CVE-2026-99169 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
futex: Fix mm reuse handling for FUT_OFF_MMSHARED
A FUT_OFF_MMSHARED futex is a shared futex that refers to an MM. It is possible for a process to wait on a shared futex with a different MM because a FUT_OFF_INODE waiter can be requeued onto a FUT_OFF_MMSHARED futex by another process. This can cause FUT_OFF_MMSHARED waiters on a freed MM to consume wakeups intended for a newly allocated MM at the same address.
Fix it by keying FUT_OFF_MMSHARED using a unique 64-bit per-MM ID. Leave private futexes as before to avoid influencing the performance of the hotpath.
(Multi-threaded processes typically implicitly use FUT_OFF_MMSHARED by setting clear_child_tid such that it points into anonymous memory, which causes mm_release() in a multi-threaded mm to perform FUTEX_WAKE.)
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel futex subsystem manages fast userspace mutexes and condition variables, optimizing synchronization performance by avoiding context switches when possible. A critical flaw exists in the handling of shared futexes that reference memory mappings identified as FUT_OFF_MMSHARED. This specific type of futex relies on virtual address space to identify the target object for waiting and waking operations. The vulnerability arises from a race condition involving process termination, memory unmapping, and subsequent reallocation at the same virtual address. When a multi-threaded process terminates or unmaps its shared memory segment, the associated memory management structure is freed. However, if another thread was previously queued on this futex using FUT_OFF_MMSHARED semantics, it remains in the wait queue even after the original memory mapping has been destroyed and potentially reclaimed by the kernel for new allocations.
The core technical flaw lies in the identification mechanism used to match wake-up signals with waiting threads. The system historically relied primarily on virtual addresses to identify shared futexes within a specific memory map. Because virtual address spaces are reused across different processes or after unmap/mmap cycles, a newly allocated memory region might occupy the exact same virtual address as the previously freed one. Consequently, when a FUT_OFF_INODE waiter is requeued onto a FUT_OFF_MMSHARED futex by another process, it may end up waiting on a futex object that points to a now-freed MM structure. More critically, if a new allocation occurs at that same address, wake-up signals intended for the newly created memory mapping are incorrectly consumed by stale waiters associated with the old, freed mapping. This results in lost synchronization events where threads remain blocked indefinitely or behave unpredictably because they missed their corresponding wakeup calls.
This vulnerability impacts system reliability and security by causing denial of service conditions through thread starvation. Applications relying on precise futex-based synchronization may experience deadlocks, hangs, or data corruption due to the misalignment between waiters and wakeups. In multi-threaded environments where clear_child_tid is set to point into anonymous memory, mm_release() triggers FUTEX_WAKE operations upon process exit. If these wake-ups are consumed by stale entries from a previous mapping at the same address, subsequent threads waiting on the new allocation will not receive their signals. This breaks fundamental assumptions of POSIX threading libraries and can lead to application crashes or unresponsive services. The issue is particularly insidious because it depends on specific timing conditions involving memory reuse, making it difficult to reproduce consistently but highly impactful when triggered in production environments with high churn rates for shared memory regions.
The resolution involves modifying the futex keying mechanism for FUT_OFF_MMSHARED objects by introducing a unique 64-bit per-MM ID. This identifier is generated and maintained alongside each memory management structure, ensuring that even if virtual addresses are reused, the underlying object identity remains distinct across different lifecycles of the memory mapping. By tying the futex wait queue to this persistent identifier rather than solely relying on volatile virtual addresses, the kernel ensures that wake-up signals are correctly routed only to threads waiting on the active, valid memory map. The fix deliberately leaves private futexes unchanged to preserve performance in hot paths where address-based identification is sufficient and less prone to such cross-process reuse conflicts. This approach maintains backward compatibility while strictly enforcing object identity for shared synchronization primitives.
From a standards perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as the race condition stems from improper handling of shared state during resource lifecycle transitions. It also relates to CWE-415: Double Free, in the context that stale references to freed memory structures were not properly invalidated before reuse occurred. In terms of MITRE ATT&CK, this could be categorized under Tactic TA0004 (Privilege Escalation) or TA0008 (Lateral Movement) if exploited by a malicious user to cause denial of service against other users on the same system, although its primary classification is as a reliability and integrity flaw. Mitigation strategies include applying the kernel patch that implements the 64-bit per-MM ID keying for shared futexes. System administrators should ensure their Linux kernels are updated to versions containing this fix. For developers utilizing complex multi-threaded applications with frequent memory mapping changes, reviewing synchronization logic and ensuring robust error handling around futex operations is recommended until patches are deployed. Regular security audits focusing on inter-process communication mechanisms can help identify similar race conditions in custom kernel modules or userspace libraries that interact directly with low-level system calls.