CVE-2026-72034fhandle: reject detached mounts in capable_wrt_mount()

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/15104234c267ebe04b9f9a73e5c2179cc265ff60
• https://git.kernel.org/stable/c/6c52226072a3c61337b1eec1799bb987748884fa
• https://git.kernel.org/stable/c/6ee183d89261bf1d1cf9f06d80a40dab8f36ee55
• https://git.kernel.org/stable/c/6c732471740bc2ac9b0946134f9f551dc75f4369