CVE-2024-26956 is a medium-severity vulnerability affecting the Linux kernel's implementation of the NILFS2 (New Implementation of a Log-structured File System) file system. The vulnerability arises from two related flaws in the handling of corrupted metadata within the NILFS2 file system, specifically in the functions nilfs_get_block() and submit_bh_wbc(). The first flaw occurs when nilfs_get_block() attempts to locate a data block via btree or direct mapping, and the disk address translation routine nilfs_dat_translate() fails with an internal error code (-ENOENT) due to corrupted DAT metadata. This failure is incorrectly propagated back to nilfs_get_block(), causing it to misidentify an existing block as non-existent. Consequently, data block lookup and insertion operations fail inconsistently. The second flaw is that nilfs_get_block() returns a success status despite this inconsistent state, leading the caller function __block_write_begin_int() or others to request a read operation on an unmapped buffer. This triggers a kernel BUG in submit_bh_wbc() due to a failed BH_Mapped flag check, resulting in a kernel panic or crash. The patch fixes these issues by changing the return value from -ENOENT to -EINVAL when DAT metadata corruption is detected, signaling a file system error that is properly handled and converted to an I/O error (-EIO) downstream. This prevents the kernel from entering the inconsistent state that causes the crash. The vulnerability was reported by syzbot, an automated kernel bug detection tool, and affects Linux kernel versions containing the specified commit hashes. Exploitation requires local privileges with at least low-level permissions (PR:L) and does not require user interaction. The vulnerability impacts availability by causing kernel crashes but does not affect confidentiality or integrity. There are no known exploits in the wild at this time.

For European organizations, this vulnerability poses a risk primarily to systems running Linux kernels with NILFS2 file systems, which are less common but may be used in specialized environments requiring log-structured file systems. The main impact is on system availability due to potential kernel panics triggered by corrupted file system metadata, which could lead to denial of service conditions. This can disrupt critical services, especially in environments where uptime is essential such as financial institutions, telecommunications, and public sector infrastructure. Since exploitation requires local privileges, the threat is higher in multi-user systems or where attackers can gain limited access. The absence of confidentiality or integrity impact reduces the risk of data breaches, but availability disruptions can still have significant operational and reputational consequences. Organizations relying on Linux servers with NILFS2 should prioritize patching to avoid unexpected crashes and service interruptions.

1. Apply the official Linux kernel patches that address CVE-2024-26956 as soon as they become available for your distribution. Monitor vendor advisories for updated kernel packages. 2. Audit systems to identify any use of the NILFS2 file system and assess the necessity of its use; consider migrating critical data to more widely supported and tested file systems if feasible. 3. Implement strict access controls and monitoring to limit local user privileges and detect suspicious activities that could lead to exploitation attempts. 4. Regularly check file system integrity using tools compatible with NILFS2 to detect early signs of metadata corruption and prevent triggering the vulnerability. 5. Employ kernel crash dump and monitoring solutions to quickly identify and respond to kernel panics related to this issue. 6. In environments where NILFS2 is essential, consider deploying redundancy and failover mechanisms to mitigate availability impacts from potential crashes.