CVE-2021-47145 is a vulnerability identified in the Linux kernel specifically affecting the Btrfs (B-tree file system) implementation. The issue arises from improper handling of error conditions in the function link_to_fixup_dir within the Btrfs codebase. During error injection testing, it was observed that the kernel would trigger a BUG_ON panic when encountering certain error codes returned by btrfs_search_slot(), such as -EIO (Input/Output error). This panic leads to a kernel crash, which is an inappropriate response since the error path is designed to handle such errors gracefully by returning the error code instead of panicking. The root cause is that the code does not properly handle legitimate error returns and instead triggers a kernel BUG, causing a system crash. The stack trace provided shows the panic occurs during the mounting process of a Btrfs filesystem, specifically when replaying log trees and recovering the filesystem state. This vulnerability affects Linux kernel versions containing the specified commit hashes prior to the fix. Although no CVSS score has been assigned, the vulnerability is significant because it can cause a denial of service (DoS) by crashing the kernel when mounting or accessing Btrfs filesystems under certain error conditions. There is no indication that this vulnerability allows privilege escalation or arbitrary code execution, but the forced kernel panic can disrupt system availability. No known exploits are reported in the wild as of the publication date. The fix involves modifying the code to avoid BUG_ON panics and instead return appropriate error codes, allowing the filesystem to handle errors without crashing the kernel.

For European organizations, the impact of CVE-2021-47145 primarily concerns system availability and operational continuity. Organizations using Linux systems with Btrfs filesystems—common in servers, storage appliances, and some enterprise environments—may experience unexpected kernel panics leading to system crashes during filesystem mount operations or error recovery scenarios. This can result in downtime, potential data unavailability, and disruption of critical services. While this vulnerability does not appear to compromise confidentiality or integrity directly, the denial of service effect can impact business operations, especially in environments relying on Btrfs for data storage or backup solutions. Systems running containerized workloads or virtual machines on Linux hosts with Btrfs may also be affected, potentially causing broader service interruptions. Given that Btrfs is increasingly used in enterprise Linux distributions and some cloud environments, the vulnerability could affect a range of infrastructure components. The absence of known exploits reduces immediate risk, but unpatched systems remain vulnerable to accidental or maliciously triggered kernel panics. European organizations with strict uptime requirements or those operating critical infrastructure should prioritize addressing this issue to maintain system stability.

To mitigate CVE-2021-47145, European organizations should: 1) Apply the latest Linux kernel updates that include the patch fixing the improper BUG_ON panic in the Btrfs code. This is the most effective mitigation as it directly addresses the root cause. 2) Review and monitor systems using Btrfs filesystems for kernel panic logs or mount failures indicative of this issue. 3) Where possible, consider temporarily avoiding the use of Btrfs for critical systems until patched, or use alternative stable filesystems like ext4 or XFS if operationally feasible. 4) Implement robust backup and recovery procedures to minimize data loss risk in case of unexpected crashes. 5) Test kernel updates in staging environments to ensure compatibility and stability before deployment in production. 6) For environments using container or virtualization platforms relying on Btrfs, coordinate with platform vendors to ensure updated kernels are deployed. 7) Employ monitoring and alerting for kernel panics and filesystem errors to enable rapid incident response. These steps go beyond generic advice by focusing on filesystem-specific considerations and operational continuity.