The Linux kernel powers millions of devices, from servers to smartphones. It must be robust, especially when it comes to file system code since any bug can threaten data safety. Recently, a vulnerability tracked as CVE-2024-53147 was found and patched in the exFAT file system driver. Let's break down this issue, see its impact, understand how it could be abused, and show you the FIX with code snippets and links for further reading.

What Is exFAT?

exFAT (Extended File Allocation Table) is a Microsoft-designed file system, commonly used on SD cards and USB flash drives due to its compatibility and large file support. Linux added exFAT support into the kernel, but handling corrupted file systems securely is always a challenge.

Background

exFAT directories are organized in clusters. The code needs to read and process all directory entries when, say, you run ls on a folder. There's a field in memory called ei->hint_femp.eidx, which is a hint referencing an index inside the directory.

What went wrong:
If the cluster chain for a directory is corrupted such that start_clu (the starting cluster for the directory) becomes an EOF (end-of-file) marker or similarly invalid, the code used that bad value to calculate where in memory to look for entries. In cases where the directory size is bigger than the cluster size, this could mean accessing memory outside the allocated range — classic out-of-bounds access.

Exploit Details

A malicious user could craft an exFAT image (say, a USB stick) with a corrupted cluster chain so that, when plugged into a Linux machine, the kernel tries to read past the directory boundaries. This could:

Cause further file system corruption

- Potentially lead to information disclosure or other undefined behavior, depending on subsequent kernel memory layout and error handling (no privilege escalation was reported, but memory safety is always a gateway...)

The Patch

The fix is simple and direct. Before using start_clu, check if it’s an invalid cluster. If it is, treat the directory as empty and avoid accessing forbidden memory.

Code Snippet (From the Patch)

if (EXFAT_IS_LAST_CLUSTER(start_clu))
    dir->size = ;

Now, before the code walks through directory entries, it simply ensures the cluster is valid.

Old Vulnerable Code

// start_clu can be EOF -- vulnerability!
lookup_dir_entry(start_clu, ei->hint_femp.eidx, ...);

New Safe Code

if (EXFAT_IS_LAST_CLUSTER(start_clu)) {  // New: validate cluster
    dir->size = ;
    return ; // or equivalent no-op
}
lookup_dir_entry(start_clu, ei->hint_femp.eidx, ...);

Example Exploit Scenario

1. Attacker builds an exFAT image. Directory chain is broken so that a directory's start_clu points to an invalid/EOF value, but directory size is set unusually large.

While we're not providing full PoC code for responsible reasons, here's a pseudocode flow

# Pseudocode for an exFAT image generator
image = create_exfat_image()
root_dir = image.add_directory("MALICIOUS")
root_dir.start_cluster = EOF_CLUSTER # set cluster to invalid
root_dir.size = 8192 # set size > cluster size
save_image(image, "exploit.img")

After mounting this image (mount -t exfat /dev/sdX /mnt/usb), the vulnerable kernel might do an out-of-bounds memory access.

Impact and Mitigation

Severity: Moderate to Serious (depends on usage).
Affected: All Linux systems with exFAT support prior to the patch.
Mitigation:
- Update your Linux kernel to one containing the patch (upstream commit ref)

References and Further Reading

- Official kernel patch
- exFAT Documentation
- CVE Record at NVD (may take a while to update)
- Report thread on oss-sec (search for CVE-2024-53147)

Conclusion

CVE-2024-53147 reminds us why file system code must be super-vigilant. A simple sanity check saves Linux from potential crashes and further file system corruption when dealing with bad exFAT media.

Upgrade your kernel — and, as always, beware the "trusted" USB!


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Timeline

Published on: 12/24/2024 12:15:22 UTC
Last modified on: 05/04/2025 09:54:14 UTC