[D/E][PATCH 0/1] Fix apparmor reference leak via AF_ALG

[Mauricio Faria de Oliveira]

BugLink: https://bugs.launchpad.net/bugs/1883962

Regarding the submission policies for this kernel SRU cycle:
this patch does not have to be applied, but review/ack would be
useful, if at all possible.

This has already been submitted/reviewed for B/F but not D/E.

I've recently learned that D's 5.0 kernel and E's 5.3 kernel
should still be supported for a while on some custom kernels,
thus sending it for D and E. Apologies for the inconvenience
and late submission.

[Impact]

 * Users of the Crypto (user-space) API (i.e., AF_ALG)
   can trigger refcount errors in AppArmor under high
   load (might lead to memory leak or use after free.)

 * There is a reference leak in AppArmor when af_alg_accept()
   calls security_sock_graft() and then security_sk_clone().

 * Both acquire a reference to a label, to assign it to the
   same pointer, but the latter does not release the former's
   acquired reference (before overwriting the pointer value.)

 * This reference leak builds up over time, and under high
   load can eventually overflow/underflow/saturate refcount,
   depending on which value it has when a program hits that.

 * The fix just checks if the pointer has an assigned label,
   then releases its acquired reference.

[Test Case]

 * See comment #1 for the test-case 'aa-refcnt-af_alg.c'.

 * Exercise that code path indefinitely until it hits
   the refcount_t overflow/underflow/saturate message
   (or not, with the patch.) (see comment #4)

 * It's possible to monitor refcount values with kprobes,
   to confirm whether or not the problem is happening.
   (see comments #2 and #3)

[Other Info]

 * Patch applied upstream on v5.8-rc1 [1]
 * Applied on Unstable (tag Ubuntu-5.8-5.8.0-0.1)
 * Not required on Groovy (still 5.4; should sync from Unstable)
 * Already sent/reviewed for Bionic and Focal.
 * Now sending for Disco and Eoan (for custom kernels support)
   both have been build tested.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=3b646abc5bc6c0df649daea4c2c976bd4d47e4c8

Mauricio Faria de Oliveira (1):
  apparmor: check/put label on apparmor_sk_clone_security()

 security/apparmor/lsm.c | 5 +++++
 1 file changed, 5 insertions(+)

-- 
2.25.1