namespace: queue a mount only once for mount notifications

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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.

Fixes: bf630c4016 ("vfs: add notifications for mount attach and detach")
Cc: stable@vger.kernel.org # v6.15+
Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-1-be34c83956ae@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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Christian Brauner 2026-09-30 15:31:53 +02:00

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3

fs/mount.h

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3

fs/namespace.c

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