CVE-2026-74658: futex: Prevent robust futex exit race some more

From: Greg Kroah-Hartman <[email protected]>

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 ]

The Linux kernel CVE team has assigned CVE-2026-74658 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 5.10.265 with commit 83b0f71d5a313a765754acab51d2ecc5de76e0b9
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 5.15.216 with commit a1c2b7b86a946b6b172bce44d74553da2323a36c
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 6.1.183 with commit 33bfa85458105d6169ebdb697f692b8bb8025bae
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 6.6.152 with commit aa5c571901c6b22b58373693a4bf889ecab11ff5
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 6.12.104 with commit 925628656b73b70930972ccde421de4f758d8650
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 6.18.45 with commit 7b8c53263f8878bdd12c87e147ac6feca5c05211
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 7.1.9 with commit 7cf710e70f9bb8ea75f759ebed09871801315992
	Issue introduced in 5.5 with commit ca16d5bee59807bf04deaab0a8eccecd5061528c and fixed in 7.2 with commit 6d4514ca9cdf61fec4ec634cf50386f6f7e69748
	Issue introduced in 3.16.82 with commit 8dd558881e0f4d6942c19bd8f7b1a7c19becb59e
	Issue introduced in 4.9.264 with commit b90aa237f469c3575190a5e6a855b76ad1d2ce94
	Issue introduced in 4.14.158 with commit 3e24098da750991f75819069c79e090dfd029219
	Issue introduced in 4.19.87 with commit 2819f4030f43057238992a4adcd950d7c95aff65
	Issue introduced in 5.3.14 with commit 2c60b44d8ba9d62c2693d2692f118177f212b1a8
	Issue introduced in 5.4.1 with commit 82ca3ab31b9cf23b86436a85381e4c5757bc6b80

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74658
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/futex/core.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/83b0f71d5a313a765754acab51d2ecc5de76e0b9
	https://git.kernel.org/stable/c/a1c2b7b86a946b6b172bce44d74553da2323a36c
	https://git.kernel.org/stable/c/33bfa85458105d6169ebdb697f692b8bb8025bae
	https://git.kernel.org/stable/c/aa5c571901c6b22b58373693a4bf889ecab11ff5
	https://git.kernel.org/stable/c/925628656b73b70930972ccde421de4f758d8650
	https://git.kernel.org/stable/c/7b8c53263f8878bdd12c87e147ac6feca5c05211
	https://git.kernel.org/stable/c/7cf710e70f9bb8ea75f759ebed09871801315992
	https://git.kernel.org/stable/c/6d4514ca9cdf61fec4ec634cf50386f6f7e69748