locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

Summary

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().

Timeline

1. Jun 25, 2026, 8:38 AM UTCCVE published
2. Jul 24, 2026, 2:33 PM UTCRecord last updated

Affected versions

• Linux Linux

  Version range not reported

• Linux Linux

  From 6.1.175 · fixed in 6.1.177; From 6.6.140 · fixed in 6.6.144; From 6.12.86 · fixed in 6.12.95; From 6.18.27 · fixed in 6.18.36; From 7.0.4 · fixed in 7.0.13

References 8