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

[Colin Ian King]

On 29/06/2020 20:41, Mauricio Faria de Oliveira wrote:
> 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(+)
> 
Acked-by: Colin Ian King <colin.king at canonical.com>