namespace: queue a mount only once for mount notifications

Summary

In the Linux kernel, the following vulnerability has been resolved: namespace: queue a mount only once for mount notifications mnt_notify_add() puts a mount on notify_list. It doesn't check whether the mount is on the list already. That's fine as long as every mount is queued at most once per namespace_sem hold. It isn't. propagate_umount() moves a surviving overmount off a stack of mounts that are going away and queues it as moved. shrink_submounts() and mark_mounts_for_expiry() call umount_tree() for several mounts under a single namespace_sem hold. So the second umount_tree() can take down exactly the mount the first one reparented, reparent it once more and so end up queueing it a second time. The second list_add_tail() cuts the mounts queued in between out of the list while the head still points at the last of them. notify_mnt_list() then only visits that one mount and the others are freed after the grace period with notify_list still pointing at them. Every later mount operation on the host walks freed memory: BUG: KASAN: slab-use-after-free in __list_add_valid_or_report Read of size 8 at addr ffff8881003d57e0 by task notify_dq/162 __list_add_valid_or_report+0x15c/0x1a0 mnt_add_to_ns+0x322/0x890 attach_recursive_mnt.isra.0+0xf2a/0x1a00 path_mount+0x139c/0x1d40 This needs a mount with MNT_SHRINKABLE to bind from. That's what finish_automount() creates (submounts of NFS, AFS and CIFS and the tracefs mount below debugfs). Bind mounts inherit the flag. A mount namespace with a FAN_MARK_MNTNS mark does the rest and root in a user namespace can have both. Initialize to_notify in alloc_vfsmnt() and leave a mount alone that is queued already. mnt_notify() looks at the state the mount has when the list is drained so a single entry per mount is enough. The move event for a mount that is unmounted under the same hold is lost. That's fine. The detach is what matters.

Timeline

1. Oct 23, 2026, 12:20 PM UTCCVE published
2. Oct 23, 2026, 12:45 PM UTCFirst seen by cve.now
3. Oct 24, 2026, 2:09 PM UTCRecord last updated

Affected versions

• Linux Linux

  From bf630c40164162ba1d3933c2f5e3397d083e0948 · fixed in acbe9e4418e2ef7bef1a12e62e0e7d6145a15190; From bf630c40164162ba1d3933c2f5e3397d083e0948 · fixed in 702ff3fe499876e09c45558a05366d21a5bc41ab; From bf630c40164162ba1d3933c2f5e3397d083e0948 · fixed in bd2feb7796b6585938a4241e7b7c4148a6f7b762

• Linux Linux

  6.15

• KernelLinux

  From 6.15.0 · fixed in 6.18.57; From 6.19.0 · fixed in 7.2.11

References 3