fuse: Fix crash if superblock of submount gets killed early

As soon as fuse_dentry_automount() does up_write(&sb->s_umount), the
superblock can theoretically be killed. If this happens before the
submount was added to the &fc->mounts list, fuse_mount_remove() later
crashes in list_del_init() because it assumes the submount to be
already there.

Add the submount before dropping sb->s_umount to fix the inconsistency.
It is okay to nest fc->killsb under sb->s_umount, we already do this
on the ->kill_sb() path.
Signed-off-by: Greg Kurz <groug@kaod.org>
Fixes: bf109c64 ("fuse: implement crossmounts")
Cc: stable@vger.kernel.org # v5.10+
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -346,15 +346,15 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 		goto out_put_sb;
 	}
 
+	down_write(&fc->killsb);
+	list_add_tail(&fm->fc_entry, &fc->mounts);
+	up_write(&fc->killsb);
+
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);
 	/* We are done configuring the superblock, so unlock it */
 	up_write(&sb->s_umount);
 
-	down_write(&fc->killsb);
-	list_add_tail(&fm->fc_entry, &fc->mounts);
-	up_write(&fc->killsb);
-
 	/* Create the submount */
 	mnt = vfs_create_mount(fsc);
 	if (IS_ERR(mnt)) {