xfs: fix UAF in xchk_btree_check_block_owner

We cannot dereference bs->cur when trying to determine if bs->cur
aliases bs->sc->sa.{bno,rmap}_cur after the latter has been freed.
Fix this by sampling before type before any freeing could happen.
The correct temporal ordering was broken when we removed xfs_btnum_t.

Cc: r772577952@gmail.com
Cc: <stable@vger.kernel.org> # v6.9
Fixes: ec793e6 ("xfs: remove xfs_btnum_t")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Jiaming Zhang <r772577952@gmail.com>

Author: Darrick J. Wong

fs/xfs/scrub/btree.c

@@ -372,12 +372,15 @@ xchk_btree_check_block_owner(
 {
 	xfs_agnumber_t		agno;
 	xfs_agblock_t		agbno;
+	bool			is_bnobt, is_rmapbt;
 	bool			init_sa;
 	int			error = 0;
 
 	if (!bs->cur)
 		return 0;
 
+	is_bnobt = xfs_btree_is_bno(bs->cur->bc_ops);
+	is_rmapbt = xfs_btree_is_rmap(bs->cur->bc_ops);
 	agno = xfs_daddr_to_agno(bs->cur->bc_mp, daddr);
 	agbno = xfs_daddr_to_agbno(bs->cur->bc_mp, daddr);
 
@@ -400,11 +403,11 @@ xchk_btree_check_block_owner(
 	 * have to nullify it (to shut down further block owner checks) if
 	 * self-xref encounters problems.
 	 */
-	if (!bs->sc->sa.bno_cur && xfs_btree_is_bno(bs->cur->bc_ops))
+	if (!bs->sc->sa.bno_cur && is_bnobt)
 		bs->cur = NULL;
 
 	xchk_xref_is_only_owned_by(bs->sc, agbno, 1, bs->oinfo);
-	if (!bs->sc->sa.rmap_cur && xfs_btree_is_rmap(bs->cur->bc_ops))
+	if (!bs->sc->sa.rmap_cur && is_rmapbt)
 		bs->cur = NULL;
 
 out_free: