CVE-2026-53163: locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-25 · updated 2026-07-24
Weakness: CWE-476

Description

In the Linux kernel, the following vulnerability has been resolved: locking/rtmutex: Skip remove_waiter() when waiter is not enqueued syzbot triggered the following splat in remove_waiter() via FUTEX_CMP_REQUEUE_PI: KASAN: null-ptr-deref in range [0x0000000000000a88-0x0000000000000a8f] class_raw_spinlock_constructor remove_waiter+0x159/0x1200 kernel/locking/rtmutex.c:1561 rt_mutex_start_proxy_lock+0x103/0x120 futex_requeue+0x10e4/0x20d0 __x64_sys_futex+0x34f/0x4d0 task_blocks_on_rt_mutex() does not arm the waiter upon deadlock detection, leaving waiter->task nil, where 3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()") made this fatal. Furthermore, rt_mutex_start_proxy_lock() should not be calling into remove_waiter() upon a successfully grabbing the rtmutex. 1a1fb985f2e2 ("futex: Handle early deadlock return correctly"), moved the remove_waiter() out of __rt_mutex_start_proxy_lock() (where 'ret' was only ever 0 or < 0) into the wrapper. Tighten this check to account for try_to_take_rt_mutex().

Affected products

Linux kernel: from 6.1.175 before 6.1.177; from 6.6.140 before 6.6.144; from 6.12.86 before 6.12.95; from 6.18.27 before 6.18.36; from 7.0.4 before 7.0.13; 7.1-rc1, 7.1-rc2, 7.1-rc3, 7.1-rc4, 7.1-rc5, 7.1-rc6 — fixed in 6.1.177, 6.12.95, 6.18.36, 6.6.144, 7.0.13

How to fix

Upgrade to a fixed version: Linux kernel 6.1.177, 6.12.95, 6.18.36, 6.6.144. Check the vendor advisory for mitigations.

References

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