CVE-2024-41092

In the Linux kernel, the following vulnerability has been resolved:

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

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] … <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().

(cherry picked from commit 24bb052d3dd499c5956abad5f7d8e4fd07da7fb1)

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/06dec31a0a5112a91f49085e8a8fa1a82296d5c7
• https://git.kernel.org/stable/c/29c0fdf49078ab161570d3d1c6e13d66f182717d
• https://git.kernel.org/stable/c/414f4a31f7a811008fd9a33b06216b060bad18fc
• https://git.kernel.org/stable/c/996c3412a06578e9d779a16b9e79ace18125ab50
• https://git.kernel.org/stable/c/ca0fabd365a27a94a36e68a7a02df8ff3c13dac6
• https://git.kernel.org/stable/c/f771b91f21c46ad1217328d05e72a2c7e3add535