CVE-2026-80776

Summary

In the Linux kernel, the following vulnerability has been resolved:

futex: Fix race in futex_pivot_pending() during private hash resize

A task performing a custom private hash resize can remain blocked in uninterruptible sleep indefinitely. The hung-task detector reports:

INFO: task futex-resizer:314 blocked for more than 10 seconds. task:futex-resizer state:D stack:14824 pid:314 tgid:312 ppid:311

Call Trace: __schedule+0x521/0xf30 schedule+0x22/0xa0 futex_hash_allocate+0x3db/0x490 __do_sys_prctl+0x6f5/0xbd0 do_syscall_64+0xf9/0x530 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Kernel panic - not syncing: hung_task: blocked tasks

futex_pivot_pending() allows the resize request to continue when either no replacement hash is pending (hash_new == NULL) or the current hash reference count has reached zero.

After the final-reference wake, another futex task can complete the pivot between the two observations:

T1 T2

futex_hash_allocate() wait_var_event(mm, …) futex_pivot_pending(mm) hash_new != NULL futex_hash() futex_ref_get(old) -> false futex_pivot_hash(mm) hash_new = NULL __futex_pivot_hash(mm, new) rcu_assign_pointer(hash, new) fph = rcu_dereference(hash) /* new */ futex_ref_is_dead(fph) -> false schedule()

The pivot changes the state from hash_new != NULL with a dead current hash to hash_new == NULL with a live current hash. Because futex_pivot_pending() reads hash_new and hash without serialization, the resize task can observe hash_new in the pre-pivot state and hash in the post-pivot state, causing futex_pivot_pending() to return false even though the pivot has completed. The task then goes to sleep after the wakeup has already been consumed.

Serialize state reads in futex_pivot_pending() using futex_mm_phash::lock. This guarantees that futex_pivot_pending() observes hash_new and hash atomically, eliminating the race condition.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/4a7e941ca29a608c6244cbd028d3599ecaef7207
• https://git.kernel.org/stable/c/19b4be0717fa83265d66aea836b7022d898422cf
• https://git.kernel.org/stable/c/8e7ff730dd96519a333d1570edf1c3fabb6d3629

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