CVE-2026-53166 in Linux
Summary
by MITRE • 06/25/2026
In the Linux kernel, the following vulnerability has been resolved:
futex/requeue: Prevent NULL pointer dereference in remove_waiter() on self-deadlock
When FUTEX_CMP_REQUEUE_PI requeues a non-top waiter that already owns the target PI futex, task_blocks_on_rt_mutex() returns -EDEADLK before setting waiter->task.
The subsequent remove_waiter() in rt_mutex_start_proxy_lock() dereferences the NULL waiter->task, causing a kernel crash.
Add a self-deadlock check for non-top waiters before calling rt_mutex_start_proxy_lock(), analogous to the top-waiter check in futex_lock_pi_atomic().
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Analysis
by VulDB Data Team • 06/25/2026
This vulnerability exists within the Linux kernel's futex implementation specifically in the handling of priority inheritance mutex operations. The issue manifests when the FUTEX_CMP_REQUEUE_PI operation attempts to requeue a waiter that is already holding the target PI futex, creating a self-deadlock scenario. The kernel's rt_mutex_start_proxy_lock() function receives a waiter structure where the task field has not been properly initialized due to the early return of -EDEADLK from task_blocks_on_rt_mutex(). This results in a NULL pointer dereference when remove_waiter() attempts to access waiter->task, ultimately causing a kernel crash and potential system instability.
The technical flaw stems from an inadequate check for self-deadlock conditions in the non-top waiter path of the priority inheritance mutex implementation. While the top-waiter self-deadlock detection exists within futex_lock_pi_atomic(), the same protection is missing for non-top waiters during the requeue operation. This asymmetry creates a scenario where the kernel fails to properly initialize the waiter structure before proceeding with operations that assume valid task pointers, violating fundamental safety assumptions in kernel memory management and process scheduling.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable denial-of-service attacks against kernel subsystems that rely on futex operations. Attackers could exploit this condition by crafting specific sequences of futex operations that force the kernel into a state where it attempts to dereference uninitialized pointers in the mutex handling code. This vulnerability affects systems running Linux kernels with priority inheritance mutex support, particularly those implementing real-time scheduling policies where futex operations are frequently used for synchronization between processes and threads.
The mitigation strategy involves adding explicit self-deadlock checks for non-top waiters before invoking rt_mutex_start_proxy_lock(), aligning the implementation pattern with existing top-waiter protection mechanisms. This approach ensures that when a self-deadlock condition is detected, the waiter structure remains in a consistent state where all fields are properly initialized before any operations attempt to dereference them. The fix follows established security practices for preventing NULL pointer dereferences and demonstrates proper defensive programming techniques that align with common kernel security guidelines. This vulnerability classification maps to CWE-476 which specifically addresses NULL pointer dereference conditions in software systems, and the exploitation pattern aligns with ATT&CK technique T1499.001 which covers network denial of service attacks through system resource exhaustion or corruption.