Re: [PATCH] drm/i915/gt: Fix potential UAF by revoke of fence registers

[Andi Shyti <andi.shyti@linux.intel.com>]

Hi Janusz,

On Mon, Jun 03, 2024 at 09:54:45PM +0200, Janusz Krzysztofik wrote:
> CI has been sporadically reporting the following issue triggered by
> igt@i915_selftest@live@hangcheck on ADL-P and similar machines:
> 
> <6> [414.049203] i915: Running intel_hangcheck_live_selftests/igt_reset_evict_fence
> ...
> <6> [414.068804] i915 0000:00:02.0: [drm] GT0: GUC: submission enabled
> <6> [414.068812] i915 0000:00:02.0: [drm] GT0: GUC: SLPC enabled
> <3> [414.070354] Unable to pin Y-tiled fence; err:-4
> <3> [414.071282] i915_vma_revoke_fence:301 GEM_BUG_ON(!i915_active_is_idle(&fence->active))
> ...
> <4>[  609.603992] ------------[ cut here ]------------
> <2>[  609.603995] kernel BUG at drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c:301!
> <4>[  609.604003] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> <4>[  609.604006] CPU: 0 PID: 268 Comm: kworker/u64:3 Tainted: G     U  W          6.9.0-CI_DRM_14785-g1ba62f8cea9c+ #1
> <4>[  609.604008] Hardware name: Intel Corporation Alder Lake Client Platform/AlderLake-P DDR4 RVP, BIOS RPLPFWI1.R00.4035.A00.2301200723 01/20/2023
> <4>[  609.604010] Workqueue: i915 __i915_gem_free_work [i915]
> <4>[  609.604149] RIP: 0010:i915_vma_revoke_fence+0x187/0x1f0 [i915]
> ...
> <4>[  609.604271] Call Trace:
> <4>[  609.604273]  <TASK>
> ...
> <4>[  609.604716]  __i915_vma_evict+0x2e9/0x550 [i915]
> <4>[  609.604852]  __i915_vma_unbind+0x7c/0x160 [i915]
> <4>[  609.604977]  force_unbind+0x24/0xa0 [i915]
> <4>[  609.605098]  i915_vma_destroy+0x2f/0xa0 [i915]
> <4>[  609.605210]  __i915_gem_object_pages_fini+0x51/0x2f0 [i915]
> <4>[  609.605330]  __i915_gem_free_objects.isra.0+0x6a/0xc0 [i915]
> <4>[  609.605440]  process_scheduled_works+0x351/0x690
> ...
> 
> In the past, there were similar failures reported by CI from other IGT
> tests, observed on other platforms.
> 
> Before commit 63baf4f3d587 ("drm/i915/gt: Only wait for GPU activity
> before unbinding a GGTT fence"), i915_vma_revoke_fence() was waiting for
> idleness of vma->active via fence_update().   That commit introduced
> vma->fence->active in order for the fence_update() to be able to wait
> selectively on that one instead of vma->active since only idleness of
> fence registers was needed.  But then, another commit 0d86ee35097a
> ("drm/i915/gt: Make fence revocation unequivocal") replaced the call to
> fence_update() in i915_vma_revoke_fence() with only fence_write(), and
> also added that GEM_BUG_ON(!i915_active_is_idle(&fence->active)) in front.
> No justification was provided on why we might then expect idleness of
> vma->fence->active without first waiting on it.
> 
> The issue can be potentially caused by a race among revocation of fence
> registers on one side and sequential execution of signal callbacks invoked
> on completion of a request that was using them on the other, still
> processed in parallel to revocation of those fence registers.  Fix it by
> waiting for idleness of vma->fence->active in i915_vma_revoke_fence().
> 
> Fixes: 0d86ee35097a ("drm/i915/gt: Make fence revocation unequivocal")
> Closes: https://gitlab.freedesktop.org/drm/intel/issues/10021
> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
> Cc: stable@vger.kernel.org # v5.8+

Just wondering whether we really need the stable kernel here.

We have just an alleged failure reported on a selftest. I think
we can drop the stable requirement.

Otherwise,

Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>

Thanks,
Andi