[PATCH] xfs: verify sb_sectorsize before using it for crc calcs

[Eric Sandeen]

xfs_sb_read_verify() uses the sector size in the superblock
as input to the CRC calculations - but if the value is corrupted,
we can have a catastrophic failure - a value of 0 for length
in:
	length - (cksum_offset + sizeof(__be32))

passes a very large (unsigned) value to crc32c, which will cause
an oops, today.  (Hardening crc32c is above my pay grade).

Doing a quick validation of sectorsize before the crc calculation
solves this problem.

Signed-off-by: Eric Sandeen <sandeen at redhat.com>
---

diff --git a/fs/xfs/xfs_sb.c b/fs/xfs/xfs_sb.c
index 6d81ece..4b62ad2 100644
--- a/fs/xfs/xfs_sb.c
+++ b/fs/xfs/xfs_sb.c
@@ -599,9 +600,19 @@ xfs_sb_read_verify(
 {
 	struct xfs_mount *mp = bp->b_target->bt_mount;
 	struct xfs_dsb	*dsb = XFS_BUF_TO_SBP(bp);
+	__uint16_t	sectsize = be16_to_cpu(dsb->sb_sectsize);
 	int		error;
 
 	/*
+	 * We use the sb sectorsize in crc calculations before general sb 
+	 * validation, so check that value first.
+	 */
+	if (sectsize < XFS_MIN_SECTORSIZE || sectsize > XFS_MAX_SECTORSIZE) {
+		error = EFSCORRUPTED;
+		goto out_error;
+	}
+
+	/*
 	 * open code the version check to avoid needing to convert the entire
 	 * superblock from disk order just to check the version number
 	 */
@@ -610,8 +621,7 @@ xfs_sb_read_verify(
 						XFS_SB_VERSION_5) ||
 	     dsb->sb_crc != 0)) {
 
-		if (!xfs_verify_cksum(bp->b_addr, be16_to_cpu(dsb->sb_sectsize),
-				      XFS_SB_CRC_OFF)) {
+		if (!xfs_verify_cksum(bp->b_addr, sectsize, XFS_SB_CRC_OFF)) {
 			/* Only fail bad secondaries on a known V5 filesystem */
 			if (bp->b_bn == XFS_SB_DADDR ||
 			    xfs_sb_version_hascrc(&mp->m_sb)) {