A vulnerability (CVE-2024-41935) has been identified and resolved in the Linux kernel, specifically within the filesystem module f2fs. This post will provide an in-depth explanation of the vulnerability, a code snippet demonstrating the fix, and links to original references and exploit details.

The issue discovered in the kernel concerns the handling of read extent nodes in the extent tree of the f2fs filesystem. Due to the misuse of an rwlock (read-write lock), the system could potentially hang under specific circumstances - such as when there are a large number of extent nodes in the extent tree.

The following sections will explore the implications of this vulnerability, the fix that has been implemented to address it, and discuss relevant source material.

The Vulnerability

The problem lies in the way the extent tree is managed during shrinking operations. In the presence of a large number of extent nodes in the extent tree, the rwlock can be held for a very long time, which may lead to a system hang.

To mitigate the risk of a system hang, the implemented fix divides the shrinking operation into smaller batches, thus reducing the critical region of the rwlock and minimizing its hold time. This prevents scenarios where the kernel would hang due to prolonged rwlock hold times.

The Fix

The original fix in the Linux kernel source code can be found in the commit 3df1c999b.

Here is the relevant code snippet that resolves the vulnerability

--- a/fs/f2fs/shrinker.c
+++ b/fs/f2fs/shrinker.c
@@ -760,7 +760,8 @@ void f2fs_shrink_extent_tree(struct f2fs_sb_info *sbi, int nr_shrink)
 {
    down_read(&sbi->umount_rwsem);

-   __f2fs_shrink_extent_tree(sbi, nr_shrink);
+   while (nr_shrink > )
+       nr_shrink -= __f2fs_shrink_extent_tree(sbi, nr_shrink);
+
    up_read(&sbi->umount_rwsem);
 }

This code change ensures that the extent tree is being shrunk in batches to help reduce the time taken by the rwlock.

For more information on this vulnerability, consider exploring the following resources

1. The original mailing list discussion of the vulnerability and patch can be found here.
2. The Linux kernel stable repo containing the commit that resolves this vulnerability can be accessed here.

As of now, there is no available exploit specific to this vulnerability. However, the community should be aware of this issue to ensure the stability and security of their systems.

Conclusion

CVE-2024-41935 poses a potential risk to the stability of systems running the Linux kernel, more specifically those using the f2fs filesystem. It is essential for developers and administrators to apply the necessary patches and keep their systems up-to-date to mitigate the risks associated with this vulnerability.

By monitoring and addressing security concerns like this one, the open-source community continues to support the safety and stability of the Linux ecosystem. Users and developers are encouraged to follow discussions in the links provided and keep abreast of new developments to ensure the ongoing protection of their systems.

Timeline

Published on: 01/11/2025 13:15:21 UTC
Last modified on: 01/20/2025 06:18:58 UTC