CVE-2026-74658 (GCVE-0-2026-74658)
Vulnerability from cvelistv5
Published
2026-08-22 15:32
Modified
2026-08-22 15:32
Severity ?
VLAI Severity ?
EPSS score ?
Summary
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 ]
References
Impacted products
| Vendor | Product | Version | ||
|---|---|---|---|---|
| Linux | Linux |
Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: ca16d5bee59807bf04deaab0a8eccecd5061528c Version: 8dd558881e0f4d6942c19bd8f7b1a7c19becb59e Version: b90aa237f469c3575190a5e6a855b76ad1d2ce94 Version: 3e24098da750991f75819069c79e090dfd029219 Version: 2819f4030f43057238992a4adcd950d7c95aff65 Version: 2c60b44d8ba9d62c2693d2692f118177f212b1a8 Version: 82ca3ab31b9cf23b86436a85381e4c5757bc6b80 Version: 3.16.82 ≤ Version: 4.9.264 ≤ Version: 4.14.158 ≤ Version: 4.19.87 ≤ Version: 5.3.14 ≤ Version: 5.4.1 ≤ |
||
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"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 -\u003e D)\n uval == D, no FUTEX_WAITERS\n B killed before acting on the wakeup\n B exit walk, pending op: owner D != B -\u003e no action\n D unlock: no FUTEX_WAITERS -\u003e 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 ]"
}
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"dateUpdated": "2026-08-22T15:32:32.190Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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{
"url": "https://git.kernel.org/stable/c/7cf710e70f9bb8ea75f759ebed09871801315992"
},
{
"url": "https://git.kernel.org/stable/c/6d4514ca9cdf61fec4ec634cf50386f6f7e69748"
}
],
"title": "futex: Prevent robust futex exit race some more",
"x_generator": {
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"cveMetadata": {
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"assignerShortName": "Linux",
"cveId": "CVE-2026-74658",
"datePublished": "2026-08-22T15:32:32.190Z",
"dateReserved": "2026-08-15T05:44:03.924Z",
"dateUpdated": "2026-08-22T15:32:32.190Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
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Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.
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