CVE-2026-31656

Status: UPDATED  |  Advisory ID: CVE-2026-31656

Key Details

Affected Products, Subsystems & Sectors

What to Know

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

drm/i915/gt: fix refcount underflow in intel_engine_park_heartbeat

A use-after-free / refcount underflow is possible when the heartbeat

worker and intel_engine_park_heartbeat() race to release the same

engine->heartbeat.systole request.

The heartbeat worker reads engine->heartbeat.systole and calls

i915_request_put() on it when the request is complete, but clears

the pointer in a separate, non-atomic step. Concurrently, a request

retirement on another CPU can drop the engine wakeref to zero, triggering

__engine_park() -> intel_engine_park_heartbeat(). If the heartbeat

timer is pending at that point, cancel_delayed_work() returns true and

intel_engine_park_heartbeat() reads the stale non-NULL systole pointer

and calls i915_request_put() on it again, causing a refcount underflow:

```

<4> [487.221889] Workqueue: i915-unordered engine_retire [i915]

<4> [487.222640] RIP: 0010:refcount_warn_saturate+0x68/0xb0

...

<4> [487.222707] Call Trace:

<4> [487.222711] <TASK>

<4> [487.222716] intel_engine_park_heartbeat.part.0+0x6f/0x80 [i915]

<4> [487.223115] intel_engine_park_heartbeat+0x25/0x40 [i915]

<4> [487.223566] __engine_park+0xb9/0x650 [i915]

<4> [487.223973] ____intel_wakeref_put_last+0x2e/0xb0 [i915]

<4> [487.224408] __intel_wakeref_put_last+0x72/0x90 [i915]

<4> [487.224797] intel_context_exit_engine+0x7c/0x80 [i915]

<4> [487.225238] intel_context_exit+0xf1/0x1b0 [i915]

<4> [487.225695] i915_request_retire.part.0+0x1b9/0x530 [i915]

<4> [487.226178] i915_request_retire+0x1c/0x40 [i915]

<4> [487.226625] engine_retire+0x122/0x180 [i915]

<4> [487.227037] process_one_work+0x239/0x760

<4> [487.227060] worker_thread+0x200/0x3f0

<4> [487.227068] ? __pfx_worker_thread+0x10/0x10

<4> [487.227075] kthread+0x10d/0x150

<4> [487.227083] ? __pfx_kthread+0x10/0x10

<4> [487.227092] ret_from_fork+0x3d4/0x480

<4> [487.227099] ? __pfx_kthread+0x10/0x10

<4> [487.227107] ret_from_fork_asm+0x1a/0x30

<4> [487.227141] </TASK>

```

Fix this by replacing the non-atomic pointer read + separate clear with

xchg() in both racing paths. xchg() is a single indivisible hardware

instruction that atomically reads the old pointer and writes NULL. This

guarantees only one of the two concurrent callers obtains the non-NULL

pointer and performs the put, the other gets NULL and skips it.

(cherry picked from commit 13238dc0ee4f9ab8dafa2cca7295736191ae2f42) (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References