xfs: avoid buffer deadlocks when walking fs inodes

When we're servicing an INUMBERS or BULKSTAT request or running
quotacheck, grab an empty transaction so that we can use its inherent
recursive buffer locking abilities to detect inode btree cycles without
hitting ABBA buffer deadlocks.  This patch requires the deferred inode
inactivation patchset because xfs_irele cannot directly call
xfs_inactive when the iwalk itself has an (empty) transaction.

Found by fuzzing an inode btree pointer to introduce a cycle into the
tree (xfs/365).
Signed-off-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>

fs/xfs/xfs_itable.c

@@ -19,6 +19,7 @@
 #include "xfs_error.h"
 #include "xfs_icache.h"
 #include "xfs_health.h"
+#include "xfs_trans.h"
 
 /*
  * Bulk Stat
@@ -163,6 +164,7 @@ xfs_bulkstat_one(
 		.formatter	= formatter,
 		.breq		= breq,
 	};
+	struct xfs_trans	*tp;
 	int			error;
 
 	if (breq->mnt_userns != &init_user_ns) {
@@ -178,9 +180,18 @@ xfs_bulkstat_one(
 	if (!bc.buf)
 		return -ENOMEM;
 
-	error = xfs_bulkstat_one_int(breq->mp, breq->mnt_userns, NULL,
-				     breq->startino, &bc);
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(breq->mp, &tp);
+	if (error)
+		goto out;
 
+	error = xfs_bulkstat_one_int(breq->mp, breq->mnt_userns, tp,
+			breq->startino, &bc);
+	xfs_trans_cancel(tp);
+out:
 	kmem_free(bc.buf);
 
 	/*
@@ -244,6 +255,7 @@ xfs_bulkstat(
 		.formatter	= formatter,
 		.breq		= breq,
 	};
+	struct xfs_trans	*tp;
 	int			error;
 
 	if (breq->mnt_userns != &init_user_ns) {
@@ -259,9 +271,18 @@ xfs_bulkstat(
 	if (!bc.buf)
 		return -ENOMEM;
 
-	error = xfs_iwalk(breq->mp, NULL, breq->startino, breq->flags,
-			xfs_bulkstat_iwalk, breq->icount, &bc);
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(breq->mp, &tp);
+	if (error)
+		goto out;
 
+	error = xfs_iwalk(breq->mp, tp, breq->startino, breq->flags,
+			xfs_bulkstat_iwalk, breq->icount, &bc);
+	xfs_trans_cancel(tp);
+out:
 	kmem_free(bc.buf);
 
 	/*
@@ -374,13 +395,24 @@ xfs_inumbers(
 		.formatter	= formatter,
 		.breq		= breq,
 	};
+	struct xfs_trans	*tp;
 	int			error = 0;
 
 	if (xfs_bulkstat_already_done(breq->mp, breq->startino))
 		return 0;
 
-	error = xfs_inobt_walk(breq->mp, NULL, breq->startino, breq->flags,
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(breq->mp, &tp);
+	if (error)
+		goto out;
+
+	error = xfs_inobt_walk(breq->mp, tp, breq->startino, breq->flags,
 			xfs_inumbers_walk, breq->icount, &ic);
+	xfs_trans_cancel(tp);
+out:
 
 	/*
 	 * We found some inode groups, so clear the error status and return

fs/xfs/xfs_iwalk.c

@@ -83,6 +83,9 @@ struct xfs_iwalk_ag {
 
 	/* Skip empty inobt records? */
 	unsigned int			skip_empty:1;
+
+	/* Drop the (hopefully empty) transaction when calling iwalk_fn. */
+	unsigned int			drop_trans:1;
 };
 
 /*
@@ -352,7 +355,6 @@ xfs_iwalk_run_callbacks(
 	int				*has_more)
 {
 	struct xfs_mount		*mp = iwag->mp;
-	struct xfs_trans		*tp = iwag->tp;
 	struct xfs_inobt_rec_incore	*irec;
 	xfs_agino_t			next_agino;
 	int				error;
@@ -362,10 +364,15 @@ xfs_iwalk_run_callbacks(
 	ASSERT(iwag->nr_recs > 0);
 
 	/* Delete cursor but remember the last record we cached... */
-	xfs_iwalk_del_inobt(tp, curpp, agi_bpp, 0);
+	xfs_iwalk_del_inobt(iwag->tp, curpp, agi_bpp, 0);
 	irec = &iwag->recs[iwag->nr_recs - 1];
 	ASSERT(next_agino >= irec->ir_startino + XFS_INODES_PER_CHUNK);
 
+	if (iwag->drop_trans) {
+		xfs_trans_cancel(iwag->tp);
+		iwag->tp = NULL;
+	}
+
 	error = xfs_iwalk_ag_recs(iwag);
 	if (error)
 		return error;
@@ -376,8 +383,15 @@ xfs_iwalk_run_callbacks(
 	if (!has_more)
 		return 0;
 
+	if (iwag->drop_trans) {
+		error = xfs_trans_alloc_empty(mp, &iwag->tp);
+		if (error)
+			return error;
+	}
+
 	/* ...and recreate the cursor just past where we left off. */
-	error = xfs_inobt_cur(mp, tp, iwag->pag, XFS_BTNUM_INO, curpp, agi_bpp);
+	error = xfs_inobt_cur(mp, iwag->tp, iwag->pag, XFS_BTNUM_INO, curpp,
+			agi_bpp);
 	if (error)
 		return error;
 
@@ -390,7 +404,6 @@ xfs_iwalk_ag(
 	struct xfs_iwalk_ag		*iwag)
 {
 	struct xfs_mount		*mp = iwag->mp;
-	struct xfs_trans		*tp = iwag->tp;
 	struct xfs_perag		*pag = iwag->pag;
 	struct xfs_buf			*agi_bp = NULL;
 	struct xfs_btree_cur		*cur = NULL;
@@ -469,7 +482,7 @@ xfs_iwalk_ag(
 	error = xfs_iwalk_run_callbacks(iwag, &cur, &agi_bp, &has_more);
 
 out:
-	xfs_iwalk_del_inobt(tp, &cur, &agi_bp, error);
+	xfs_iwalk_del_inobt(iwag->tp, &cur, &agi_bp, error);
 	return error;
 }
 
@@ -599,8 +612,18 @@ xfs_iwalk_ag_work(
 	error = xfs_iwalk_alloc(iwag);
 	if (error)
 		goto out;
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(mp, &iwag->tp);
+	if (error)
+		goto out;
+	iwag->drop_trans = 1;
 
 	error = xfs_iwalk_ag(iwag);
+	if (iwag->tp)
+		xfs_trans_cancel(iwag->tp);
 	xfs_iwalk_free(iwag);
 out:
 	xfs_perag_put(iwag->pag);