mount: keep a copied mount unbindable

It's groundhog day.

MNT_UNBINDABLE used to be set in in mnt->mnt.mnt_flags and was not part
of MNT_INTERNAL_FLAGS. clone_mnt() copied it with all the other flags to
the new mount. This guaranteed that the copy of an unbindable mount
became unbindable as well. Unbindable copies that ended up as shared
lost the unbindable property.

But then commit 406fea7999 ("mount: separate the flags accessed only
under namespace_sem") moved MNT_UNBINDABLE from mnt->mnt.mnt_flags into
mnt->mnt_t_flags as T_UNBINDABLE.

clone_mnt() doesn't copy from mnt_t_flags and specifically doesn't copy
T_UNBINDABLE. The only place a mount gets marked unbindable is in
change_mnt_propagation() for MS_UNBINDABLE.

This reintroduced an earlier bug we had already fixed. It resurfaces in
copy_mnt_ns() which clones unbindable mounts via CL_COPY_UNBINDABLE. So
since v6.17 a mount namespace created via clone(CLONE_NEWNS) or
unshare(CLONE_NEWNS) contain private and bindable copies of every
unbindable mount. The following snippet:

  mount --make-unbindable /mnt
  unshare -m --propagation unchanged
  mount --bind /mnt /tmp/x

succeeds. This used to fail with EINVAL. This also applies to
recursively binding a tree that contains an unbindable the mount. Such
subtrees used to be pruned when they were moved onto a shared mount.

And Documentation/filesystems/sharedsubtree.rst still says that the copy
of an unbindable mount is unbindable.

So once again let's fix this. Copy T_UNBINDABLE in clone_mnt(). Let
set_mnt_shared() mask it off whenever a mount is made shared.
This restores what the old MNT_INTERNAL_FLAGS mask did.

Link: https://patch.msgid.link/20260923-work-mount-fixes-v1-1-f424cf8d3242@kernel.org
Fixes: 406fea7999 ("mount: separate the flags accessed only under namespace_sem")
Cc: stable@vger.kernel.org # v6.17+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/namespace.c

@@ -1254,6 +1254,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 
 	mnt->mnt.mnt_flags = READ_ONCE(old->mnt.mnt_flags) &
 			     ~MNT_INTERNAL_FLAGS;
+	mnt->mnt_t_flags = old->mnt_t_flags & T_UNBINDABLE;
 
 	if (flag & (CL_SLAVE | CL_PRIVATE))
 		mnt->mnt_group_id = 0; /* not a peer of original */