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CVE-2026-53039 is a vulnerability in the Linux kernel’s OCFS2 (Oracle Cluster File System) component that arises from a missing validation step before caching a user‑supplied group block during a group add operation. The flaw resides in the `ocfs2_group_add()` function, which handles the `OCFS2_IOC_GROUP_ADD` ioctl command. This ioctl is used to add a new group descriptor to the filesystem’s block group chain, typically during online resize operations.
The root cause is an incorrect order of operations: `ocfs2_group_add()` calls `ocfs2_set_new_buffer_uptodate()` on a group block number that is fully controlled by the user, before invoking `ocfs2_verify_group_and_input()` to validate that block number. The helper `ocfs2_set_new_buffer_uptodate()` is designed exclusively for newly allocated metadata; it contains a `BUG_ON()` assertion that verifies the block is not already present in the metadata cache. Because the input is not validated beforehand, an unprivileged user can supply a block number that already exists in the cache, or that points to an invalid or unexpected location, triggering the `BUG_ON()` and causing a kernel panic.
Additionally, the code incorrectly uses `INODE_CACHE(inode)` for the group descriptor, even though the descriptor actually belongs to `main_bm_inode` (the main bitmap inode). This inconsistency means that later journal accesses use the wrong cache context, further complicating the buffer lifetime and increasing the risk of corruption or inconsistent state. The bug was introduced in kernel version 2.6.25 with commit `7909f2bf8353` and has been present in all subsequent releases until the fix was applied.
The vulnerability is triggered locally by invoking the `OCFS2_IOC_GROUP_ADD` ioctl with a crafted argument. No special privileges are required beyond the ability to open an OCFS2 file descriptor and issue ioctl commands. Successful exploitation leads to a kernel panic (denial of service), as the `BUG_ON()` is hit and the system crashes. There is no evidence of privilege escalation or arbitrary code execution, but the crash can be used to disrupt services in multi‑tenant or high‑availability environments where OCFS2 is deployed.
The fix, already merged into the mainline kernel, reorders the operations so that `ocfs2_verify_group_and_input()` is called before any caching takes place. Only after validation passes is the group descriptor added to the metadata cache, using the correct `INODE_CACHE(main_bm_inode)` context. The validation and cleanup paths are also separated to ensure that buffers are only removed from the cache after they have been successfully inserted, maintaining consistency across validation, journaling, and cleanup. The patch has been backported to all stable kernel branches, with fixed versions including 5.10.258, 5.15.209, and later.
DailyCVE Form:
Platform: ……. Linux Kernel
Version: …….. 2.6.25 – 5.10.257, 5.15.208
Vulnerability :.. Improper Input Validation
Severity: ……. Medium (Local DoS)
date: ……….. 2026‑06‑24
Prediction: ….. Patched (2026‑06‑24)
What Undercode Say:
Check your kernel version uname -r Check if your OCFS2 module is loaded lsmod | grep ocfs2 Attempt to trigger the vulnerability (requires an OCFS2 filesystem) This is a conceptual example – do not run on production systems. The ioctl number for OCFS2_IOC_GROUP_ADD is typically defined in <linux/ocfs2_fs.h> The following C code snippet illustrates the vulnerable call path:
// Vulnerable code path in fs/ocfs2/resize.c (before fix)
static int ocfs2_group_add(struct inode inode, struct ocfs2_group_add group_add)
{
// ...
// BUG: caching before validation
group_bh = ocfs2_set_new_buffer_uptodate(INODE_CACHE(inode),
group_add->group_blkno);
// Validation happens after caching – too late!
status = ocfs2_verify_group_and_input(inode, group_add, group_bh);
// ...
}
To check if your kernel includes the fix, look for the commit: git log --oneline --grep="ocfs2: validate group add input before caching" The fixing commits are: mainline: <commit hash> stable 5.10: f7e139d7563f stable 5.15: 22544ddedf38
Exploit:
An attacker with local access and the ability to open an OCFS2 file descriptor can craft a `struct ocfs2_group_add` where the `group_blkno` field points to an already‑cached block or an invalid block number. By issuing the `OCFS2_IOC_GROUP_ADD` ioctl, the kernel will attempt to cache this block using ocfs2_set_new_buffer_uptodate(), which will trigger the `BUG_ON()` assertion because the block is already present in the cache (or because the block number is invalid). This results in a kernel panic, crashing the system and causing a denial of service. No special capabilities are required beyond standard file system access, making this a low‑complexity local attack.
Protection:
- Upgrade your kernel to a version that includes the fix:
- For 5.10 series: upgrade to 5.10.258 or later.
- For 5.15 series: upgrade to 5.15.209 or later.
- For newer kernels, ensure you have pulled the latest stable updates.
- If upgrading is not immediately possible, restrict access to the OCFS2 ioctl interface by removing the `ocfs2` module or using LSM (e.g., SELinux) policies to block unprivileged users from issuing
OCFS2_IOC_GROUP_ADD. - Monitor system logs for any unexpected kernel panics or BUG traces related to
ocfs2_set_new_buffer_uptodate; these may indicate attempted exploitation.
Impact:
- Denial of Service (DoS): Successful exploitation causes a kernel panic, rendering the system unusable until a reboot. This can be particularly disruptive in clustered or high‑availability environments where OCFS2 is commonly used.
- System Instability: The panic interrupts all running processes and may lead to data loss or corruption if the filesystem was in the middle of critical operations.
- No Privilege Escalation: The vulnerability does not allow arbitrary code execution or privilege escalation; it is strictly a local crash vector.
- Wide Affected Range: Because the bug has existed since kernel 2.6.25, a large number of enterprise and production systems running older kernels are potentially vulnerable.
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Sources:
Reported By: nvd.nist.gov
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