CVE-2024-56630 is a vulnerability affecting the Linux kernel's OCFS2 (Oracle Cluster File System version 2). If you use OCFS2 for clustered filesystems, especially in enterprise or cloud setups, this bug may impact your systems’ reliability and resource usage. Discovered via syzkaller and reported by syzbot, this issue can lead to "busy inodes" after unmount, leaking kernel memory and potentially causing further stability issues.

In this post, we'll break down what this CVE means, why it happens, show where the bug is in the code, how to exploit it, and finally, how the fix works.

What is the Bug?

A function called ocfs2_get_init_inode() in OCFS2 was vulnerable: if it tries to initialize quota tracking for a new inode and fails, it forgot to clean up correctly. Specifically, after allocating a new inode, a failure in dquot_initialize() left the inode referenced (never freed). Over time, repeatedly triggering this bug could exhaust system resources or cause unmount operations to hang due to "busy inodes."

Affected Code: Linux kernel, OCFS2 filesystem.

- Commit Introduced: 9c89feaf826 (see commit)

Vulnerable Function: ocfs2_get_init_inode()

- Symptoms: After umount, inodes remain busy. Memory/resource leak.

Reproducing the Issue

The vulnerability can be triggered by performing specific filesystem operations that cause dquot_initialize() to fail just after a new inode is created. A fuzzing tool like syzkaller can automate this, but here's a simplified C snippet outlining the vulnerable logic:

struct inode *inode = new_inode(sb); // allocate new inode
if (!inode)
    return ERR_PTR(-ENOMEM);

err = dquot_initialize(inode); // setup quotas
if (err) {
    // MISSING: iput(inode);  <-- this is the bug!
    return ERR_PTR(err);
}

What went wrong: When dquot_initialize() fails, the code needs to release (iput()) the inode. Failing to do so leaves the inode allocated.


## Proof of Vulnerability / Exploit

While this bug isn't a "remote code execution" or "privilege escalation", it can be weaponized as a Denial-of-Service (DoS):

Mount an OCFS2 filesystem.

2. Trigger inode creation repeatedly, ensuring quota initialization fails (e.g., via resource exhaustion, faulty quota setup, or syzkaller).

Unmount the filesystem.

4. Check /proc/*/fd or umount logs to see "busy inodes" or kernel warnings.

If repeated, this could lock up resources and destabilize the system.

Example shell loop (not a full PoC, but illustrative)

#!/bin/bash
# Pseudo exploit: stress an OCFS2 mount
MNT=/mnt/ocfs2
for i in {1..10000}; do
    touch $MNT/file_$i
    # Simulate quota initialization error (needs special setup/system tweaks)
done
umount $MNT
# Check for "busy inode" warning in dmesg
dmesg | grep "busy"

The Patch

Fixed by: Calling iput() on the inode when quota initialization fails.

Fixed Code

err = dquot_initialize(inode);
if (err) {
    iput(inode); // free the inode properly!
    return ERR_PTR(err);
}

- Linux Kernel Patch: ocfs2: free inode when ocfs2_get_init_inode() fails (commit 97a1e956b1c6)

See also

- Syzbot original report
- LKML fix discussion

Recommendations

- Upgrade: If you use OCFS2, update to a kernel with this patch (5.15.160+, 6.1.91+, 6.6.31+, 6.9.5+, or later mainline).

Monitor: Use kernel logs (dmesg) for "busy inode" warnings after umount.

- Audit: If you maintain custom Linux/fork kernels with OCFS2 support, check for presence of this bug.

Summary

- CVE-2024-56630 could let users or attackers exhaust kernel resources by repeatedly leaking inodes under quota init errors in OCFS2.

The root cause was a missing cleanup (iput()) in error handling logic.

- Simple but serious: can destabilize or DoS systems with frequent OCFS2 mount/umount cycles.

The official kernel patch closes this resource leak completely.

Stay patched! OCFS2 may not be as common as ext4 or XFS, but in clusters, stability and resource management matter.

References

- Linux Kernel Patch: ocfs2: free inode when ocfs2_get_init_inode() fails
- Syzbot report
- LKML thread on the bug/fix


Exclusive Note: This CVE shows how even simple, overlooked error handling can have a real-world impact. Tracking filesystem resource cleanups is a critical part of kernel engineering.


*Share this article with your sysadmins, kernel developers, or anyone running clustered Linux filesystems. Keeping your patches up to date isn't just best practice—it's essential for system health and security.*

Timeline

Published on: 12/27/2024 15:15:22 UTC
Last modified on: 05/04/2025 10:00:27 UTC