CVE-2024-56780 - Race Condition in Linux Kernel Quota Subsystem—Exploit & Fix Explained

The Linux kernel is continuously evolving, and every line of code protecting your data matters. In CVE-2024-56780, a subtle but real vulnerability in the quota management code could lead to dangerous race conditions on filesystems—threatening stability and causing headaches for sysadmins and developers alike. Let’s break down what happened, how you might exploit it, and how the Linux kernel team fixed it.

What is the Issue?

Modern filesystems like ext4 often use quotas to control how much disk space users or groups can use. Whenever these quotas are updated or released, the kernel manages them carefully in the background so your filesystem stays consistent and stable.

But, the Linux kernel’s quota_release_work mechanism had a race condition during certain quota sync operations. This could result in kernel warnings and, in rare cases, filesystem instability when freezing a filesystem (like for backups).

Here’s the problematic execution path

1. During normal operations, in-memory disk quota (dquot) objects are released in the background and put onto a *work queue*.

Kernel completes the freeze, but the background work *might* still be happening.

5. If the kernel tries to finish up these quota-release tasks while the filesystem is deeply frozen, the operation triggers a kernel warning (WARN_ON) because this isn’t allowed during a frozen state.

Here’s a simple pseudocode summary

// Pseudocode: Freezing misses flush of quota work
freeze_super() {
    sync_filesystem();
    // Quota workqueue not always flushed here!
}

If quota_release_work isn’t flushed before the FS freeze, the kernel can hit an assert

WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);

Here’s a snippet from a real kernel trace on a PowerPC system

ext4_journal_check_start+x28/x110 [ext4] (unreliable)
__ext4_journal_start_sb+x64/x1c [ext4]
ext4_release_dquot+x90/x1d [ext4]
quota_release_workfn+x43c/x4d

Why Is This a Problem?

- Kernel Warnings: Repeated warnings pollute system logs (e.g., dmesg), which makes debugging *real* problems harder.
- Filesystem Instability: Rarely, if this race corrupts internal kernel state, it can even cause system instability.
- Test Failures: Automated filesystems tests like generic/390 can repeatedly fail, making it hard to trust kernel builds.

Exploitation Details

This is not a classic “remote code execution” bug, but it could be triggered locally as a denial-of-service (DoS) by an unprivileged user if that user is allowed to freeze and unfreeze filesystems (rare, but possible in some container or VM environments).

Sample Bash commands (run as root!)

# Create a big file to exceed quota
dd if=/dev/zero of=/mnt/test/somefile bs=1M count=200

# Mark quota for sync and release work
sync; sleep .5

# Freeze the filesystem fast
fsfreeze -f /mnt/test

# Observe dmesg for WARN_ONs:
dmesg | grep WARN

The Fix

Solution: Always flush the quota_release_work workqueue whenever the kernel is about to write back quotas during a filesystem sync or freeze.

The fix lands in dquot_writeback_dquots() as an explicit flush of the workqueue, ensuring that *no* release work remains after the filesystem is frozen.

Relevant code snip (simplified)

// Old: No flush here
void dquot_writeback_dquots(...) {
    // There may be pending quota work items at freeze time!
}

// New: Flush the quota release workqueue
void dquot_writeback_dquots(...) {
    // Ensure all quota release work is done before FS freeze
    flush_workqueue(quota_release_workqueue);
}

Now, when the filesystem is frozen, all pending quota work is completed first, eliminating the race.

Linux Kernel Commit Fix:

kernel.org: flush quota_release_work upon quota writeback
- Filesystem Test Generic/390:
xfstests generic/390

CVE Details Page:

CVE-2024-56780 — cve.org

Especially under heavy load or in multi-core environments,

Upgrade to a kernel version including this fix (June 2024 and later).
If you maintain your own branch, cherry-pick the above commit.

TL;DR

CVE-2024-56780 is a kernel bug where quota-release work could race with filesystem freeze, causing kernel warnings and instability. The fix forces Linux to flush all quota-release tasks during certain syncs, making your filesystem and quota management safer.

Timeline

Published on: 01/08/2025 18:15:18 UTC
Last modified on: 01/09/2025 21:50:02 UTC