CVE-2026-72034

This CVE patches a NULL dereference in the kernel's file handle lookup path when encountering detached mounts. The technical root cause is straightforward: `capable_wrt_mount()` previously assumed mounts remained namespace-attached, and when `umount_tree()` clears `mnt_ns`, a subsequent handle lookup could dereference NULL. The fix makes `capable_wrt_mount()` return false for any detached mount—a policy change, not just a bug fix. The deeper implication is that the patch fundamentally changes what `open_by_handle_at()` guarantees. File handles obtained from cloned mounts via `open_tree(..., OPEN_TREE_CLONE)` are now subject to silent authorization failure if their mount becomes detached—even temporarily, even during legitimate orchestration. Handles don't corrupt; they simply become unauthorized. This breaks an implicit atomicity guarantee userspace relied upon: the handle was valid when created, and nothing corrupted it, yet access is now denied based on mount lifecycle state. Defenders should note this isn't an isolated flaw. This is the third installment in a regression cycle beginning with CVE-2018-1128 and CVE-2018-1129, which introduced the same "detached means denied" semantics this patch extends. The container ecosystem already developed workarounds for earlier tightening; expect another round of API gymnastics. The unaddressed risk: any code with CAP_SYS_ADMIN can force mount detachment to invalidate file handles held by other principals. The patch trades a kernel panic for an availability failure without acknowledging the substitution. If you run container runtimes that manipulate mounts aggressively, monitor for handle-invalidation events—the new policy creates a deterministic denial-of-service vector that didn't exist before.

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