CVE-2022-50668

Vulnerability

In the Linux kernel's ext4 filesystem, a deadlock can occur due to corruption of mbcache entries when manipulating xattr blocks. The root cause is a non-atomic update of the e_reusable bit in the cache entry's bitfield, which can race with updates to e_referenced, causing inconsistencies. This leads to an infinite loop in ext4_xattr_block_set() where the system repeatedly finds a candidate xattr block in the mbcache but cannot reuse it because its reference count is too high, while the entry is incorrectly marked as reusable.

Exploitation

An attacker would need local access to a system with the ext4 filesystem and the ability to trigger xattr operations (e.g., setting extended attributes) on files. The vulnerability is triggered through normal filesystem operations involving extended attributes. The commit 65f8b80053a1 ("ext4: fix race when reusing xattr blocks") made the bug much easier to hit, but the underlying issue existed for many years. No authentication beyond local user access is required.

Impact

Successful exploitation leads to a deadlock, effectively causing a denial of service (DoS) as the filesystem thread hangs indefinitely, potentially affecting the entire system's availability. There is no indication of data corruption or privilege escalation from the description.

Mitigation

The vulnerability is fixed by using atomic bitops for the e_reusable update. The fix has been backported to stable kernels via commits referenced in the advisory. Users should update their Linux kernel to a patched version. No workaround is mentioned.