aa_change_hat (2) manual page

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aa_change_hat - change to or from a “hat” within a AppArmor profile


#include <sys/apparmor.h>

int aa_change_hat (char *subprofile, unsigned long magic_token);

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 hats” to a different role, also known as a subprofile. To change into a new hat, it calls the aa_change_hat() function to do so. It passes in a pointer to the subprofile which it wants to change into, and a 64bit magic_token. The magic_token is used to return out of the subprofile at a later time.

If a program wants to return out of the current subprofile to the original profile, it calls aa_change_hat() with a pointer to NULL as the subprofile, and the original magic_token value. If the magic_token does not match the original magic_token passed into the kernel when the program entered the subprofile, the change back to the original profile will not happen, and the current task will be killed. If the magic_token matches the original token, then the process will change back to the original profile.

If the program wants to change to a subprofile that it can never change back out of, the application should call aa_change_hat() with a magic_token of 0.

As both read(2) and write(2) are mediated, a file must be listed in a subprofile definition if the file is to be accessed while the process is in a “hat".

Return Value

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


The apparmor kernel module is not loaded or the communication via the /proc/*/attr/current file did not conform to protocol.

Insufficient kernel memory was available.

The calling application is not confined by apparmor.

The application’s profile has no hats defined for it.

The specified subprofile does not exist in this profile or the process tried to change another process’s domain.


The following code examples shows simple, if contrived, uses of aa_change_hat(); a typical use of aa_change_hat() will separate privileged portions of a process from unprivileged portions of a process, such as keeping unauthenticated network traffic handling separate from authenticated network traffic handling in OpenSSH or executing user-supplied CGI scripts in apache.

The use of random(3) is simply illustrative. Use of /dev/urandom is recommended.

First, a simple high-level overview of aa_change_hat() use:

void foo (void) {
unsigned long magic_token;

/* get a random magic token value from our huge entropy pool */
magic_token = random_function();

/* change into the subprofile while * we do stuff we don’t trust */ aa_change_hat("stuff_we_dont_trust", magic_token);

/* Go do stuff we don’t trust -- this is all * done in *this* process space, no separate * fork()/exec()’s are done. */ interpret_perl_stuff(stuff_from_user);

/* now change back to our original profile */ aa_change_hat(NULL, magic_token);

Second, an example to show that files not listed in a subprofile ("hat") aren’t accessible after an aa_change_hat() call:

#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;
unsigned long tok;
char buf[10];

/* random() is a poor choice */ tok = random();

/* open /etc/passwd outside of any hat */ if ((fd=open("/etc/passwd", O_RDONLY)) < 0) perror("Failure opening /etc/passwd");

/* 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 pre-hat"); _exit(1) ;
buf[9] = ‘\0’;
printf("/etc/passwd: %s\n", buf);

/* change hat to the “hat” subprofile, which should not have * read access to /etc/passwd -- even though we have a valid * file descriptor at the time of the aa_change_hat() call. */ if (aa_change_hat("hat", tok)) { perror("Failure changing hat -- aborting"); _exit(1) ;

/* confirm that we cannot read /etc/passwd */ lseek(fd,0,SEEK_SET);
memset(&buf, 0, 10);
if (read(fd, &buf, 10) == -1)
perror("Failure reading /etc/passwd post-hat"); buf[9] = ‘\0’;
printf("/etc/passwd: %s\n", buf);

return 0;

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

/tmp/ch {

mr, /usr/lib/gconv/gconv-modules* mr,

mr, /lib/libapparmor*.so* mr,


^hat {

rw, } }

The output when run:

$ /tmp/ch
/etc/passwd: root:x:0:
Failure reading /etc/passwd post-hat: Permission denied /etc/passwd:


None known. If you find any, please report them to bugzilla at <http://bugzilla.novell.com>. Note that aa_change_hat(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) , and <http://forge.novell.com/modules/xfmod/project/?apparmor>.

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