xfs: sanity check attr fork size

Recently we have quite a few kerneloops reports about dereferencing a NULL
if_data in the attribute fork.  From looking over the code this can only
happen if we pass a 0 size argument to xfs_iformat_local.  This implies some
sort of corruption and in fact the only mailinglist report about this from
earlier this year was after a powerfail presumably on a system with write
cache and without barriers.

Add a quick sanity check for the attr fork size in xfs_iformat to catch
these early and without an oops.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_inode.c

@@ -424,6 +424,19 @@ xfs_iformat(
 	case XFS_DINODE_FMT_LOCAL:
 		atp = (xfs_attr_shortform_t *)XFS_DFORK_APTR(dip);
 		size = be16_to_cpu(atp->hdr.totsize);
+
+		if (unlikely(size < sizeof(struct xfs_attr_sf_hdr))) {
+			xfs_fs_repair_cmn_err(CE_WARN, ip->i_mount,
+				"corrupt inode %Lu "
+				"(bad attr fork size %Ld).",
+				(unsigned long long) ip->i_ino,
+				(long long) size);
+			XFS_CORRUPTION_ERROR("xfs_iformat(8)",
+					     XFS_ERRLEVEL_LOW,
+					     ip->i_mount, dip);
+			return XFS_ERROR(EFSCORRUPTED);
+		}
+
 		error = xfs_iformat_local(ip, dip, XFS_ATTR_FORK, size);
 		break;
 	case XFS_DINODE_FMT_EXTENTS: