CVE-2024-26939

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

drm/i915/vma: Fix UAF on destroy against retire race

Object debugging tools were sporadically reporting illegal attempts to free 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 is deactivating the VMA inside __active_retire() helper, after the VMAs active counter has been already decremented to 0, but before deactivation of the VMAs 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 wouldnt stop the VMA from being used, e.g. from __i915_vma_retire() called at the end of __active_retire(), after that VMA has been already freed by a concurrent i915_vma_destroy() on return from the i915_active_fini(). Then, we should rather fix the issue at the VMA level, not in i915_active.

Since __i915_vma_parked() is called from __gt_park() on last put of the GTs wakeref, the issue could be addressed by holding the GT wakeref long enough for __active_retire() to complete before that wakeref is released and 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() from a 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 a bug 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 doesnt acquire a GT wakeref by itself. Instead, it depends on a wakeref held directly by the requests active intel_context for a GT associated with its VM, and indirectly on that intel_contexts engine wakeref if the engine belongs to the same GT as the VMAs VM. Those wakerefs are released asynchronously to VMA deactivation.

Fix the issue by getting a wakeref for the VMAs 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 goes idle. Since __i915_vma_retire() may be called from atomic contexts, use async 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—

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/0e45882ca829b26b915162e8e86dbb1095768e9e
• https://git.kernel.org/stable/c/59b2626dd8c8a2e13f18054b3530e0c00073d79f
• https://git.kernel.org/stable/c/5e3eb862df9f972ab677fb19e0d4b9b1be8db7b5
• https://git.kernel.org/stable/c/704edc9252f4988ae1ad7dafa23d0db8d90d7190