CVE-2026-74658
Full vulnerability record from the local CVENow database.
Description
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. 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: 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 forever 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 ]
Severity
Not scored
CVSS score
Not available
Vulnerability status
Received
Source identifier
416baaa9-dc9f-4396-8d5f-8c081fb06d67
Published
2026-08-22 16:16:39.453000
Last modified
2026-11-06 17:19:52.940000
View original CVE JSON
{
"id": "CVE-2026-74658",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"published": "2026-08-22T16:16:39.453",
"lastModified": "2026-11-06T17:19:52.940",
"vulnStatus": "Received",
"cveTags": [],
"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 ]"
},
{
"lang": "es",
"value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nfutex: Prevenir aún más la condición de carrera de salida de futex robusto\n\nUn desbloqueo de futex robusto almacena 0 sobre todo el valor del futex - borrando FUTEX_WAITERS - y despierta a un único esperador. Ese despertar es una notificación de un solo uso: el protocolo se basa en que su destinatario adquiera el futex (y eventualmente desbloquee siendo consciente de la contención restante) o rearme FUTEX_WAITERS antes de volver a dormir. Si el esperador despertado es eliminado antes de que pueda hacer cualquiera de las dos cosas, el kernel debe intervenir y despertar a la siguiente tarea en la línea.\n\nEsta es una complicación conocida del protocolo futex con una corrección parcial previa en el commit ca16d5bee598 ('futex: Prevenir la condición de carrera de salida de futex robusto'). Desafortunadamente, esa corrección es insuficiente.\n\nSi una tercera tarea readquirió el futex a través de la ruta rápida sin contención mientras tanto, la notificación se pierde: el procesamiento de salida robusto ve que es propiedad de otra tarea y no hace nada, mientras que el nuevo propietario no ve FUTEX_WAITERS cuando desbloquea y no despierta a nadie. Los esperadores restantes duermen para siempre detrás de un futex libre:\n\n A posee el futex, B y C duermen en FUTEX_WAIT\n uval == A | FUTEX_WAITERS\n Un desbloqueo robusto de A: almacena 0, FUTEX_WAKE(1) despierta a B\n uval == 0\n Adquisición por ruta rápida de D: cmpxchg(0 -> D)\n uval == D, no FUTEX_WAITERS\n B eliminado antes de actuar sobre el despertar\n Recorrido de salida de B, operación pendiente: propietario D != B -> ninguna acción\n Desbloqueo de D: no FUTEX_WAITERS -> no despertar\n C duerme para siempre\n\nEsto es claramente una deficiencia en la implementación, que no logra mantener el bit FUTEX_WAITERS consistente.\n\nSolucionar esto aumentando el procesamiento de salida de la lista robusta para también realizar el despertar extra si la palabra futex es propiedad de otro hilo pero FUTEX_WAITERS no está establecido.\n\nEsto no soluciona el problema de una secuencia de toma de control/liberación y liberación sin contención, que ha sido discutido durante años y ha sido abordado por el commit 3ca9595d9fb6 ('futex: Añadir soporte para desbloquear futexes robustos') y cambios posteriores, pero no tuvo en cuenta el problema descrito anteriormente.\n\nUna solución más completa que se basa en el desbloqueo en el kernel de futexes robustos con contención ha sido discutida en el contexto de este cambio y debería aparecer en la rama principal más pronto que tarde.\n\n[ tglx: Modificar ligeramente el registro de cambios y corregir el estilo de codificación ]"
}
],
"affected": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"affectedData": [
{
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"defaultStatus": "unaffected",
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],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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{
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},
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],
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},
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"metrics": {
"ssvcV203": [
{
"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
"ssvcData": {
"timestamp": "2026-11-06T17:00:33.818677Z",
"id": "CVE-2026-74658",
"options": [
{
"exploitation": "none"
},
{
"automatable": "no"
},
{
"technicalImpact": "partial"
}
],
"role": "CISA Coordinator",
"version": "2.0.3"
}
}
]
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