# This publication is intellectual property of Canonical Ltd. Its contents
# can be duplicated, either in part or in whole, provided that a copyright
# label is visibly located on each copy.
#
# All information found in this book has been compiled with utmost
# attention to detail. However, this does not guarantee complete accuracy.
# Neither Canonical Ltd, the authors, nor the translators shall be held
# liable for possible errors or the consequences thereof.
#
# Many of the software and hardware descriptions cited in this book
# are registered trademarks. All trade names are subject to copyright
# restrictions and may be registered trade marks. Canonical Ltd.
# essentially adhere to the manufacturer's spelling.
#
# Names of products and trademarks appearing in this book (with or without
# specific notation) are likewise subject to trademark and trade protection
# laws and may thus fall under copyright restrictions.
#


=pod

=head1 NAME

aa_stack_profile, aa_stack_onexec - combine multiple profiles to confine a task

=head1 SYNOPSIS

B<#include E<lt>sys/apparmor.hE<gt>>

B<int aa_stack_profile(const char *profile);>

B<int aa_stack_onexec(const char *profile);>

Link with B<-lapparmor> when compiling.

=head1 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 I<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.

=head1 RETURN VALUE

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

=head1 ERRORS

=over 4

=item B<EINVAL>

AppArmor is not loaded, neither a profile nor a namespace was specified,
or the communication via the F</proc/*/attr/current> file did not conform
to protocol.

=item B<ENOMEM>

Insufficient kernel memory was available.

=item B<ENOENT>

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

=back

=head1 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.

=head1 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
 $

=head1 BUGS

None known. If you find any, please report them at
L<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.

=head1 SEE ALSO

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

=cut
