CVE-2026-74658
In the Linux kernel, the following vulnerability has been resolved:
futex: Prevent robust futex exit race some more
A robust futex unlock stores 0 over the whole futex value - wiping FUTEX_WAITERS - and wakes a single waiter. That wakeup is a one-shot notification: the protocol relies on its recipient to either acquire the futex (and eventually unlock while aware of the remaining contention) or re-arm FUTEX_WAITERS before sleeping again. If the woken waiter is killed before it can do either, the kernel must jump in and wake the next task down the line.
This is a known complication of the futex protocol with a previous partial fix in commit ca16d5bee598 ("futex: Prevent robust futex exit race"). Unfortunately, that fix is insufficient.
Leer descripción completaMostrar menos
If a third task re-acquired the futex through the uncontended fast path in the meantime, the notification is lost: robust exit processing sees that it is owned by another task and does nothing, while the new owner sees no FUTEX_WAITERS when it unlocks and wakes nobody. The remaining waiters sleep forever behind a free futex:
This is clearly a shortcoming in the implementation, which fails to keep the FUTEX_WAITERS bit consistent.
Work around this by augmenting the robust list exit processing to also perform the extra wakeup if the futex word is owned by another thread but FUTEX_WAITERS is not set.
This does not fix the problem of a non-contended take over/release and free sequence, which has been discussed for years and has been addressed by commit 3ca9595d9fb6 ("futex: Add support for unlocking robust futexes") and subsequent changes, but failed to take the problem described above into account.
A more complete solution which is based on the in kernel unlock of contended robust futexes has been discussed in the context of this change and should show up in mainline sooner than later.
[ tglx: Amend change log slightly and fixup coding style ]
Detalles técnicos trazas, registros y código del informe original
A owns the futex, B and C sleep in FUTEX_WAIT
uval == A | FUTEX_WAITERS
A robust unlock: store 0, FUTEX_WAKE(1) wakes B
uval == 0
D fast path acquire: cmpxchg(0 -> D)
uval == D, no FUTEX_WAITERS
B killed before acting on the wakeup
B exit walk, pending op: owner D != B -> no action
D unlock: no FUTEX_WAITERS -> no wake
C sleeps foreverCVSS
NVD no ha asignado puntuación CVSS a esta CVE (habitual desde el cambio de política de abril de 2026).
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.18%
- Percentil entre todas las CVEs puntuadas: 7
- Fecha de la puntuación: 5/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
Tecnologías afectadas (1)
⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.
Referencias
- https://git.kernel.org/stable/c/33bfa85458105d6169ebdb697f692b8bb8025bae
- https://git.kernel.org/stable/c/6d4514ca9cdf61fec4ec634cf50386f6f7e69748
- https://git.kernel.org/stable/c/7b8c53263f8878bdd12c87e147ac6feca5c05211
- https://git.kernel.org/stable/c/7cf710e70f9bb8ea75f759ebed09871801315992
- https://git.kernel.org/stable/c/83b0f71d5a313a765754acab51d2ecc5de76e0b9
- https://git.kernel.org/stable/c/925628656b73b70930972ccde421de4f758d8650
- https://git.kernel.org/stable/c/a1c2b7b86a946b6b172bce44d74553da2323a36c
- https://git.kernel.org/stable/c/aa5c571901c6b22b58373693a4bf889ecab11ff5
JSON original (NVD)
Mostrar
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],
"vulnStatus": "Received",
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfutex: Prevent robust futex exit race some more\n\nA robust futex unlock stores 0 over the whole futex value - wiping\nFUTEX_WAITERS - and wakes a single waiter. That wakeup is a one-shot\nnotification: the protocol relies on its recipient to either acquire the\nfutex (and eventually unlock while aware of the remaining contention) or\nre-arm FUTEX_WAITERS before sleeping again. If the woken waiter is killed\nbefore it can do either, the kernel must jump in and wake the next task\ndown the line.\n\nThis is a known complication of the futex protocol with a previous\npartial fix in commit ca16d5bee598 (\"futex: Prevent robust futex exit\nrace\"). Unfortunately, that fix is insufficient.\n\nIf a third task re-acquired the futex through the uncontended fast\npath in the meantime, the notification is lost: robust exit processing\nsees that it is owned by another task and does nothing, while the new\nowner sees no FUTEX_WAITERS when it unlocks and wakes nobody.\nThe remaining waiters sleep forever behind a free futex:\n\n A owns the futex, B and C sleep in FUTEX_WAIT\n uval == A | FUTEX_WAITERS\n A robust unlock: store 0, FUTEX_WAKE(1) wakes B\n uval == 0\n D fast path acquire: cmpxchg(0 -> D)\n uval == D, no FUTEX_WAITERS\n B killed before acting on the wakeup\n B exit walk, pending op: owner D != B -> no action\n D unlock: no FUTEX_WAITERS -> no wake\n C sleeps forever\n\nThis is clearly a shortcoming in the implementation, which fails to keep\nthe FUTEX_WAITERS bit consistent.\n\nWork around this by augmenting the robust list exit processing to also\nperform the extra wakeup if the futex word is owned by another thread but\nFUTEX_WAITERS is not set.\n\nThis does not fix the problem of a non-contended take over/release and free\nsequence, which has been discussed for years and has been addressed by\ncommit 3ca9595d9fb6 (\"futex: Add support for unlocking robust futexes\") and\nsubsequent changes, but failed to take the problem described above into\naccount.\n\nA more complete solution which is based on the in kernel unlock of\ncontended robust futexes has been discussed in the context of this change\nand should show up in mainline sooner than later.\n\n[ tglx: Amend change log slightly and fixup coding style ]"
}
],
"lastModified": "2026-08-22T16:16:39.453",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}