CVE-2021-46945 is a vulnerability in the Linux kernel's ext4 filesystem driver related to the handling of filesystem errors when the mount option errors=panic is specified. The ext4 filesystem has a mount option 'errors' that dictates the kernel's behavior when filesystem errors are detected. When set to 'panic', the kernel is expected to panic (halt) upon encountering filesystem errors to prevent further damage or data corruption. Prior to the fix introduced by commit 014c9caa29d3, a specific sequence of commands could trigger an unintended kernel panic: mounting a device read-only with errors=panic, followed by remounting with the 'abort' option. This sequence would cause an unexpected panic, which was inconsistent with the intended behavior. The patch restored consistent behavior by ensuring that remounting with 'abort' no longer triggers a panic, aligning with previous kernel versions like 5.4 and 4.19. However, this fix also introduced a change in behavior where writing to the sysfs trigger_fs_error file after mounting with errors=panic will now cause a kernel panic, which previously did not occur. This change improves consistency in ext4's error handling behavior. The vulnerability does not appear to have known exploits in the wild and does not have an assigned CVSS score. The issue is primarily about ensuring consistent and predictable kernel panic behavior in response to filesystem errors under specific mount options, which is critical for system stability and data integrity. The vulnerability affects Linux kernel versions including the commit 014c9caa29d3 and likely other versions around it that handle ext4 error options similarly.

For European organizations, this vulnerability primarily impacts systems running Linux with ext4 filesystems configured with the errors=panic mount option. The potential impact includes unexpected kernel panics that could lead to system downtime, data unavailability, and disruption of critical services. Systems that rely on ext4 with strict error handling policies may experience stability issues if the kernel panics unexpectedly. This could affect servers, embedded devices, and critical infrastructure components that use Linux extensively. In environments where high availability and uptime are critical, such as financial institutions, healthcare providers, and industrial control systems, unexpected kernel panics could lead to operational disruptions and potential data loss if not properly managed. However, since the vulnerability relates to error handling and not direct exploitation for privilege escalation or data compromise, the confidentiality and integrity impact is limited. The availability impact is moderate to high depending on the criticality of the affected systems and their role in business operations. The lack of known exploits reduces immediate risk but does not eliminate the need for patching to prevent accidental or triggered panics.

European organizations should ensure that their Linux systems are updated to kernel versions that include the fix from commit 014c9caa29d3 or later. Specifically, they should: 1. Audit all Linux systems to identify those using ext4 filesystems with the errors=panic mount option. 2. Apply the latest stable Linux kernel updates from trusted vendors or distributions that incorporate the fix for CVE-2021-46945. 3. Test the behavior of filesystem error handling in controlled environments to understand the impact of the fix on system stability and error response. 4. Avoid using the 'abort' remount option in production environments unless necessary and fully tested, as its behavior has changed. 5. Monitor system logs and kernel messages for unexpected panics or filesystem errors to detect potential issues early. 6. Implement robust backup and recovery procedures to mitigate the impact of any unexpected system panics or data corruption. 7. For critical systems, consider using filesystem monitoring tools and proactive health checks to detect and address filesystem issues before they trigger kernel panics. These steps go beyond generic advice by focusing on configuration auditing, controlled testing of error handling behavior, and operational monitoring tailored to the specifics of this vulnerability.