fuse: Fix crash in fuse_dentry_automount() error path

If fuse_fill_super_submount() returns an error, the error path
triggers a crash:

[   26.206673] BUG: kernel NULL pointer dereference, address: 0000000000000000
[...]
[   26.226362] RIP: 0010:__list_del_entry_valid+0x25/0x90
[...]
[   26.247938] Call Trace:
[   26.248300]  fuse_mount_remove+0x2c/0x70 [fuse]
[   26.248892]  virtio_kill_sb+0x22/0x160 [virtiofs]
[   26.249487]  deactivate_locked_super+0x36/0xa0
[   26.250077]  fuse_dentry_automount+0x178/0x1a0 [fuse]

The crash happens because fuse_mount_remove() assumes that the FUSE
mount was already added to list under the FUSE connection, but this
only done after fuse_fill_super_submount() has returned success.

This means that until fuse_fill_super_submount() has returned success,
the FUSE mount isn't actually owned by the superblock. We should thus
reclaim ownership by clearing sb->s_fs_info, which will skip the call
to fuse_mount_remove(), and perform rollback, like virtio_fs_get_tree()
already does for the root sb.

Fixes: bf109c64 ("fuse: implement crossmounts")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -339,8 +339,12 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 
 	/* Initialize superblock, making @mp_fi its root */
 	err = fuse_fill_super_submount(sb, mp_fi);
-	if (err)
+	if (err) {
+		fuse_conn_put(fc);
+		kfree(fm);
+		sb->s_fs_info = NULL;
 		goto out_put_sb;
+	}
 
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);