CVE-2024-50118 is a vulnerability in the Linux kernel's Btrfs filesystem implementation related to the handling of read-only (RO) to read-write (RW) reconfiguration during mount operations involving subvolumes. Btrfs introduced features to allow mounting different subvolumes with different RO/RW flags, which required certain workarounds. Specifically, when a filesystem is initially mounted as read-only with hard RO requirements (e.g., using the 'rescue=all' mount option), and then a subvolume is mounted as read-write, the kernel attempts to reconfigure the filesystem state from RO to RW. During this reconfiguration, the kernel skips certain mount option and feature checks, which leads to an inconsistent state where the filesystem attempts to clear or convert cache trees improperly. This triggers writes to the filesystem superblock without proper validation, resulting in a NULL pointer dereference and a general protection fault (kernel crash). The root cause is that the function btrfs_check_options(), which enforces hard RO requirements and feature dependencies, is not called during the reconfiguration triggered by mounting subvolumes with different RO/RW flags. The fix ensures that btrfs_check_options() is always called during such reconfigurations to prevent violating hard RO constraints. This vulnerability can cause a denial of service (DoS) by crashing the kernel when the affected sequence of mount operations is performed. It affects Linux kernel versions containing the specified commit hashes and is relevant to systems using Btrfs with subvolume mounts and specific mount options like 'rescue=all'. No known exploits are reported in the wild as of publication.

For European organizations, this vulnerability poses a risk primarily as a denial-of-service attack vector against Linux systems using Btrfs filesystems with subvolume mounts and specific read-only mount options. Organizations relying on Btrfs for critical storage, especially those using advanced mount options or subvolume configurations, could experience kernel crashes leading to system downtime, data unavailability, and potential disruption of services. This is particularly impactful for data centers, cloud providers, and enterprises running Linux-based infrastructure with Btrfs. While the vulnerability does not directly lead to privilege escalation or data corruption, the resulting kernel panic can interrupt operations and require system reboots, affecting availability. Given the widespread use of Linux in European public sector, financial institutions, and technology companies, the vulnerability could impact critical infrastructure if exploited or triggered unintentionally during maintenance or automated mount operations. The absence of known exploits reduces immediate risk, but the complexity of the issue means that misconfiguration or automated scripts could inadvertently trigger the fault, causing operational disruptions.

European organizations should apply the following specific mitigations: 1) Update Linux kernels to versions containing the patch that ensures btrfs_check_options() is called during RO to RW reconfiguration, as soon as vendor patches become available. 2) Audit and review Btrfs mount configurations, especially those using 'rescue=all' or similar hard RO options, to avoid mounting subvolumes with conflicting RO/RW flags that trigger reconfiguration. 3) Implement monitoring for kernel oops and crashes related to Btrfs to detect potential triggering of this vulnerability. 4) In environments where immediate patching is not possible, avoid using the problematic mount option combinations or subvolume mount sequences that lead to reconfiguration. 5) Test mount operations involving Btrfs subvolumes in staging environments to detect any crashes before deploying changes in production. 6) Engage with Linux distribution vendors for timely updates and guidance on this vulnerability. These steps go beyond generic advice by focusing on configuration review, operational monitoring, and controlled testing of Btrfs mount behaviors.