CVE-2026-31656

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)