namespace: remove the fsnotify marks of a mount namespace in process context

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mnt_ns_release() drops the last passive reference of a mount namespace
and removes its fanotify marks via fsnotify_mntns_delete(). That takes
the mutex of every group with a mark on the namespace and the spinlock
of the connector. Fine from process context. But mnt_ns_tree_remove()
hands the reference the namespace was allocated with to call_rcu() and
so the marks are removed from the RCU softirq:

  BUG: sleeping function called from invalid context at kernel/locking/mutex.c:623
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 0, name: swapper/3
   __mutex_lock+0x113/0x24b0
   fsnotify_destroy_marks+0x11b/0x3d0
   mnt_ns_release_rcu+0x57/0xa0
   rcu_core+0x6b6/0x1e40

and lockdep complains about the connector lock being taken from softirq
context. A fanotify group with a FAN_MARK_MNTNS mark on the mount
namespace of another task is all that's needed. Root in a user
namespace can do that for a mount namespace it owns. The task exits and
the namespace is freed with the mark still on it.

Remove the marks in free_mnt_ns() before the namespace is handed to RCU.
That runs in process context once the last active reference is gone. A
mark is added through a file descriptor to the namespace which holds an
active reference so no mark can show up after that.

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-v1-13-be34c83956ae@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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Christian Brauner 2026-09-30 15:32:05 +02:00

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commit 1a116222ac

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fs/namespace.c

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