile_a"), the equivalent of this shell command can be used:

 $ echo -n "stack profile_a" > /proc/self/attr/exec

These raw AppArmor filesystem operations must only be used when using libapparmor is not a viable option.

EXAMPLE
The following example shows a simple, if contrived, use of aa_stack_profile().

 #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>

 static void read_passwd()
 {
         int fd;
         char buf[10];

         if ((fd=open("/etc/passwd", O_RDONLY)) < 0) {
                perror("Failure opening /etc/passwd");
                _exit(1);
         }

         /* Verify that we can read /etc/passwd */
         memset(&buf, 0, 10);
         if (read(fd, &buf, 10) == -1) {
                 perror("Failure reading /etc/passwd");
                 _exit(1);
         }
         buf[9] = '\0';
         printf("/etc/passwd: %s\n", buf);
         close(fd);
 }

 int main(int argc, char * argv[])
 {
         printf("Before aa_stack_profile():\n");
         read_passwd();

         /* stack the "i_cant_be_trusted_anymore" profile, which
          * should not have read access to /etc/passwd. */
         if (aa_stack_profile("i_cant_be_trusted_anymore") < 0) {
             perror("Failure changing profile -- aborting");
             _exit(1);
         }

         printf("After aa_stack_profile():\n");
         read_passwd();
         _exit(0);
 }

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

 # Confine stack_p to be able to read /etc/passwd and aa_stack_profile()
 # to the 'i_cant_be_trusted_anymore' profile.
 /tmp/stack_p {
     /etc/ld.so.cache          mr,
     /lib/ld-*.so*             mrix,
     /lib/libc*.so*            mr,

     /etc/passwd               r,

     # Needed for aa_stack_profile()
     change-profile -> &i_cant_be_trusted_anymore,
     /usr/lib/libapparmor*.so* mr,
     /proc/[0-9]*/attr/current w,
 }

As well as the profile to stack:

 profile i_cant_be_trusted_anymore {
     /etc/ld.so.cache      mr,
     /lib/ld-*.so*         mrix,
     /lib/libc*.so*        mr,
 }

The output when run:

 $ /tmp/stack_p
 Before aa_stack_profile():
 /etc/passwd: root:x:0:
 After aa_stack_profile():
 Failure opening /etc/passwd: Permission denied
 $

BUGS
None known. If you find any, please report them at <https://gitlab.com/apparmor/apparmor/-/issues>. Note that using aa_stack_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.

SEE ALSO
apparmor(7), apparmor.d(5), apparmor_parser(8), aa_change_profile(2), aa_getcon(2) and <https://wiki.apparmor.net>.