CVE-2026-68149

The fix landing in the VFS layer rather than FUSE is the significant detail that most engineers will miss. This placement reveals the real vulnerability: ACL_DONT_CACHE was stored alongside transient cache metadata when it actually represents a filesystem's categorical policy decision—a declaration that ACL caching should never occur for that inode. The VFS helper forget_cached_acl() was treating this flag as routine cache state and clearing it during normal invalidation, which conflates cache cleanup with policy modification—two concerns that should be entirely separate. The consequence is concrete and security-relevant. When FUSE (or any filesystem using this pattern) calls forget operations, ACL_DONT_CACHE gets wiped. Subsequent get_acl() calls then return cached ACLs when they should have triggered a fresh filesystem lookup. This means permission decisions—LSM hooks, capability checks, namespace enforcement—operate on stale authorization state. The race window is practical enough to be exploitable, not merely theoretical. More critically, this bug is a design pattern vulnerability, not a one-off FUSE quirk. The kernel has a recurring history of policy flags (MS_RDONLY, filesystem-specific inode->i_flags, and now ACL_DONT_CACHE) being physically co-located with operational state and subsequently cleared by generic cleanup paths. The ACL_DONT_CACHE case is asymmetrically dangerous because stale permissions compound over time—every authorization decision on that inode inherits the corruption—while stale MS_RDONLY causes a single discrete failure. Check whether any custom filesystems or out-of-tree modules you maintain use ACL_DONT_CACHE or similar policy flags stored in ACL cache structures. Verify the fix is present in your kernel version. Treat any flag representing a filesystem's categorical choice as permanently owned by the filesystem layer and never modified by VFS invalidation helpers—this invariant should guide future audit of similar caching paths.

Reviewed through automated stages and approved by a human before publication.