{"id":"CVE-2026-31656","summary":"drm/i915/gt: fix refcount underflow in intel_engine_park_heartbeat","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)","modified":"2026-05-18T05:59:53.356658148Z","published":"2026-04-24T14:45:07.738Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31656.json"},"references":[{"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"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/31xxx/CVE-2026-31656.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-31656"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"058179e72e0956a2dfe4927db6cbe5fbfb2406aa"},{"fixed":"70d3e622b10092fc483e28e57b4e8c49d9cc7f68"},{"fixed":"8ce44d28a84fd5e053a88b04872a89d95c0779d4"},{"fixed":"ca3f48c3567dd49efdc55b80029ae74659c682ee"},{"fixed":"a00e92bf6583d019a4fb2c2df7007e6c9b269ce7"},{"fixed":"2af8b200cae3fdd0e917ecc2753b28bb40c876c1"},{"fixed":"455d98ed527fc94eed90406f90ab2391464ca657"},{"fixed":"4c71fd099513bfa8acab529b626e1f0097b76061"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-31656.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.5.0"},{"fixed":"5.15.203"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.169"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.135"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.82"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.23"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"6.19.13"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-31656.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"}]}