The Linux kernel is the beating heart of millions of systems, powering everything from cloud servers to smartphones. A recent issue, tracked as CVE-2024-26601, surfaced in the ext4 filesystem code—one of the most widely used filesystems in Linux. In this post, I’ll break down what happened, how the vulnerability arose, why it's a big deal, and, most importantly, what developers have done to fix it.

Background: What Is ext4 and Fast Commit Replay?

The ext4 filesystem is known for its robustness and performance. One of its features, "fast commit" (fc), is designed to speed up journaling so data is reliably written to disk without slowing down your machine.

When a crash or power loss occurs, the filesystem replays pending "fast commits" at mount time to ensure it’s consistent.

But even the best designs can have edge cases, and for ext4, that edge case popped up in its buddy allocator logic—a core bit of code responsible for keeping track of which blocks on disk are free or in use.

Technical Glitch

In the block-allocator code, a change (commit 6bd97bf273bd, for those tracking the kernel git) removed what seemed like a redundant call to a function called mb_regenerate_buddy(). At first glance, this made sense and removed some code that looked unnecessary. But reality was a tad more tricky.

Scenario:
During a fast commit replay (fc replay), ext4 might try to free a block that is already free. Normally, that should be harmless. But in the way ext4’s "buddy allocator" works, this could mess up its *bitmap*—the in-memory map marking which blocks are available. Corruption here can lead to … well, really bad things: filesystem corruption, files that disappear, and even potential kernel crashes.

The Smoking Gun: Why Regenerate the Buddy?

mb_regenerate_buddy() is a function that can rebuild or repair this in-memory map when things get wonky. By removing it from key places, the original patch introduced a subtle but critical bug:
*During fast commit replay, if freeing a block failed (the block was already free), the buddy bitmap could become out-of-sync and corrupt.*

The Fix: "We Gotta Regenerate, Buddy!"

The fix is simple but effective: put mb_regenerate_buddy() back in!
This mostly reverts the earlier change, ensuring that after a failed block-free operation during fast commit replay, the code will regenerate the buddy.

Old (buggy) code (after commit 6bd97bf273bd)

if (mb_free_blocks(...)) {
    // If freeing blocks failed, do nothing
    return;
}

Fixed code (as of commit for CVE-2024-26601)

if (mb_free_blocks(...)) {
    // If freeing blocks failed, regenerate buddy bitmap!
    mb_regenerate_buddy();
    return;
}

This addition ensures that the in-memory map is rebuilt if fast commit replay gets confused about block states.

Impact

This isn’t a “remote code execution” bug in the classic sense. However, if an attacker can trigger repeated fast commit replays, they might reliably induce filesystem corruption, potentially leading to data loss, denial of service, or a system crash.

Attack Scenario

Imagine a server repeatedly power-cycled by an attacker (in a datacenter, for example) or a multi-tenant system where malicious software can cause crashes.

- Under such pressure, an attacker might trick ext4 into freeing the same block more than once during fc replay.
- If the buddy bitmap isn’t regenerated, the filesystem loses track of which disk blocks are valid, leading to unexpected allocations or potential privilege escalation if reused blocks are read as confidential data.

Upstream fix commit:

ext4: regenerate buddy after block freeing failed if under fc replay

Bug report & discussion:

LKML: Fix regression from commit 6bd97bf273bd

Explainer on ext4 fast commits:

LWN.net: Next-level journaling in ext4

Am I Vulnerable? And What Should I Do?

- If you run a Linux system (especially servers or storage appliances) on an unpatched ext4 kernel newer than commit 6bd97bf273bd, you should update to the latest kernel as soon as possible.
- Always back up your data, and use reliable power supplies to reduce the chance of interrupted fast commit replays.

Summary

CVE-2024-26601 shows how dangerous even small changes in filesystem logic can be. The original removal of a regenerating function seemed fine—until reality reminded us that edge cases happen, and data integrity depends on solid failsafes. Thanks to vigilant kernel engineers, the vulnerability is closed, and your files are safe again… as long as you keep your system updated.

Timeline

Published on: 02/26/2024 16:27:59 UTC
Last modified on: 04/17/2024 18:03:41 UTC