The Linux kernel recently fixed an important data corruption vulnerability, now tracked as CVE-2024-27036. This issue affected the CIFS (Common Internet File System) writeback process, exposing users to silent file corruption when copying files to CIFS-mounted shares, especially with certain write size configurations. In this article, we’ll break down how it happened, what the patch changes, test steps, and how attackers or testers could exploit it in practice.

What is CIFS and Why This Matters

CIFS, also known as SMB (Server Message Block), lets you mount network shares in Linux and treat them like regular files or folders. In businesses or homes using NAS devices or Windows shares, this is a common way to store and access data.

A bug in this mechanism could mean files copied to such network drives get silently corrupted — especially dangerous because there is often little or no warning until someone notices the bad file later!

Technical Root Cause

When writing files to a mounted CIFS share, Linux splits the work into chunks based on wsize (write size). However, the kernel’s code for extending a write operation (cifs_extend_writeback()) was flawed. When the write operation was about to exceed wsize with another memory *folio* (a "chunk" of file data), it would try to pause the scan loop over the file’s pages, but incorrectly advanced the iterator, skipping a chunk of the file. This caused those bytes to not be written at all — resulting in missing data (corruption) on the remote share.

The core issue was an incorrect use of xas_pause(), which moves the internal pointer forward instead of just pausing. The fix is to use xas_reset() when you want to *stop* and *resume* correctly, without skipping anything.

How the Bug Would Show Up

You could see the corruption like this:

First, before the fix, you could run

dd if=/dev/urandom of=/tmp/64K bs=64k count=1
mount -t cifs //192.168.6.1/test /mnt -o user=USER,pass=PASS,wsize=64000
cp /tmp/64K /mnt/64K
cmp /tmp/64K /mnt/64K

If you hit the bug, cmp would output

/tmp/64K /mnt/64K differ: byte 64000, line 2

Before

if (would_overrun_wsize) {
    xas_pause(&xas);
    break;
}

After

if (would_overrun_wsize) {
    xas_reset(&xas);
    break;
}

This one-line logic change is the heart of the fix:

Exploit Details: Practical Impact

An attacker with write access to a CIFS-mounted network drive could ensure data is *silently corrupted* (e.g., partial uploads of malware, images, or documents). While there is no remote code execution, it can cause:

Data loss

- Failure of backup/restore operations

Attackers might use this for a *timebomb effect* — users think their data is safe, but it's corrupted.

How to Fix

Update your Linux kernel!
Any current mainline kernel after February 2024 includes the fix. Some distributions have backported the patch.

Check for updates with

uname -r
sudo dnf/apt/yum update

and confirm with your distribution's security advisories.

- Kernel Patch
- CVE Page at Mitre (when available)
- Linux CIFS Maintainers Mailing List

Summary

CVE-2024-27036 is a subtle, but dangerous, data corruption bug in Linux’s CIFS kernel code when writing files. If you use Samba or Windows shares on Linux, check you’re up to date.

Files copied to or from network shares could silently go bad — protect your data with the latest updates!


*If you found this exclusive breakdown helpful, please share with your team. For more kernel and security insights, keep an eye on official Linux kernel and your distro’s security pages.*

Timeline

Published on: 05/01/2024 13:15:49 UTC
Last modified on: 09/18/2025 16:04:59 UTC