CVE-2024-26616 is a vulnerability in the Linux kernel's Btrfs filesystem scrub functionality. The issue arises specifically when a Btrfs filesystem has been converted from ext4 and contains data chunks whose lengths are not aligned to the expected 64KB boundary. During a scrub operation, which is intended to verify and repair filesystem integrity, the kernel attempts to read these chunks. However, due to the misalignment, the bio (block I/O) submitted for reading is split into two halves, each triggering an end I/O (endio) callback. The scrub code incorrectly assumes only a single endio call per bio, leading to a use-after-free condition when the first endio frees the bio and the second endio attempts to access already freed memory. This results in kernel memory corruption, reported by Kernel Address Sanitizer (KASAN) as slab-use-after-free, and can cause system crashes or unrecoverable filesystem errors such as "unable to find chunk map" errors. The root cause is that scrub_read_endio() updates bits beyond the chunk boundary and scrub_submit_initial_read() does not correctly limit the read range to the chunk size. The fix involves ensuring that scrub_read_endio() only updates bits within the chunk range and that scrub_submit_initial_read() calculates and submits bio requests strictly within chunk boundaries. This prevents the double endio calls on the same bio and eliminates the use-after-free condition. The vulnerability affects Linux kernel versions containing the vulnerable Btrfs scrub implementation prior to the fix, notably kernel version 6.7.0-x64v3-dbg as referenced in the report. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on Btrfs filesystems on Linux servers for critical data storage and integrity verification. The use-after-free bug can lead to kernel crashes (denial of service), potential data corruption, and unrecoverable filesystem errors during scrub operations. This can disrupt services, cause data loss, and require time-consuming recovery efforts. Organizations using Btrfs on converted ext4 filesystems are particularly at risk. Since the vulnerability is triggered during scrub operations, which are often scheduled for routine maintenance or data integrity checks, the risk of unexpected downtime increases. Additionally, if attackers gain local access, they might be able to trigger the vulnerability to cause denial of service or potentially escalate privileges by exploiting kernel memory corruption, although no direct exploit is known yet. The vulnerability affects confidentiality, integrity, and availability by risking data corruption and system stability. Given the widespread use of Linux in European data centers, cloud providers, and enterprise environments, the vulnerability poses a moderate to high operational risk if unpatched.

European organizations should take the following specific mitigation steps: 1) Identify Linux systems using Btrfs filesystems, especially those converted from ext4, and verify kernel versions for vulnerability. 2) Apply the official Linux kernel patches that fix CVE-2024-26616 as soon as they become available, or upgrade to a fixed kernel version. 3) Temporarily disable or avoid running Btrfs scrub operations on affected systems until patches are applied to prevent triggering the use-after-free. 4) Implement monitoring for kernel crash logs and KASAN reports related to Btrfs scrub to detect potential exploitation or triggering of the vulnerability. 5) For critical systems, consider filesystem backups and recovery plans to mitigate data loss risk from potential corruption. 6) Limit local user access to systems running vulnerable kernels to reduce risk of intentional triggering. 7) Coordinate with Linux distribution vendors for timely patch deployment and security advisories. These targeted steps go beyond generic advice by focusing on the specific conditions and operations (scrub on converted ext4 Btrfs) that trigger the vulnerability.