CVE-2021-46989 is a medium-severity vulnerability affecting the Linux kernel's HFS+ filesystem implementation. The vulnerability arises from a logic error introduced by commit 31651c607151, which aimed to avoid deadlocks during file truncation operations on HFS+ filesystems. HFS+ uses extent records, each containing 8 extents, to track file data allocation. When a file is truncated (shrunk) in the middle of an extent record located in the extents overflow file, the flawed logic in the hfsplus_file_truncate() function causes the last extent record to be removed entirely rather than partially truncated. This occurs because the call to hfs_brec_remove() is no longer properly guarded by a conditional check (blk_cnt > start), leading to unconditional removal of the extent record. The consequence is filesystem corruption and data loss, as extents that should remain are erroneously discarded. Additionally, there is a locking issue where the tree_lock is dropped before certain checks, potentially causing race conditions or premature unlocking, which could lead to further data integrity problems. The vulnerability can be reproduced by creating a file with at least 10 extents and performing a shrinking truncate into the middle of the last extent record, where the number of remaining extents is not divisible by 8. The fix involves reintroducing the conditional check before removing extent records and carefully managing the tree_lock to avoid race conditions. This vulnerability does not affect confidentiality or integrity directly but impacts availability by causing data loss and filesystem corruption. Exploitation requires local privileges (PR:L) and no user interaction, with low attack complexity (AC:L) and local attack vector (AV:L).

For European organizations, this vulnerability poses a risk primarily to systems using the HFS+ filesystem on Linux, which is less common but may exist in environments that interact with macOS-formatted drives or legacy storage devices. The impact is significant in scenarios where critical data resides on HFS+ volumes, as truncation operations could lead to unexpected data loss and filesystem corruption, potentially disrupting business operations and causing downtime. Since exploitation requires local privileges, the threat is more relevant in multi-user systems or environments where attackers can gain limited access. The data loss could affect backup integrity, forensic investigations, or any workflows relying on HFS+ volumes. Organizations with mixed OS environments or those using Linux systems to mount HFS+ drives for data exchange should be particularly vigilant. The medium severity rating reflects the limited attack vector and requirement for local access but acknowledges the high impact on availability and data integrity.

1. Apply the official Linux kernel patches that address CVE-2021-46989 as soon as they become available to ensure the truncation logic and locking mechanisms are corrected. 2. Avoid performing file truncation operations on HFS+ volumes from Linux systems until patched, especially shrinking truncates in the middle of extent records. 3. Implement strict access controls and monitoring on systems that mount HFS+ filesystems to prevent unauthorized local access that could trigger this vulnerability. 4. Regularly back up data stored on HFS+ volumes to mitigate potential data loss from corruption. 5. For environments requiring HFS+ support, consider isolating these volumes or using dedicated systems with limited user access to reduce risk. 6. Monitor kernel updates and security advisories related to filesystem vulnerabilities and test patches in staging environments before deployment. 7. Educate system administrators about the risks of manipulating HFS+ filesystems on Linux and encourage cautious operational procedures.