In the Linux kernel, the following vulnerability has been resolved:drm/i915/vma: Fix UAF on destroy against retire raceObject debugging tools were sporadically reporting illegal attempts tofree a still active i915 VMA object when parking a GT believed to be idle.[161.359441] ODEBUG: free active (active state 0) object: ffff88811643b958 object type: i915_active hint: __i915_vma_active+0x0/0x50 [i915][161.360082] WARNING: CPU: 5 PID: 276 at lib/debugobjects.c:514 debug_print_object+0x80/0xb0...[161.360304] CPU: 5 PID: 276 Comm: kworker/5:2 Not tainted 6.5.0-rc1-CI_DRM_13375-g003f860e5577+ #1[161.360314] Hardware name: Intel Corporation Rocket Lake Client Platform/RocketLake S UDIMM 6L RVP, BIOS RKLSFWI1.R00.3173.A03.2204210138 04/21/2022[161.360322] Workqueue: i915-unordered __intel_wakeref_put_work [i915][161.360592] RIP: 0010:debug_print_object+0x80/0xb0...[161.361347] debug_object_free+0xeb/0x110[161.361362] i915_active_fini+0x14/0x130 [i915][161.361866] release_references+0xfe/0x1f0 [i915][161.362543] i915_vma_parked+0x1db/0x380 [i915][161.363129] __gt_park+0x121/0x230 [i915][161.363515] ____intel_wakeref_put_last+0x1f/0x70 [i915]That has been tracked down to be happening when another thread isdeactivating the VMA inside __active_retire() helper, after the VMA'sactive counter has been already decremented to 0, but before deactivationof the VMA's object is reported to the object debugging tool.We could prevent from that race by serializing i915_active_fini() with__active_retire() via ref->tree_lock, but that wouldn't stop the VMA frombeing used, e.g. from __i915_vma_retire() called at the end of__active_retire(), after that VMA has been already freed by a concurrenti915_vma_destroy() on return from the i915_active_fini().  Then, we shouldrather fix the issue at the VMA level, not in i915_active.Since __i915_vma_parked() is called from __gt_park() on last put of theGT's wakeref, the issue could be addressed by holding the GT wakeref longenough for __active_retire() to complete before that wakeref is releasedand the GT parked.I believe the issue was introduced by commit d93939730347 ("drm/i915:Remove the vma refcount") which moved a call to i915_active_fini() froma dropped i915_vma_release(), called on last put of the removed VMA kref,to i915_vma_parked() processing path called on last put of a GT wakeref.However, its visibility to the object debugging tool was suppressed by abug in i915_active that was fixed two weeks later with commit e92eb246feb9("drm/i915/active: Fix missing debug object activation").A VMA associated with a request doesn't acquire a GT wakeref by itself.Instead, it depends on a wakeref held directly by the request's activeintel_context for a GT associated with its VM, and indirectly on thatintel_context's engine wakeref if the engine belongs to the same GT as theVMA's VM.  Those wakerefs are released asynchronously to VMA deactivation.Fix the issue by getting a wakeref for the VMA's GT when activating it,and putting that wakeref only after the VMA is deactivated.  However,exclude global GTT from that processing path, otherwise the GPU never goesidle.  Since __i915_vma_retire() may be called from atomic contexts, useasync variant of wakeref put.  Also, to avoid circular locking dependency,take care of acquiring the wakeref before VM mutex when both are needed.v7: Add inline comments with justifications for:    - using untracked variants of intel_gt_pm_get/put() (Nirmoy),    - using async variant of _put(),    - not getting the wakeref in case of a global GTT,    - always getting the first wakeref outside vm->mutex.v6: Since __i915_vma_active/retire() callbacks are not serialized, storing    a wakeref tracking handle inside struct i915_vma is not safe, and    there is no other good place for that.  Use untracked variants of    intel_gt_pm_get/put_async().v5: Replace "tile" with "GT" across commit description (Rodrigo),  - ---truncated---
CWE-416 Use After Free