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CVE-2026-53040 is a vulnerability in the Linux kernel’s OCFS2 (Oracle Cluster File System) driver, specifically within the `ocfs2_info_handle_freefrag` function that handles the `OCFS2_IOC_INFO` ioctl command. This vulnerability arises when a crafted filesystem image triggers an out-of-bounds read during a free-fragment scan, leading to a use-after-free condition that can be exploited to cause a kernel panic or potentially leak sensitive kernel memory.
The root cause lies in the `ocfs2_info_freefrag_scan_chain()` function, which uses the on-disk `bg_bits` field directly as the bitmap scan limit without proper validation. Under normal operation, the coherent path reads group descriptors through ocfs2_read_group_descriptor(), which performs validation before use. However, when the `OCS2_INFO_FL_NON_COHERENT` flag is set, the non-coherent path uses `ocfs2_read_blocks_sync()` instead, which skips all validation checks. This allows an attacker with local access and the ability to mount a malicious filesystem to set an impossibly large `bg_bits` value, driving the bitmap walk past the end of the allocated buffer.
When the scan exceeds the buffer boundary, the `test_bit()` macro attempts to read memory outside the valid range, triggering a KASAN (Kernel Address Sanitizer) use-after-free report. The crash trace shows the issue occurring at ocfs2_info_freefrag_scan_chain+0x495, with the call stack ultimately leading to the `ocfs2_ioctl` system call handler. The vulnerability affects systems that have OCFS2 support enabled and allow unprivileged users to issue the `OCFS2_IOC_INFO` ioctl on OCFS2 filesystems.
The fix implemented in the kernel patch computes the bitmap capacity using `ocfs2_group_bitmap_size()` based on the filesystem format, reports descriptors whose `bg_bits` exceeds this limit, and clamps the scan to the computed capacity. This ensures that even with a malformed on-disk structure, the scan cannot read beyond the allocated buffer, maintaining system stability while still providing free-fragment reporting functionality.
DailyCVE Form
Platform: Linux Kernel
Version: 2.6.37-6.14
Vulnerability: Out-of-bounds read
Severity: Medium
date: 2026-06-24
Prediction: 2026-07-14
What Undercode Say
Analytics & Bash Commands:
The vulnerability can be triggered by crafting a malicious OCFS2 filesystem image with an invalid `bg_bits` value and mounting it, then issuing the `OCFS2_IOC_INFO` ioctl with the `OCFS2_INFO_FL_NON_COHERENT` flag. Below are commands to check if your system is affected and to reproduce the issue in a controlled environment.
Check if OCFS2 is loaded and vulnerable lsmod | grep ocfs2 Check kernel version (vulnerable range: 2.6.37 to 6.14) uname -r Mount a malicious OCFS2 image (for testing only) mount -t ocfs2 -o loop malicious.img /mnt/ocfs2 Trigger the vulnerability via ioctl (requires custom C program) Example using strace to observe the ioctl call strace -e ioctl ./trigger_ioctl /mnt/ocfs2 Check dmesg for KASAN report dmesg | grep -i "use-after-free|ocfs2" Verify patch status (if backported) grep ocfs2_info_freefrag_scan_chain /proc/kallsyms
Exploit
To exploit this vulnerability, an attacker must have local access to the system and the ability to mount a crafted OCFS2 filesystem. The exploitation steps are:
1. Craft a malicious OCFS2 image: Modify the on-disk group descriptor to set `bg_bits` to a value larger than the actual bitmap size, e.g., 0xffffffff.
2. Mount the image: Use the `mount` command with the `-t ocfs2` option to mount the crafted image.
3. Trigger the ioctl: Execute a program that calls `ioctl(fd, OCFS2_IOC_INFO, &arg)` with the `OCFS2_INFO_FL_NON_COHERENT` flag set.
4. Observe the crash: The kernel will attempt to read beyond the bitmap buffer, causing a use-after-free panic or potentially leaking kernel memory if KASAN is not enabled.
The exploit does not require root privileges if the filesystem is mounted with user permissions, making it a local privilege escalation vector in some configurations.
Protection
Mitigation strategies for CVE-2026-53040:
- Apply kernel patch: Update to kernel version 6.15 or later, or backport the patch that validates `bg_bits` against
ocfs2_group_bitmap_size(). - Disable OCFS2: If OCFS2 is not required, blacklist the module: `echo “blacklist ocfs2” >> /etc/modprobe.d/blacklist.conf`
– Restrict ioctl access: Use SELinux or AppArmor to restrict unprivileged access to the `OCFS2_IOC_INFO` ioctl. - Mount with
noexec: Prevent execution of binaries from OCFS2 mounts to reduce attack surface. - Monitor logs: Watch for KASAN reports or kernel crashes related to OCFS2.
Impact
Successful exploitation of CVE-2026-53040 can lead to:
- Kernel panic: System crashes, resulting in denial of service.
- Information disclosure: Reading out-of-bounds kernel memory may leak sensitive data such as cryptographic keys, passwords, or other process memory.
- Local privilege escalation: In some scenarios, an attacker may combine this with other vulnerabilities to escalate privileges.
- System instability: Repeated exploitation can cause unpredictable system behavior and data corruption.
The vulnerability affects all Linux kernels from version 2.6.37 (when OCFS2 was introduced) up to 6.14, making it a widespread issue for enterprise environments using OCFS2 for shared storage.
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Sources:
Reported By: nvd.nist.gov
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