xfs: AGI length should be bounds checked

Similar to the recent patch strengthening the AGF agf_length
verification, the AGI verifier does not check that the AGI length field
is within known good bounds. This isn't currently checked by runtime
kernel code, yet we assume in many places that it is correct and verify
other metadata against it.

Add length verification to the AGI verifier. Just like the AGF length
checking, the length of the AGI must be equal to the size of the AG
specified in the superblock, unless it is the last AG in the filesystem.
In that case, it must be less than or equal to sb->sb_agblocks and
greater than XFS_MIN_AG_BLOCKS, which is the smallest AG a growfs
operation will allow to exist.

There's only one place in the filesystem that actually uses agi_length,
but let's not leave it vulnerable to the same weird nonsense that
generates syzbot bugs, eh?

Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Dave Chinner <[e-mail]>

fs/xfs/libxfs/xfs_alloc.c

···
 }
 
 /*
+ * Check that this AGF/AGI header's sequence number and length matches the AG
+ * number and size in fsblocks.
+ */
+xfs_failaddr_t
+xfs_validate_ag_length(
+	struct xfs_buf		*bp,
+	uint32_t		seqno,
+	uint32_t		length)
+{
+	struct xfs_mount	*mp = bp->b_mount;
+	/*
+	 * During growfs operations, the perag is not fully initialised,
+	 * so we can't use it for any useful checking. growfs ensures we can't
+	 * use it by using uncached buffers that don't have the perag attached
+	 * so we can detect and avoid this problem.
+	 */
+	if (bp->b_pag && seqno != bp->b_pag->pag_agno)
+		return __this_address;
+
+	/*
+	 * Only the last AG in the filesystem is allowed to be shorter
+	 * than the AG size recorded in the superblock.
+	 */
+	if (length != mp->m_sb.sb_agblocks) {
+		/*
+		 * During growfs, the new last AG can get here before we
+		 * have updated the superblock. Give it a pass on the seqno
+		 * check.
+		 */
+		if (bp->b_pag && seqno != mp->m_sb.sb_agcount - 1)
+			return __this_address;
+		if (length < XFS_MIN_AG_BLOCKS)
+			return __this_address;
+		if (length > mp->m_sb.sb_agblocks)
+			return __this_address;
+	}
+
+	return NULL;
+}
+
+/*
  * Verify the AGF is consistent.
  *
  * We do not verify the AGFL indexes in the AGF are fully consistent here
···
 {
 	struct xfs_mount	*mp = bp->b_mount;
 	struct xfs_agf		*agf = bp->b_addr;
+	xfs_failaddr_t		fa;
+	uint32_t		agf_seqno = be32_to_cpu(agf->agf_seqno);
 	uint32_t		agf_length = be32_to_cpu(agf->agf_length);
 
 	if (xfs_has_crc(mp)) {
···
 	/*
 	 * Both agf_seqno and agf_length need to validated before anything else
 	 * block number related in the AGF or AGFL can be checked.
-	 *
-	 * During growfs operations, the perag is not fully initialised,
-	 * so we can't use it for any useful checking. growfs ensures we can't
-	 * use it by using uncached buffers that don't have the perag attached
-	 * so we can detect and avoid this problem.
 	 */
-	if (bp->b_pag && be32_to_cpu(agf->agf_seqno) != bp->b_pag->pag_agno)
-		return __this_address;
-
-	/*
-	 * Only the last AGF in the filesytsem is allowed to be shorter
-	 * than the AG size recorded in the superblock.
-	 */
-	if (agf_length != mp->m_sb.sb_agblocks) {
-		/*
-		 * During growfs, the new last AGF can get here before we
-		 * have updated the superblock. Give it a pass on the seqno
-		 * check.
-		 */
-		if (bp->b_pag &&
-		    be32_to_cpu(agf->agf_seqno) != mp->m_sb.sb_agcount - 1)
-			return __this_address;
-		if (agf_length < XFS_MIN_AG_BLOCKS)
-			return __this_address;
-		if (agf_length > mp->m_sb.sb_agblocks)
-			return __this_address;
-	}
+	fa = xfs_validate_ag_length(bp, agf_seqno, agf_length);
+	if (fa)
+		return fa;
 
 	if (be32_to_cpu(agf->agf_flfirst) >= xfs_agfl_size(mp))
 		return __this_address;

fs/xfs/libxfs/xfs_alloc.h

···
 int __init xfs_extfree_intent_init_cache(void);
 void xfs_extfree_intent_destroy_cache(void);
 
+xfs_failaddr_t xfs_validate_ag_length(struct xfs_buf *bp, uint32_t seqno,
+		uint32_t length);
+
 #endif	/* __XFS_ALLOC_H__ */

fs/xfs/libxfs/xfs_ialloc.c

···
 
 static xfs_failaddr_t
 xfs_agi_verify(
-	struct xfs_buf	*bp)
+	struct xfs_buf		*bp)
 {
-	struct xfs_mount *mp = bp->b_mount;
-	struct xfs_agi	*agi = bp->b_addr;
-	int		i;
+	struct xfs_mount	*mp = bp->b_mount;
+	struct xfs_agi		*agi = bp->b_addr;
+	xfs_failaddr_t		fa;
+	uint32_t		agi_seqno = be32_to_cpu(agi->agi_seqno);
+	uint32_t		agi_length = be32_to_cpu(agi->agi_length);
+	int			i;
 
 	if (xfs_has_crc(mp)) {
 		if (!uuid_equal(&agi->agi_uuid, &mp->m_sb.sb_meta_uuid))
···
 	if (!XFS_AGI_GOOD_VERSION(be32_to_cpu(agi->agi_versionnum)))
 		return __this_address;
 
+	fa = xfs_validate_ag_length(bp, agi_seqno, agi_length);
+	if (fa)
+		return fa;
+
 	if (be32_to_cpu(agi->agi_level) < 1 ||
 	    be32_to_cpu(agi->agi_level) > M_IGEO(mp)->inobt_maxlevels)
 		return __this_address;
···
 	if (xfs_has_finobt(mp) &&
 	    (be32_to_cpu(agi->agi_free_level) < 1 ||
 	     be32_to_cpu(agi->agi_free_level) > M_IGEO(mp)->inobt_maxlevels))
-		return __this_address;
-
-	/*
-	 * during growfs operations, the perag is not fully initialised,
-	 * so we can't use it for any useful checking. growfs ensures we can't
-	 * use it by using uncached buffers that don't have the perag attached
-	 * so we can detect and avoid this problem.
-	 */
-	if (bp->b_pag && be32_to_cpu(agi->agi_seqno) != bp->b_pag->pag_agno)
 		return __this_address;
 
 	for (i = 0; i < XFS_AGI_UNLINKED_BUCKETS; i++) {