[apparmor] [RFC PATCH 1/1] libapparmor: Create man page for aa_stack_profile()/aa_stack_onexec()
Seth Arnold
seth.arnold at canonical.com
Tue Jan 12 01:02:29 UTC 2016
Thanks for getting this started; some comments inline:
On Mon, Jan 11, 2016 at 06:17:47PM -0600, Tyler Hicks wrote:
> +=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_change_profile("profile_b") while
Typo here, this should read aa_stack_profile("profile_b")
> +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.
> +
> +Stacking another profile via aa_stack_profile() is permanent and the process is not
> +permitted to revert to the previous confinement context. Unlike
I'm not sure this is true; aa_change_profile(), if properly allowed by
policy, or Px rules in profiles, would both allow returning to previous
confinement, right?
> +aa_change_profile(2), confined programs wanting to use aa_stack_profile() need
> +no special rules in their profile to stack a new profile since the operation
> +does not broaden the allowed permissions.
> +
> +Open file descriptors are not 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().
I'm not sure this is strictly true. While there's no plans for
revalidating all open files at the time of aa_stack_profile(), I don't
think we can promise (or should promise) that revalidation won't happen
on future read(), write(), sendfile(), etc. calls. John has plans for
what delegating authority looks like, and I don't want this paragraph
here to complicate those plans.
> +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
Hmm, this is an interesting point. There's something nice about the
directness and clarity here but I think this may prematurely prevent some
otherwise interesting uses of stacking.
I'm also afraid that this kind of rule might also allow these APIs to be
used by exploit code to unreasonably manipulate profile transitions in
ways that aren't expected by policy authors.
> +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>
> +
> +The AppArmor kernel module 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.
Since AppArmor must be built-in these days we should probably reduce
references to the old module style. (Nevermind the name "LSM"..)
> +=item B<ENOMEM>
> +
> +Insufficient kernel memory was available.
> +
> +=item B<EPERM>
> +
> +The calling application is not confined by AppArmor, or the no_new_privs
> +bit is set.
When would "not confined" be returned as an error in these APIs?
aa_stack_profile() was defined above to be the same as aa_change_profile()
if the process was currently unconfined; aa_stack_onexec() might be
different, but it feels like an artificial requirement. Am I overlooking
something?
> +=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 among 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 "stackprofile 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 "stackexec 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()
> + /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://bugs.launchpad.net/apparmor/+filebug>. 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<http://wiki.apparmor.net>.
> +
> +=cut
Thanks!
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