CVE-2022-50755: 
Linux Kernel vulnerability analysis and mitigation

CVE-2022-50755 is a double-free (double brelse()) vulnerability in the Linux kernel's UDF (Universal Disk Format) filesystem driver, specifically in the udf_rename() function. The flaw causes an unbalanced buffer reference count (b_count) on buffer heads, leading to kernel warnings and potential system instability. It was publicly disclosed on December 24, 2025, and affects the Linux kernel across multiple stable branches. The vulnerability carries a CVSS v3.1 base score of 5.5 (Medium), requiring only low-privilege local access with no user interaction (Red Hat CVE, Red Hat Bugzilla).

The root cause is a double-release of buffer heads (ofibh.sbh and ofibh.ebh) classified as CWE-415 (Double Free) or more broadly as improper resource management. When udf_find_entry() returns NULL, it already calls brelse() on both buffer heads internally; however, udf_rename() also calls brelse() on the same buffer heads unconditionally, causing the buffer reference count (b_count) to become unbalanced. This triggers a kernel warning: VFS: brelse: Trying to free free buffer at fs/buffer.c:1145. The fix prevents udf_rename() from calling brelse() when udf_find_entry() returns NULL, correcting the reference counting logic (Red Hat Bugzilla).

A local attacker with low privileges can trigger the double brelse() condition by performing rename operations on a UDF-formatted filesystem, causing kernel warnings and potential system instability or crash (denial of service). The vulnerability has no confidentiality or integrity impact, but carries a high availability impact, meaning it could destabilize or crash the affected system. There is no evidence of lateral movement potential or data exposure risk beyond the denial-of-service scenario (Red Hat CVE, Red Hat Bugzilla).

• Logs: Kernel log entries containing VFS: brelse: Trying to free free buffer or WARNING: CPU: ... at fs/buffer.c:1145 __brelse indicate triggering of the vulnerability.
• Process: Unusual or repeated rename operations on UDF-mounted filesystems by low-privilege users, potentially visible in audit logs or strace output.
• System: Kernel oops or warning traces in /var/log/kern.log or dmesg output referencing udf_rename, invalidate_bh_lru, or blkdev_flush_mapping in the call stack.

The fix has been committed to multiple Linux kernel stable branches (commits 4fca090, 9d2cad6, 40dba68, 78eba27). Users should update to a patched kernel version provided by their Linux distribution — Red Hat, SUSE, and other vendors have issued or are issuing kernel updates addressing this CVE. As a workaround, administrators can restrict local user access to systems with UDF filesystems mounted, or avoid mounting UDF filesystems if not required. Monitoring kernel logs for brelse-related warnings can help detect exploitation attempts (Red Hat Bugzilla, Red Hat CVE).