Re: [PATCH for-3.18.y] perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 09, 2018 at 07:12:42PM +0530, Amit Pundir wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> commit 321027c1fe77f892f4ea07846aeae08cefbbb290 upstream.
> 
> 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>
> [bwh: Backported to 3.16:
>  - Use ACCESS_ONCE() instead of READ_ONCE()
>  - Test perf_event::group_flags instead of group_caps
>  - Add the err_locked cleanup block, which we didn't need before
>  - Adjust context]
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
> ---
> This upstream patch is featured in recent Android Security bulletin.
> Picked up this backported patch from android-3.18. Build tested on 3.18.91

Thanks for this, now queued up.

greg k-h