GHSA-4J69-96H5-Q8G9

Vulnerability from github – Published: 2026-04-24 15:32 – Updated: 2026-04-24 15:32
VLAI?
Details

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)

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{
  "affected": [],
  "aliases": [
    "CVE-2026-31656"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-04-24T15:16:45Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\ndrm/i915/gt: fix refcount underflow in intel_engine_park_heartbeat\n\nA use-after-free / refcount underflow is possible when the heartbeat\nworker and intel_engine_park_heartbeat() race to release the same\nengine-\u003eheartbeat.systole request.\n\nThe heartbeat worker reads engine-\u003eheartbeat.systole and calls\ni915_request_put() on it when the request is complete, but clears\nthe pointer in a separate, non-atomic step. Concurrently, a request\nretirement on another CPU can drop the engine wakeref to zero, triggering\n__engine_park() -\u003e intel_engine_park_heartbeat(). If the heartbeat\ntimer is pending at that point, cancel_delayed_work() returns true and\nintel_engine_park_heartbeat() reads the stale non-NULL systole pointer\nand calls i915_request_put() on it again, causing a refcount underflow:\n\n```\n\u003c4\u003e [487.221889] Workqueue: i915-unordered engine_retire [i915]\n\u003c4\u003e [487.222640] RIP: 0010:refcount_warn_saturate+0x68/0xb0\n...\n\u003c4\u003e [487.222707] Call Trace:\n\u003c4\u003e [487.222711]  \u003cTASK\u003e\n\u003c4\u003e [487.222716]  intel_engine_park_heartbeat.part.0+0x6f/0x80 [i915]\n\u003c4\u003e [487.223115]  intel_engine_park_heartbeat+0x25/0x40 [i915]\n\u003c4\u003e [487.223566]  __engine_park+0xb9/0x650 [i915]\n\u003c4\u003e [487.223973]  ____intel_wakeref_put_last+0x2e/0xb0 [i915]\n\u003c4\u003e [487.224408]  __intel_wakeref_put_last+0x72/0x90 [i915]\n\u003c4\u003e [487.224797]  intel_context_exit_engine+0x7c/0x80 [i915]\n\u003c4\u003e [487.225238]  intel_context_exit+0xf1/0x1b0 [i915]\n\u003c4\u003e [487.225695]  i915_request_retire.part.0+0x1b9/0x530 [i915]\n\u003c4\u003e [487.226178]  i915_request_retire+0x1c/0x40 [i915]\n\u003c4\u003e [487.226625]  engine_retire+0x122/0x180 [i915]\n\u003c4\u003e [487.227037]  process_one_work+0x239/0x760\n\u003c4\u003e [487.227060]  worker_thread+0x200/0x3f0\n\u003c4\u003e [487.227068]  ? __pfx_worker_thread+0x10/0x10\n\u003c4\u003e [487.227075]  kthread+0x10d/0x150\n\u003c4\u003e [487.227083]  ? __pfx_kthread+0x10/0x10\n\u003c4\u003e [487.227092]  ret_from_fork+0x3d4/0x480\n\u003c4\u003e [487.227099]  ? __pfx_kthread+0x10/0x10\n\u003c4\u003e [487.227107]  ret_from_fork_asm+0x1a/0x30\n\u003c4\u003e [487.227141]  \u003c/TASK\u003e\n```\n\nFix this by replacing the non-atomic pointer read + separate clear with\nxchg() in both racing paths. xchg() is a single indivisible hardware\ninstruction that atomically reads the old pointer and writes NULL. This\nguarantees only one of the two concurrent callers obtains the non-NULL\npointer and performs the put, the other gets NULL and skips it.\n\n(cherry picked from commit 13238dc0ee4f9ab8dafa2cca7295736191ae2f42)",
  "id": "GHSA-4j69-96h5-q8g9",
  "modified": "2026-04-24T15:32:37Z",
  "published": "2026-04-24T15:32:37Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-31656"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/2af8b200cae3fdd0e917ecc2753b28bb40c876c1"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/455d98ed527fc94eed90406f90ab2391464ca657"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/4c71fd099513bfa8acab529b626e1f0097b76061"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/70d3e622b10092fc483e28e57b4e8c49d9cc7f68"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/8ce44d28a84fd5e053a88b04872a89d95c0779d4"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a00e92bf6583d019a4fb2c2df7007e6c9b269ce7"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ca3f48c3567dd49efdc55b80029ae74659c682ee"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}


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