Re: [PATCH 1/3] drm/i915/gt: Do not allow setting ring size for legacy ring submission

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On 21/06/2021 14:07, Maarten Lankhorst wrote:
Op 21-06-2021 om 14:52 schreef Tvrtko Ursulin:

On 21/06/2021 13:08, Tvrtko Ursulin wrote:

I had some questions on the trybot mailing list, let me copy&paste..

On 21/06/2021 12:41, Maarten Lankhorst wrote:
It doesn't work for legacy ring submission, and is in the best case
ignored.

Looks rejected instead of ignored:

static int set_ringsize(struct i915_gem_context *ctx,
              struct drm_i915_gem_context_param *args)
{
      if (!HAS_LOGICAL_RING_CONTEXTS(ctx->i915))
          return -ENODEV;

In the worst case we end up freeing engine->legacy.ring for all other
active engines, resulting in a use-after-free.

Worst case is cloning because ring_context_alloc is not taking a reference to engine->legacy.ring, or something else?

No can't be that, it was my incomplete analysis last week. Since ring_context_destroy does not actually free the legacy ring I don't see any use after free paths.

Regards,

Hmm, it gets stuck inside intel_context_set_ring_size when cloning engines..

I guess it can't happen in practice, just the code introduces the race by preallocating
inside intel_context_lock_pinned()..

"The code" being the rest of your series? Haven't looked in there, but can't find a problem in upstream. Since as you say, copy_ring_size will run but intel_context_set_ring_size will not free-and-allocate old/new ring since cloned context does not have a state allocated yet.

Regards,

Tvrtko

copy_ring_size() should only be called for HAS_LOGICAL_RING_CONTEXTS().
I guess that makes this patch obsolete. It can safely be dropped from the series,
I think I should probably introduce a check to only set the size when HAS_LOGICAL_RING_CONTEXTS
evaluates to true, but that wouldn't block the rest of this series.

~Maarten

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