CVE-2026-53040 Overview
CVE-2026-53040 is an out-of-bounds read vulnerability in the Linux kernel's Oracle Cluster File System version 2 (OCFS2) implementation. The flaw resides in the ocfs2_info_freefrag_scan_chain() function within fs/ocfs2/ioctl.c. A crafted filesystem can trigger a use-after-free condition when the OCFS2_IOC_INFO ioctl is issued with the OCFS2_INFO_FL_NON_COHERENT flag. The non-coherent code path uses ocfs2_read_blocks_sync() and skips group descriptor validation, allowing an attacker-controlled bg_bits value to drive the bitmap walk past the end of the block buffer.
Critical Impact
A local attacker who can mount a crafted OCFS2 filesystem image can trigger a kernel out-of-bounds read, leading to memory disclosure or denial of service.
Affected Products
- Linux kernel - OCFS2 filesystem driver (fs/ocfs2/ioctl.c)
- Multiple stable kernel branches addressed via commits 05d0cbea, 0998674e, 3e167e23, 4c2d62dd, 8f687eee, bb2906a1, bb3c54d1, and e0dcf126
- Systems exposing OCFS2 mount capability to untrusted users or accepting untrusted disk images
Discovery Timeline
- 2026-06-24 - CVE-2026-53040 published to the National Vulnerability Database
- 2026-06-24 - Last updated in NVD database
Technical Details for CVE-2026-53040
Vulnerability Analysis
The vulnerability is an out-of-bounds read triggered through the OCFS2_IOC_INFO ioctl interface. When userspace requests a free-fragment report with the OCFS2_INFO_FL_NON_COHERENT flag set, the kernel walks block group bitmaps to compute fragmentation statistics. The scan limit is taken directly from the on-disk bg_bits field of each group descriptor.
Kernel Address Sanitizer (KASAN) reports a use-after-free read of 8 bytes in instrument_atomic_read() reached through test_bit_le() inside ocfs2_info_freefrag_scan_chain() at fs/ocfs2/ioctl.c:495. The read overruns the descriptor block buffer because bg_bits is not bounded by the actual bitmap capacity that the filesystem format can support.
Root Cause
The coherent ioctl path reads group descriptors through ocfs2_read_group_descriptor(), which performs structural validation before the descriptor is consumed. The non-coherent path substitutes ocfs2_read_blocks_sync(), which performs only a raw block read with no semantic checks. As a result, a malicious or corrupt filesystem can report an bg_bits value larger than the descriptor block can hold, and the bitmap walk continues past the buffer boundary into unrelated or freed memory.
Attack Vector
Exploitation requires the ability to mount a crafted OCFS2 image and invoke the OCFS2_IOC_INFO ioctl with OCFS2_INFO_FL_NON_COHERENT. On systems where unprivileged users can mount filesystem images, or where removable media is auto-mounted, a local attacker can disclose kernel memory contents or crash the kernel. The upstream fix computes the bitmap capacity using ocfs2_group_bitmap_size(), rejects descriptors whose bg_bits exceeds that limit, and clamps the scan to the computed capacity so the freefrag report continues without reading past the buffer.
Detection Methods for CVE-2026-53040
Indicators of Compromise
- KASAN use-after-free or out-of-bounds reports referencing ocfs2_info_freefrag_scan_chain, ocfs2_info_freefrag_scan_bitmap, or ocfs2_info_handle_freefrag in kernel logs
- Unexpected kernel oops or panic traces originating from ocfs2_ioctl+0x632/0x6e0 in fs/ocfs2/ioctl.c
- Mount events for OCFS2 images from non-administrative users or removable media
Detection Strategies
- Monitor dmesg and /var/log/kern.log for KASAN reports involving the OCFS2 ioctl call chain
- Audit ioctl() syscalls targeting OCFS2 mount points with the OCFS2_IOC_INFO request code
- Track mount operations of OCFS2 filesystems from untrusted sources using auditd rules on the mount syscall
Monitoring Recommendations
- Enable kernel auditing for filesystem mount activity and ioctl invocations on cluster filesystem nodes
- Forward kernel logs to a centralized logging platform and alert on OCFS2-related fault traces
- Inventory hosts with the ocfs2 module loaded using lsmod and flag those that do not require it
How to Mitigate CVE-2026-53040
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the Kernel Git Commit 05d0cbea and the related stable branch commits
- Restrict mounting of OCFS2 filesystems to trusted administrators and disable automatic mounting of removable media
- Unload the ocfs2 kernel module on systems that do not require cluster filesystem functionality
Patch Information
The fix introduces validation in ocfs2_info_freefrag_scan_chain() by computing the bitmap capacity with ocfs2_group_bitmap_size(), reporting descriptors whose bg_bits exceeds that limit, and clamping the scan to the computed capacity. Patches have been backported across multiple stable kernel branches. See Kernel Git Commit 0998674e, Kernel Git Commit 3e167e23, Kernel Git Commit 4c2d62dd, Kernel Git Commit 8f687eee, Kernel Git Commit bb2906a1, Kernel Git Commit bb3c54d1, and Kernel Git Commit e0dcf126 for the corresponding stable backports.
Workarounds
- Blacklist the ocfs2 module on hosts that do not require it to prevent loading of the vulnerable code path
- Remove the SUID bit or restrict CAP_SYS_ADMIN capability that allows unprivileged mounting in containerized or multi-tenant environments
- Disable auto-mount handlers for removable media that could carry crafted OCFS2 images
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
