CVE-2026-98507 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish
On arm64 server, we find that a task trying to grab the anon_vma lock triggers hungtask.
INFO: task main:2354726 blocked for more than 120 seconds. Tainted: G E 5.10.0-0021.aarch64 #1 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:main state:D stack: 0 pid:2354726 ppid:2350673 flags:0x00000a01 Call trace: __switch_to+0x7c/0xbc __schedule+0x3b4/0x8a0 schedule+0x50/0xe0 rwsem_down_write_slowpath+0x3cc/0x6cc down_write+0x60/0x260 __anon_vma_prepare+0x6c/0x210 do_anonymous_page+0x258/0x660 handle_pte_fault+0x188/0x214 __handle_mm_fault+0x1b0/0x380 handle_mm_fault+0xf4/0x284 do_page_fault+0x19c/0x494 do_translation_fault+0xcc/0xf8 do_mem_abort+0x48/0xac el0_da+0x44/0x80 el0_sync_handler+0x88/0xb4 el0_sync+0x160/0x180
After analyzing the vmcore, we found the anon_vma->root->rwsem.count is -1. There is another anon_vma whose anon_vma->root->rwsem.count is 1, the anon_vma->root->rwsem.owner shows the lock is held, but the stack of the task shows the task doesn't hold the anon_vma lock.
After adding more debugging info, we found __anon_vma_prepare() reuses anon_vma and triggers the UAF of anon_vma->root due to missing memory barrier, leading to locking and unlocking two different anon_vma->root, thus leading to an anon_vma will never be unlocked, and another anon_vma couldn't be locked anymore.
This race requires two adjacent VMAs that are not merged but are anon_vma-compatible (e.g., they differ in VMA_ACCESS_FLAGS that can be changed by mprotect()). Two threads fault on each VMA concurrently, both calling __anon_vma_prepare() with only mmap_lock held for reading.
THREAD A THREAD B __anon_vma_prepare __anon_vma_prepare find_mergeable_anon_vma() -> NULL anon_vma = anon_vma_alloc(); anon_vma->root = anon_vma; // the two stores may be reordered vma->anon_vma = anon_vma; // finds A's anon_vma anon_vma = find_mergeable_anon_vma(vma); anon_vma_lock_write(anon_vma); // may still see the old root down_write(&anon_vma->root->rwsem); anon_vma_unlock_write(anon_vma); // see the new root, never unlock old up_write(&anon_vma->root->rwsem);
thread A triggers page fault and calls __anon_vma_prepare() to prepare anon_vma for the faulting vma. __anon_vma_prepare() allocates and initializes a new anon_vma, and then publishes it to the vma with a plain store. anon_vma_prepare() only requires the mmap_lock to be held for reading, so two threads can fault on adjacent VMAs at the same time. While thread A publishes a new anon_vma, thread B could find the anon_vma via find_mergeable_anon_vma() and then locks anon_vma->root->rwsem.
The store to anon_vma->root in anon_vma_alloc() and the store to vma->anon_vma can be reordered. The anon_vma_lock_write() and spin_lock() only provide acquire semantics, which do not prevent prior stores from being reordered after them. The release semantics of the corresponding spin_unlock() and anon_vma_unlock_write() come too late, the store to vma->anon_vma is already published before they take effect. As a result, thread B can observe the following order:
vma->anon_vma = anon_vma; anon_vma->root = anon_vma;
The anon_vma slab is SLAB_TYPESAFE_BY_RCU, so a newly allocated anon_vma may reuse memory from a previously freed one. The constructor (anon_vma_ctor) does not reset anon_vma->root, and __put_anon_vma() doesn't clear it either, so the old root value persists until anon_vma_alloc() overwrites it. If that store isn't v ---truncated---
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Analysis
by VulDB Data Team • 10/19/2026
The Linux kernel vulnerability identified in mm/rmap involves a critical race condition within the anonymous memory management subsystem, specifically during the initialization and publication of an anon_vma structure to a virtual memory area. This flaw arises from missing memory barriers between the internal initialization of the anon_vma root pointer and its subsequent exposure to other threads via the vma->anon_vma field. On architectures like arm64 where store reordering is permitted by the hardware, this lack of synchronization allows concurrent page fault handlers operating on adjacent virtual memory areas to observe inconsistent states of the anonymous memory structure. The vulnerability manifests when two threads simultaneously trigger faults on compatible VMAs that are not merged but share anonymity characteristics, leading to a scenario where one thread publishes an uninitialized or partially initialized anon_vma while another thread attempts to lock and use it based on stale data.
The technical root cause lies in the reordering of memory stores during the allocation process within __anon_vma_prepare(). When a new anon_vma is allocated, two critical writes occur: setting anon_vma->root to point to itself and assigning this structure to vma->anon_vma. Without explicit memory barriers ensuring ordering, the store to vma->anon_vma can be executed before the store to anon_vma->root completes on other cores due to CPU cache coherency protocols and compiler optimizations. Since the anonymous virtual memory area slab utilizes SLAB_TYPESAFE_BY_RCU semantics, newly allocated structures may reuse memory from previously freed instances that were not fully cleared by their destructors. Consequently, if a thread observes the vma pointer before seeing the updated root pointer, it accesses an anon_vma with stale or incorrect internal state, specifically regarding its root semaphore count and ownership tracking.
This race condition leads to severe operational impacts including kernel deadlocks and system hangs due to unbalanced locking primitives. Specifically, one thread may acquire a write lock on the anon_vma->root rwsem based on the assumption that it holds exclusive access, while another thread concurrently attempts to unlock what it believes is the same structure but is actually referencing a different instance or an inconsistent state. This results in reference count mismatches where locks are never properly released for one entity and cannot be acquired by others, causing tasks to block indefinitely as seen in hungtask reports with rwsem counts showing negative values or incorrect ownership flags. The system effectively enters a state of resource starvation within the memory management subsystem, preventing further page fault handling and potentially requiring a hard reset to recover stability.
From a classification perspective, this vulnerability aligns with CWE-362 Concurrent Execution Using Shared Resource with Improper Synchronization, as it involves multiple execution paths accessing shared kernel data structures without adequate synchronization primitives. It also relates to CWE-820 Missing Synchronization in Multi-threaded Programs and potentially CWE-457 Use of Uninitialized Variable if the stale root pointer leads to access of uninitialized memory regions during lock operations. In terms of adversarial tactics, this flaw could be leveraged within ATT&CK technique T1053 Scheduled Task/Job or more accurately as part of privilege escalation vectors where an attacker might exploit the resulting denial-of-service state to trigger kernel panics for availability attacks, although direct exploitation for code execution is less likely given the nature of the locking imbalance.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that introduces appropriate memory barriers, such as smp_store_release or similar ordering primitives, between the initialization of anon_vma->root and its publication to vma->anon_vma. System administrators should ensure their kernels are updated with versions containing this fix, particularly for arm64 deployments where store reordering is prevalent. Additionally, developers working on memory management subsystems must rigorously audit all paths that publish shared structures to concurrent contexts, ensuring that release semantics are correctly applied before any other thread can observe the new state. Regular stress testing involving high-concurrency page fault scenarios and validation of lock ordering invariants can help detect similar synchronization flaws early in the development lifecycle.