CVE-2026-80775 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
futex: Fix race on the initial mm->futex.phash.ref allocation
futex_hash_allocate() allocates mm->futex.phash.ref without any locking. Commit d9b05321e21e ("futex: Move futex_hash_free() back to __mmput()") moved the allocation here and assumed that the process has just a single thread at this point.
Commit ee9dce44362b ("futex: Drop CLONE_THREAD requirement for private default hash alloc") widened need_futex_hash_allocate_default() to cover any CLONE_VM clone, but left out vfork because the parent is suspended and cannot race.
That no longer holds once vfork is nested. If a vfork child calls vfork again and is then killed with SIGKILL, the parent is released from its vfork wait and runs concurrently with the grandchild in the same mm. Neither of them went through futex_hash_allocate_default().
When both call prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS) at the same time, each one sees mm->futex.phash.ref as NULL and stores its own percpu counter. Only the last store survives. The counter stored first is no longer reachable from the mm, so the references on it are not seen by __futex_ref_atomic_end(). A private hash that still has references is then considered dead and freed, and a task that still holds one of its buckets writes into freed memory in futex_q_lock().
Store the counter once with cmpxchg() and let the loser free_percpu() its own. The initial reference has to be taken before the store, otherwise another task can install a private hash while the counter is still 0.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel futex subsystem contains a race condition vulnerability related to the initialization of the per-process futex hash reference structure. This flaw arises from an improper handling of concurrent access during the allocation phase, specifically within the function responsible for allocating mm->futex.phash.ref. The root cause lies in historical changes to how and when this allocation occurs. Originally, commit d9b05321e21e moved the futex hash free operation back to __mmput() under the assumption that only a single thread would be active at that point, thereby avoiding concurrency issues. Subsequently, commit ee9dce44362b expanded the scope of need_futex_hash_allocate_default() to include any clone with CLONE_VM but excluded vfork scenarios because it was assumed that the parent process remains suspended during a vfork and thus cannot race with the child.
This assumption breaks down when vfork operations are nested. In such configurations, if a vfork child invokes another vfork call and is subsequently terminated via SIGKILL, the parent process is released from its wait state while still sharing memory space with the grandchild. Consequently, both processes may attempt to initialize their private futex hash simultaneously without any synchronization mechanism protecting the initial allocation step. When multiple threads execute prctl(PR_FUTEX_HASH_SET_SLOTS) concurrently in this context, each thread observes mm->futex.phash.ref as NULL and proceeds to store its own per-CPU counter value into that location. Since there is no atomicity guarantee during this write operation, only the last stored reference survives, while earlier references become orphaned and unreachable from the memory management structure.
The operational impact of this race condition is severe and can lead to use-after-free vulnerabilities within the kernel space. The per-CPU counter allocated by an earlier thread becomes inaccessible because it was overwritten before being properly registered in the mm structure. As a result, when __futex_ref_atomic_end() attempts to clean up references, it fails to account for these orphaned counters. This discrepancy causes the system to incorrectly determine that a private hash still holds active references and subsequently free memory associated with buckets that are actively being used by tasks executing futex_q_lock(). Writing into freed kernel memory constitutes a critical security flaw that can lead to arbitrary code execution, privilege escalation, or denial of service depending on how an attacker exploits this state corruption.
To mitigate this vulnerability, the allocation logic must be modified to ensure atomicity during the initialization phase. The recommended fix involves using cmpxchg() for storing the counter value instead of a simple assignment operation. This ensures that only one thread successfully installs its private hash while others detect the conflict and handle their own per-CPU counters via free_percpu(). Crucially, an initial reference must be taken before attempting to store the new pointer; otherwise, another task could install a valid private hash between the time the counter is zeroed out and when the actual storage occurs. This approach prevents race conditions by guaranteeing that concurrent attempts are serialized at the hardware level through atomic compare-and-swap instructions.
From a classification perspective, this vulnerability aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). The lack of proper locking during shared resource initialization allows multiple execution contexts to corrupt state in ways that compromise memory safety. In terms of the MITRE ATT&CK framework, this flaw could potentially be leveraged within techniques related to Privilege Escalation via Kernel Exploitation or Defense Evasion through Memory Corruption, particularly if an attacker can trigger nested vfork scenarios and race against system calls like prctl to manipulate kernel data structures. Security practitioners should ensure that systems are updated with patches addressing futex hash allocation synchronization issues to prevent exploitation of this concurrency flaw in multi-threaded environments involving complex process creation hierarchies.