namespace: check the target before walking it in do_mount_setattr()

When mount_setattr() changes the propagation type to MS_SHARED
do_mount_setattr() takes namespace_sem and calls invent_group_ids()
which walks the whole target tree with next_mnt() and allocates a peer
group id for every mount that doesn't have one.

Whether the caller may touch the target at all is only checked
afterwards under mount_lock. If it may not, cleanup_group_ids() walks
the tree a second time on the way out to free the ids again.

A detached tree that a file descriptor keeps alive, a mount in another
mount namespace or a mount below the root of an anonymous mount
namespace does a bunch of allocation work that is completely useless
because it will fail and revert right after.

Do the right thing and check the target right after taking namespace_sem
instead. A mount leaves its namespace only under namespace_sem so the
mount stays stable. The check under mount_lock stays for the case where
no propagation change is requested and namespace_sem isn't taken.
Callers get the same EINVAL as before.

Fixes: 2a1867219c7b ("fs: add mount_setattr()")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260926-work-mount-fixes-2-v1-5-f357abf3d17b@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions