xfs: forbid AG btrees with level == 0

There is no such thing as a zero-level AG btree since even a single-node
zero-records btree has one level.  Btree cursor constructors read
cur_nlevels straight from disk and then access things like
cur_bufs[cur_nlevels - 1] which is /really/ bad if cur_nlevels is zero!
Therefore, strengthen the verifiers to prevent this possibility.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_alloc.c

@@ -2455,12 +2455,15 @@ xfs_agf_verify(
 	      be32_to_cpu(agf->agf_flcount) <= XFS_AGFL_SIZE(mp)))
 		return false;
 
-	if (be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]) > XFS_BTREE_MAXLEVELS ||
+	if (be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]) < 1 ||
+	    be32_to_cpu(agf->agf_levels[XFS_BTNUM_CNT]) < 1 ||
+	    be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]) > XFS_BTREE_MAXLEVELS ||
 	    be32_to_cpu(agf->agf_levels[XFS_BTNUM_CNT]) > XFS_BTREE_MAXLEVELS)
 		return false;
 
 	if (xfs_sb_version_hasrmapbt(&mp->m_sb) &&
-	    be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) > XFS_BTREE_MAXLEVELS)
+	    (be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) < 1 ||
+	     be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) > XFS_BTREE_MAXLEVELS))
 		return false;
 
 	/*
@@ -2477,7 +2480,8 @@ xfs_agf_verify(
 		return false;
 
 	if (xfs_sb_version_hasreflink(&mp->m_sb) &&
-	    be32_to_cpu(agf->agf_refcount_level) > XFS_BTREE_MAXLEVELS)
+	    (be32_to_cpu(agf->agf_refcount_level) < 1 ||
+	     be32_to_cpu(agf->agf_refcount_level) > XFS_BTREE_MAXLEVELS))
 		return false;
 
 	return true;;

fs/xfs/libxfs/xfs_ialloc.c

@@ -2510,8 +2510,15 @@ xfs_agi_verify(
 	if (!XFS_AGI_GOOD_VERSION(be32_to_cpu(agi->agi_versionnum)))
 		return false;
 
-	if (be32_to_cpu(agi->agi_level) > XFS_BTREE_MAXLEVELS)
+	if (be32_to_cpu(agi->agi_level) < 1 ||
+	    be32_to_cpu(agi->agi_level) > XFS_BTREE_MAXLEVELS)
 		return false;
+
+	if (xfs_sb_version_hasfinobt(&mp->m_sb) &&
+	    (be32_to_cpu(agi->agi_free_level) < 1 ||
+	     be32_to_cpu(agi->agi_free_level) > XFS_BTREE_MAXLEVELS))
+		return false;
+
 	/*
 	 * during growfs operations, the perag is not fully initialised,
 	 * so we can't use it for any useful checking. growfs ensures we can't