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

[Janusz Krzysztofik]

Hi Andi,

Thanks for review.

On Tuesday, 4 June 2024 02:48:43 GMT+2 Andi Shyti wrote:
> 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 at i915_selftest@live at 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 at linux.intel.com>
> > Cc: stable at 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.

Please note there were similar failures from other tests reported by CI in the 
past, e.g., https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/8846, 
https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10638.  Would you like 
me to mention them in commit description.

OTOH, stable will probably pick this patch up themselves, based on the Fixes: 
commit ID, even if we drop the Cc: stable.

Anyway, please feel free to drop Cc: stable, you or anyone who will be 
pushing.

Thanks,
Janusz

> 
> Otherwise,
> 
> Reviewed-by: Andi Shyti <andi.shyti at linux.intel.com>
> 
> Thanks,
> Andi
>