Description

In the Linux kernel, the following vulnerability has been resolved: futex: Also allocate private hash on vfork() As Jann demonstrated, it is entirely feasible to access the mm through vfork(). Therefore we need to allocate a private hash on vfork() as well as any other CLONE_VM user. Specifically, it must be avoided to have (private) futex waiters before allocating the private hash.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.8",
  "pubDate": "2026-10-06T09:18:18.040Z",
  "pubdate": "2026-10-06T09:18:18.040Z",
  "executiveSummary": "This vulnerability involves an improper initialization of private hash tables within the Linux kernel's futex subsystem during vfork() operations. By failing to allocate a private hash table when a process is created via vfork(), the kernel exposes an inconsistency in how memory management (mm) structures are handled for tasks sharing the same address space.\nThe vulnerability type is categorized as an improper resource management issue, potentially leading to race conditions or memory access synchronization failures. The flaw affects the Linux kernel's futex management logic for processes utilizing CLONE_VM. An attacker capable of executing code within a child process created via vfork() can potentially exploit this state to interfere with futex waiters or trigger undefined behavior within the kernel's synchronization primitives.\nThe risk is significant as it affects the fundamental reliability of kernel-level synchronization mechanisms, which are heavily relied upon by user-space threading libraries. While exploitation requires local access, the implications involve potential privilege escalation or denial of service through kernel panic or memory corruption.",
  "technicalDetails": "The core issue resides in the kernel's handling of task duplication involving memory sharing. Specifically, the futex subsystem relies on a hash table to manage private futexes associated with a process's memory map (mm). When a task is cloned, the kernel must ensure that the child task has an appropriate mechanism to track its futex state, particularly when the CLONE_VM flag is present, implying shared address space.\nPrior to the fix, the kernel failed to allocate a private hash table during a vfork() operation. The vfork() system call creates a new process that shares the parent's memory space, but it is traditionally used to execute a new program image via execve(). Jann Horn demonstrated that it is possible to bypass the expected constraints of vfork() to perform operations on the shared mm before the address space is replaced by execve().\nThe attack flow occurs as follows: 1) An attacker triggers a vfork() system call, creating a child process that shares the address space with the parent. 2) Because the kernel did not allocate a private futex hash for the vfork() child, the child lacks the necessary infrastructure to manage private futex waiters correctly. 3) The attacker initiates private futex operations (waiters) within the child process. 4) Due to the missing hash allocation, these operations interact with a null pointer or an improperly initialized hash structure. 5) This creates a race condition or invalid memory access, as the kernel attempts to manage futexes for the shared mm without the designated hash table.\nThe vulnerable component is the futex subsystem in the Linux kernel. The root cause is the inconsistent policy regarding when the private hash is allocated; it was previously restricted to standard cloning procedures but omitted for vfork(), which also results in CLONE_VM behavior. The lack of this allocation creates a window of vulnerability where a process can register futex waiters in an improperly initialized environment. This failure violates the kernel's invariant that all tasks sharing an mm must have a valid path for private futex synchronization, leading to potential kernel memory corruption or unexpected state transitions."
}