CVE-2024-26774

What it actually does

Division by zero in ext4 mb_update_avg_fragment_size() when a corrupted block bitmap causes bb_fragments to be zero, leading to kernel crash or denial of service.

Why it matters to the business

Unexpected system crashes or downtime on Linux servers and devices using ext4 filesystems when metadata corruption occurs; low urgency due to no known exploitation and long publication time.

How an attack unfolds

An attacker with local access corrupts the ext4 block bitmap (via unprivileged writes or other filesystem bugs), mounts the filesystem, and triggers block allocation code paths that invoke the vulnerable averaging function, resulting in a divide-by-zero fault.

How to fix it

Upgrade the Linux kernel to a version containing commit 8b40eb2e716b503f7a4e1090815a17b1341b2150 or later stable releases that add the bb_fragments zero check before division.

How to confirm the fix worked

Verify kernel source contains the bb_fragments check in mb_update_avg_fragment_size(), confirm running kernel version includes the fix commit, and test with artificially corrupted bitmaps to ensure no crash occurs.

Typical attack path

1. Obtain local access to a system with ext4 filesystem
2. Corrupt block bitmap metadata through crafted writes or auxiliary vulnerabilities
3. Trigger filesystem operations that call mb_update_avg_fragment_size()
4. Cause divide-by-zero kernel fault and system instability

How to detect it

Search kernel logs and dmesg for divide-by-zero errors or oops traces mentioning ext4 and mb_update_avg_fragment_size; inspect for unexpected filesystem corruption events.

• divide error
• mb_update_avg_fragment_size
• ext4

What your security team should do

• Apply the referenced kernel commit or upgrade to a patched distribution kernel immediately on all systems
• Monitor ext4 filesystem integrity with fsck and enable metadata checksums where available
• Review logs for prior divide-by-zero faults in ext4 code paths