Overview
About vulnerability
In the Linux kernel, the following vulnerability has been resolved:
fhandle: reject detached mounts in capable_wrt_mount()
The recent fhandle RCU fix moved the mount namespace capability check into capable_wrt_mount(), so a non-NULL mnt_namespace survives the ns_capable() dereference. The helper still assumes the later READ_ONCE(mount->mnt_ns) must be non-NULL because may_decode_fh() checked is_mounted() first.
That assumption is not stable. A detached mount from open_tree(…, OPEN_TREE_CLONE) can be dissolved on fput while open_by_handle_at() is between those checks, and umount_tree() can clear mount->mnt_ns. If the helper observes NULL, it dereferences mnt_ns->user_ns and panics.
Return false when the RCU read observes a detached mount. This keeps the relaxed permission path conservative: a mount no longer attached to a namespace cannot authorize open_by_handle_at() access.
Details
- Affected product:
- Debian 10 ELS
- Affected packages:
- linux @ 4.19.0