---
id: CVE-2026-31656
title: 'drm/i915/gt: fix refcount underflow in intel_engine_park_heartbeat'
summary: |-
  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_he…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    82034799c6c14b3104668878c3f3e5786f777126
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    70d3e622b10092fc483e28e57b4e8c49d9cc7f68
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    8ce44d28a84fd5e053a88b04872a89d95c0779d4
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    ca3f48c3567dd49efdc55b80029ae74659c682ee
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    a00e92bf6583d019a4fb2c2df7007e6c9b269ce7
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    2af8b200cae3fdd0e917ecc2753b28bb40c876c1
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    455d98ed527fc94eed90406f90ab2391464ca657
  - >-
    Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa <
    4c71fd099513bfa8acab529b626e1f0097b76061
  - Linux 5.5
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-10-16T15:11:35.318678Z'
published: '2026-04-24'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T15:20:19.352Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-31656'
references:
  - url: 'https://git.kernel.org/stable/c/82034799c6c14b3104668878c3f3e5786f777126'
  - url: 'https://git.kernel.org/stable/c/70d3e622b10092fc483e28e57b4e8c49d9cc7f68'
  - url: 'https://git.kernel.org/stable/c/8ce44d28a84fd5e053a88b04872a89d95c0779d4'
  - url: 'https://git.kernel.org/stable/c/ca3f48c3567dd49efdc55b80029ae74659c682ee'
  - url: 'https://git.kernel.org/stable/c/a00e92bf6583d019a4fb2c2df7007e6c9b269ce7'
  - url: 'https://git.kernel.org/stable/c/2af8b200cae3fdd0e917ecc2753b28bb40c876c1'
  - url: 'https://git.kernel.org/stable/c/455d98ed527fc94eed90406f90ab2391464ca657'
  - url: 'https://git.kernel.org/stable/c/4c71fd099513bfa8acab529b626e1f0097b76061'
tags:
  - cve.org
epss: 0.00117
epssPercentile: 0.01547
ingestedAt: '2026-10-16T17:24:46.666Z'
---

## Overview

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)

## Affected

- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < 82034799c6c14b3104668878c3f3e5786f777126`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < 70d3e622b10092fc483e28e57b4e8c49d9cc7f68`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < 8ce44d28a84fd5e053a88b04872a89d95c0779d4`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < ca3f48c3567dd49efdc55b80029ae74659c682ee`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < a00e92bf6583d019a4fb2c2df7007e6c9b269ce7`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < 2af8b200cae3fdd0e917ecc2753b28bb40c876c1`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < 455d98ed527fc94eed90406f90ab2391464ca657`
- `Linux >= 058179e72e0956a2dfe4927db6cbe5fbfb2406aa < 4c71fd099513bfa8acab529b626e1f0097b76061`
- `Linux 5.5`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.