CVE-2022-49850

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix deadlock in nilfs_count_free_blocks()

A semaphore deadlock can occur if nilfs_get_block() detects metadata corruption while locating data blocks and a superblock writeback occurs at the same time:

task 1 task 2

• A file operation * nilfs_truncate() nilfs_get_block() down_read(rwsem A) <– nilfs_bmap_lookup_contig() … generic_shutdown_super() nilfs_put_super() * Prepare to write superblock * down_write(rwsem B) <– nilfs_cleanup_super() * Detect b-tree corruption * nilfs_set_log_cursor() nilfs_bmap_convert_error() nilfs_count_free_blocks() __nilfs_error() down_read(rwsem A) <– nilfs_set_error() down_write(rwsem B) <–

                       *** DEADLOCK ***
  

Here, nilfs_get_block() readlocks rwsem A (= NILFS_MDT(dat_inode)->mi_sem) and then calls nilfs_bmap_lookup_contig(), but if it fails due to metadata corruption, __nilfs_error() is called from nilfs_bmap_convert_error() inside the lock section.

Since __nilfs_error() calls nilfs_set_error() unless the filesystem is read-only and nilfs_set_error() attempts to writelock rwsem B (= nilfs->ns_sem) to write back superblock exclusively, hierarchical lock acquisition occurs in the order rwsem A -> rwsem B.

Now, if another task starts updating the superblock, it may writelock rwsem B during the lock sequence above, and can deadlock trying to readlock rwsem A in nilfs_count_free_blocks().

However, there is actually no need to take rwsem A in nilfs_count_free_blocks() because it, within the lock section, only reads a single integer data on a shared struct with nilfs_sufile_get_ncleansegs(). This has been the case after commit aa474a220180 (nilfs2: add local variable to cache the number of clean segments), that is, even before this bug was introduced.

So, this resolves the deadlock problem by just not taking the semaphore in nilfs_count_free_blocks().

Weakness

The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.

Affected Software

Extended Description

Locking is a type of synchronization behavior that ensures that multiple independently-operating processes or threads do not interfere with each other when accessing the same resource. All processes/threads are expected to follow the same steps for locking. If these steps are not followed precisely - or if no locking is done at all - then another process/thread could modify the shared resource in a way that is not visible or predictable to the original process. This can lead to data or memory corruption, denial of service, etc.

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/25.html
• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/27.html

References

• https://git.kernel.org/stable/c/1d4ff73062096c21b47954d2996b4df259777bda
• https://git.kernel.org/stable/c/36ff974b0310771417c0be64b64aa221bd70d63d
• https://git.kernel.org/stable/c/3c89ca6d3dfa6c09c515807a7a97a521f5d5147e
• https://git.kernel.org/stable/c/8ac932a4921a96ca52f61935dbba64ea87bbd5dc
• https://git.kernel.org/stable/c/8b4506cff6630bb474bb46a2a75c31e533a756ba
• https://git.kernel.org/stable/c/abc082aac0d9b6b926038fc3adb7008306581be2
• https://git.kernel.org/stable/c/cb029b54953420f7a2d65100f1c5107f14411bdc
• https://git.kernel.org/stable/c/f0cc93080d4c09510b74ecba87fd778cca390bb1