fhandle: reject detached mounts in capable_wrt_mount()

Summary

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.

Timeline

1. Aug 15, 2026, 5:51 AM UTCCVE published
2. Oct 16, 2026, 3:50 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Oct 19, 2026, 9:19 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 620c266f394932e5decc4b34683a75dfc59dc2f4 · fixed in 15104234c267ebe04b9f9a73e5c2179cc265ff60; From 620c266f394932e5decc4b34683a75dfc59dc2f4 · fixed in 6c52226072a3c61337b1eec1799bb987748884fa; From 620c266f394932e5decc4b34683a75dfc59dc2f4 · fixed in 6ee183d89261bf1d1cf9f06d80a40dab8f36ee55; From 620c266f394932e5decc4b34683a75dfc59dc2f4 · fixed in 6c732471740bc2ac9b0946134f9f551dc75f4369

• Linux Linux

  6.11

• KernelLinux

  From 6.11.0 · fixed in 6.12.97; From 6.13.0 · fixed in 6.18.40; From 6.19.0 · fixed in 7.1.5

References 4