CVE-2026-98507: Linux Kernel Race Condition Vulnerability

CVE-2026-98507 Overview

CVE-2026-98507 is a Linux kernel race condition in the memory management reverse mapping (mm/rmap) subsystem. The flaw stems from a missing memory barrier between anon_vma initialization and the publication of vma->anon_vma. Two threads faulting concurrently on adjacent, anon_vma-compatible VMAs can observe reordered stores, causing one thread to lock a stale anon_vma->root while another unlocks a different root. The result is a use-after-free of anon_vma->root and a permanently held read-write semaphore, producing hung tasks and kernel-level denial of service on weakly-ordered architectures such as arm64.

Critical Impact
Concurrent page faults on adjacent VMAs can corrupt anon_vma locking state, causing indefinite task hangs and a kernel denial-of-service condition on arm64 systems.

Affected Products

Linux kernel (mainline, mm/rmap subsystem)
Linux kernel 5.10.x stable series (reproduced on 5.10.0-0021.aarch64)
Additional stable branches addressed in fix commits 465e222, 4cef512, a500df4, b6ac0b3, and d348a71

Discovery Timeline

2026-10-18 - CVE-2026-98507 published to NVD
2026-10-21 - Last updated in NVD database

Technical Details for CVE-2026-98507

Vulnerability Analysis

The defect lives in __anon_vma_prepare(), which allocates and publishes a new anon_vma structure for a faulting VMA while holding only mmap_lock for reading. Because the read lock permits concurrent faulters, two threads operating on adjacent anon_vma-compatible VMAs can race. Thread A allocates a new anon_vma, sets anon_vma->root = anon_vma, and then publishes it via vma->anon_vma = anon_vma. Thread B, faulting on the neighboring VMA, calls find_mergeable_anon_vma(), discovers Thread A's newly published pointer, and attempts to lock it.

On weakly-ordered CPUs such as arm64, the two plain stores in Thread A can retire out of order. Thread B may therefore observe vma->anon_vma before it sees the updated anon_vma->root. Because the anon_vma slab cache is marked SLAB_TYPESAFE_BY_RCU, the allocated object may still hold a stale root pointer from a prior freed anon_vma. Thread B then calls down_write() on the stale root and later calls up_write() on the new root, leaving the stale lock permanently held and triggering hung_task warnings.

Root Cause

The root cause is missing release/acquire ordering between the initialization of a newly allocated anon_vma and the publication of its pointer into vma->anon_vma. Neither anon_vma_ctor() nor __put_anon_vma() clears the root field, so recycled memory from SLAB_TYPESAFE_BY_RCU retains stale values. The subsequent anon_vma_lock_write() provides only acquire semantics, which cannot prevent prior stores from being reordered after the lock acquisition.

Attack Vector

Exploitation requires local code execution with the ability to create adjacent VMAs that are anon_vma-compatible but not merged, such as regions differing in access flags set by mprotect(). Two threads must fault on each VMA concurrently. The race is reliably reproducible on arm64 server hardware under load. The practical impact is denial of service via unkillable hung tasks; the described write-side locking inconsistency does not grant memory disclosure or code execution in the reported scenario, but the underlying use-after-free of anon_vma->root warrants treatment as a kernel memory safety issue.

No verified public proof-of-concept code is available. The upstream commit messages and the referenced kernel.org git commits document the race in detail.

Detection Methods for CVE-2026-98507

Indicators of Compromise

Kernel hung_task warnings naming tasks blocked in rwsem_down_write_slowpath with call paths through __anon_vma_prepare, do_anonymous_page, and handle_mm_fault.
Processes stuck in uninterruptible sleep (state D) during page fault handling on arm64 hosts.
Vmcore analysis showing an anon_vma->root->rwsem.count of -1 or an owner field referencing a task whose stack does not hold the lock.

Detection Strategies

Enable CONFIG_DETECT_HUNG_TASK and monitor /proc/sys/kernel/hung_task_timeout_secs events through system logs.
Correlate repeated memory-management stack traces across arm64 fleet nodes to surface the race pattern.
Audit kernel versions against the fix commits listed on git.kernel.org to identify unpatched hosts.

Monitoring Recommendations

Forward kernel ring buffer and journald events to a centralized log pipeline and alert on blocked for more than 120 seconds messages.
Track per-host rates of page-fault latency and D-state process counts to detect emerging hangs before service impact.
Review workloads that heavily use mprotect() across many threads for correlated anomalies.

How to Mitigate CVE-2026-98507

Immediate Actions Required

Apply the upstream kernel patches corresponding to commits 465e222, 4cef512, a500df4, b6ac0b3, and d348a71 as shipped by your distribution.
Prioritize arm64 and other weakly-ordered architectures where the reordering is observable in practice.
Reboot affected hosts after upgrading, since the fix changes kernel memory ordering semantics that cannot be hot-patched reliably.

Patch Information

The upstream fix introduces a release/acquire pairing so that vma->anon_vma is only published after anon_vma->root is fully initialized, and consumers observe the correct root before acquiring the lock. Review the fix in Kernel Git Commit 465e222, Kernel Git Commit 4cef512, Kernel Git Commit a500df4, Kernel Git Commit b6ac0b3, and Kernel Git Commit d348a71.

Workarounds

No reliable workaround exists for the underlying race; the memory barrier fix must be applied in kernel code.
As a partial risk reduction, limit workloads that repeatedly mprotect() adjacent regions across multiple threads on unpatched arm64 hosts.
Where feasible, pin latency-sensitive workloads to patched kernels or x86_64 nodes until arm64 fleets are upgraded.

bash

# Verify running kernel against fixed stable branches
uname -r
# Example: upgrade on Debian/Ubuntu derivatives
sudo apt update && sudo apt install --only-upgrade linux-image-$(uname -r | cut -d- -f3-)
sudo reboot