Linux: CVE-2017-6001: Incomplete fix for CVE-2016-6786: perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race

[Salvatore Bonaccorso <carnil () debian org>]

Hi

The original fix for CVE-2016-6786 ws incomplete. Upstream has
commited

https://git.kernel.org/linus/321027c1fe77f892f4ea07846aeae08cefbbb290

which is in v4.10-rc4 (and also backported to 4.9.x in v4.9.7). This
has been assigned a new CVE identifier: CVE-2017-6001 (assigned via ->
https://cveform.mitre.org/).

Commit message reads as:

> commit 321027c1fe77f892f4ea07846aeae08cefbbb290
> Author: Peter Zijlstra <peterz () infradead org>
> Date:   Wed Jan 11 21:09:50 2017 +0100
>
>     perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race
>     
>     Di Shen reported a race between two concurrent sys_perf_event_open()
>     calls where both try and move the same pre-existing software group
>     into a hardware context.
>     
>     The problem is exactly that described in commit:
>     
>       f63a8daa5812 ("perf: Fix event->ctx locking")
>     
>     ... where, while we wait for a ctx->mutex acquisition, the event->ctx
>     relation can have changed under us.
>     
>     That very same commit failed to recognise sys_perf_event_context() as an
>     external access vector to the events and thereby didn't apply the
>     established locking rules correctly.
>     
>     So while one sys_perf_event_open() call is stuck waiting on
>     mutex_lock_double(), the other (which owns said locks) moves the group
>     about. So by the time the former sys_perf_event_open() acquires the
>     locks, the context we've acquired is stale (and possibly dead).
>     
>     Apply the established locking rules as per perf_event_ctx_lock_nested()
>     to the mutex_lock_double() for the 'move_group' case. This obviously means
>     we need to validate state after we acquire the locks.
>     
>     Reported-by: Di Shen (Keen Lab)
>     Tested-by: John Dias <joaodias () google com>
>     Signed-off-by: Peter Zijlstra (Intel) <peterz () infradead org>
>     Cc: Alexander Shishkin <alexander.shishkin () linux intel com>
>     Cc: Arnaldo Carvalho de Melo <acme () kernel org>
>     Cc: Arnaldo Carvalho de Melo <acme () redhat com>
>     Cc: Jiri Olsa <jolsa () redhat com>
>     Cc: Kees Cook <keescook () chromium org>
>     Cc: Linus Torvalds <torvalds () linux-foundation org>
>     Cc: Min Chong <mchong () google com>
>     Cc: Peter Zijlstra <peterz () infradead org>
>     Cc: Stephane Eranian <eranian () google com>
>     Cc: Thomas Gleixner <tglx () linutronix de>
>     Cc: Vince Weaver <vincent.weaver () maine edu>
>     Fixes: f63a8daa5812 ("perf: Fix event->ctx locking")
>     Link: http://lkml.kernel.org/r/20170106131444.GZ3174 () twins programming kicks-ass net
>     Signed-off-by: Ingo Molnar <mingo () kernel org>

Regards,
Salvatore