[Intel-gfx] [PATCH] drm/i915: Avoid use-after-free of ctx in request tracepoints

[Tvrtko Ursulin]

On 17/03/2017 07:56, Chris Wilson wrote:
> On Fri, Mar 17, 2017 at 07:47:23AM +0000, Tvrtko Ursulin wrote:
>>
>> On 16/03/2017 20:42, Chris Wilson wrote:
>>> trace_i915_gem_request_out may be used after the request is completed,
>>> and so the request may have been retired on another thread, invalidating
>>> the rq->ctx. Avoid dereferencing rq->ctx in the tracepoint by switching
>>> to the fence context id instead, updating all tracepoints to match.
>>>
>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>> ---
>>> drivers/gpu/drm/i915/i915_trace.h | 8 ++++----
>>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
>>> index 5503f5ab1e98..66404c5aee82 100644
>>> --- a/drivers/gpu/drm/i915/i915_trace.h
>>> +++ b/drivers/gpu/drm/i915/i915_trace.h
>>> @@ -590,7 +590,7 @@ TRACE_EVENT(i915_gem_request_queue,
>>> 	    TP_fast_assign(
>>> 			   __entry->dev = req->i915->drm.primary->index;
>>> 			   __entry->ring = req->engine->id;
>>> -			   __entry->ctx = req->ctx->hw_id;
>>> +			   __entry->ctx = req->fence.context;
>>> 			   __entry->seqno = req->fence.seqno;
>>> 			   __entry->flags = flags;
>>> 			   ),
>>> @@ -637,8 +637,8 @@ DECLARE_EVENT_CLASS(i915_gem_request,
>>>
>>> 	    TP_fast_assign(
>>> 			   __entry->dev = req->i915->drm.primary->index;
>>> -			   __entry->ctx = req->ctx->hw_id;
>>> 			   __entry->ring = req->engine->id;
>>> +			   __entry->ctx = req->fence.context;
>>> 			   __entry->seqno = req->fence.seqno;
>>> 			   __entry->global = req->global_seqno;
>>> 			   ),
>>> @@ -681,7 +681,7 @@ DECLARE_EVENT_CLASS(i915_gem_request_hw,
>>> 		    TP_fast_assign(
>>> 			           __entry->dev = req->i915->drm.primary->index;
>>> 			           __entry->ring = req->engine->id;
>>> -			           __entry->ctx = req->ctx->hw_id;
>>> +			           __entry->ctx = req->fence.context;
>>> 			           __entry->seqno = req->fence.seqno;
>>> 			           __entry->global_seqno = req->global_seqno;
>>> 			           __entry->port = port;
>>> @@ -776,7 +776,7 @@ TRACE_EVENT(i915_gem_request_wait_begin,
>>> 	    TP_fast_assign(
>>> 			   __entry->dev = req->i915->drm.primary->index;
>>> 			   __entry->ring = req->engine->id;
>>> -			   __entry->ctx = req->ctx->hw_id;
>>> +			   __entry->ctx = req->fence.context;
>>> 			   __entry->seqno = req->fence.seqno;
>>> 			   __entry->global = req->global_seqno;
>>> 			   __entry->flags = flags;
>>>
>>
>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> req->engine is fine? No chance of request re-use in the window?
>
> We still hold a reference on the rq, so we don't need to play rcu games
> to acquire a stable rq for the tracepoint. We only need to worry about
> objects whose lifetime is coupled to rq retirement. engine/i915 outlive
> all requests.

Yes I know, but I thought the problem was request re-cycling. Becuase 
how does context go away, dont' we have it pinned until the following 
request completes any longer? (To avoid the unpin during context save.)

Regards,

Tvrtko