CVE-2026-31597: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31597 Overview

CVE-2026-31597 is a use-after-free vulnerability [CWE-416] in the Linux kernel's Oracle Cluster File System version 2 (ocfs2) implementation. The flaw resides in the ocfs2_fault() page fault handler. When filemap_fault() returns VM_FAULT_RETRY, it may drop the mmap_lock before returning. A concurrent munmap() call can then invoke remove_vma() and free the associated vm_area_struct via RCU. The saved vma pointer in ocfs2_fault() becomes a dangling reference, and the subsequent trace_ocfs2_fault() call dereferences freed memory.

Critical Impact
A local authenticated user can trigger memory corruption in the kernel, potentially leading to denial of service or privilege escalation on systems using the ocfs2 filesystem.

Affected Products

Linux Kernel (multiple stable branches, see vendor commits)
Systems mounting Oracle Cluster File System v2 (ocfs2) volumes
Linux distributions shipping vulnerable kernel builds with ocfs2 enabled

Discovery Timeline

2026-04-24 - CVE-2026-31597 published to NVD
2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-31597

Vulnerability Analysis

The vulnerability is a use-after-free in the ocfs2 page fault handler ocfs2_fault(). The function calls filemap_fault() to handle file-backed memory faults. As documented in mm/filemap.c, filemap_fault() may drop the mmap_lock before returning VM_FAULT_RETRY, specifically when it performs I/O or when lock_folio_maybe_drop_mmap() releases the lock.

Once the mmap_lock is released, the calling thread no longer has synchronization protecting the vm_area_struct (vma) it referenced. A concurrent munmap() syscall on the same process can proceed to call remove_vma(), which schedules the vma for release through RCU. The original vma pointer held on the stack in ocfs2_fault() is now stale.

After filemap_fault() returns, ocfs2_fault() invokes trace_ocfs2_fault() passing the saved vma pointer. The tracepoint reads fields from the freed structure, triggering a use-after-free read in kernel memory.

Root Cause

The root cause is the assumption in ocfs2_fault() that the vma pointer remains valid for the entire function lifetime. This assumption breaks because filemap_fault() is permitted to drop mmap_lock under the VM_FAULT_RETRY contract, but ocfs2_fault() continues to dereference vma afterward via the trace event.

Attack Vector

Exploitation requires local access and the ability to map an ocfs2-backed file into the process address space. An attacker races a thread performing page faults against another thread issuing munmap() on the same mapping. Repeatedly forcing VM_FAULT_RETRY paths through I/O pressure or folio locking maximizes the race window. Successful exploitation produces a kernel read of freed memory, which can be shaped into information disclosure or, when combined with heap grooming, memory corruption suitable for privilege escalation.

No public proof-of-concept is currently available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-31597

Indicators of Compromise

Kernel oops or KASAN use-after-free reports referencing ocfs2_fault or trace_ocfs2_fault in the call stack
Unexpected kernel panics on hosts mounting ocfs2 volumes under concurrent fault and munmap() workloads
Anomalous local processes repeatedly mapping and unmapping ocfs2-backed files

Detection Strategies

Enable KASAN on test or canary systems to catch the use-after-free at the point of dereference inside trace_ocfs2_fault()
Monitor dmesg and /var/log/kern.log for stack traces involving ocfs2_fault, filemap_fault, and remove_vma
Audit installed kernel package versions against vendor advisories listing the fix commits

Monitoring Recommendations

Centralize kernel logs and alert on crash signatures referencing ocfs2 fault handling
Track mount events for ocfs2 filesystems on hosts that do not normally use clustered storage
Correlate unexpected process terminations with kernel taint flags using endpoint telemetry

How to Mitigate CVE-2026-31597

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in the vendor advisory commits and reboot affected systems
Inventory hosts with ocfs2 support compiled in (CONFIG_OCFS2_FS) and prioritize those actively mounting ocfs2 volumes
Restrict local shell and code execution privileges on multi-tenant systems running vulnerable kernels

Patch Information

The upstream fix saves ip_blkno as a plain integer before calling filemap_fault() and removes vma from the trace event. Because ip_blkno is copied by value before the lock can be dropped, it remains valid regardless of what happens to the vma or inode afterward. Fixes are available in the following stable kernel commits: 4cf2768a0291, 6f072daefcab, 76a602fdbb78, 7de554cabf16, 925bf22c1b82, and d45ff441b416. Refer to the kernel.org stable commits for the canonical patch.

Workarounds

Unmount ocfs2 filesystems where they are not required for operations
Rebuild kernels without CONFIG_OCFS2_FS on hosts that do not need Oracle Cluster File System support
Disable ocfs2 tracepoints if patching cannot be performed immediately to reduce the dereference path

bash

# Verify whether ocfs2 module is loaded and in use
lsmod | grep ocfs2
mount | grep ocfs2

# Prevent ocfs2 from auto-loading until the kernel is patched
echo 'blacklist ocfs2' | sudo tee /etc/modprobe.d/blacklist-ocfs2.conf
sudo update-initramfs -u