NAME
aa_change_hat - change to or from a "hat" within a AppArmor profile
SYNOPSIS
#include <sys/apparmor.h>
int aa_change_hat (char *subprofile, unsigned long magic_token);
Link with -lapparmor when compiling.
DESCRIPTION
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.
ERRORS
EINVAL
The apparmor kernel module is not loaded or the communication via
the /proc/*/attr/current file did not conform to protocol.
ENOMEM
Insufficient kernel memory was available.
EPERM
The calling application is not confined by apparmor.
ECHILD
The application's profile has no hats defined for it.
EACCES
The specified subprofile does not exist in this profile or the
process tried to change another process's domain.
EXAMPLE
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 {
/etc/ld.so.cache mr,
/etc/locale/** r,
/etc/localtime r,
/usr/share/locale/** r,
/usr/share/zoneinfo/** r,
/usr/lib/locale/** mr,
/usr/lib/gconv/*.so mr,
/usr/lib/gconv/gconv-modules* mr,
/lib/ld-*.so* mrix,
/lib/libc*.so* mr,
/lib/libapparmor*.so* mr,
/dev/pts/* rw,
/tmp/ch mr,
/etc/passwd r,
^hat {
/dev/pts/* rw,
}
}
The output when run:
$ /tmp/ch
/etc/passwd: root:x:0:
Failure reading /etc/passwd post-hat: Permission denied
/etc/passwd:
$
BUGS
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>.