gfs2: add validation checks for size of superblock

In gfs2_check_sb(), no validation checks are performed with regards to
the size of the superblock.
syzkaller detected a slab-out-of-bounds bug that was primarily caused
because the block size for a superblock was set to zero.
A valid size for a superblock is a power of 2 between 512 and PAGE_SIZE.
Performing validation checks and ensuring that the size of the superblock
is valid fixes this bug.

Reported-by: syzbot+af90d47a37376844e731@syzkaller.appspotmail.com
Tested-by: syzbot+af90d47a37376844e731@syzkaller.appspotmail.com
Suggested-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
[Minor code reordering.]
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/ops_fstype.c

@@ -169,15 +169,19 @@ static int gfs2_check_sb(struct gfs2_sbd *sdp, int silent)
 		return -EINVAL;
 	}
 
-	/*  If format numbers match exactly, we're done.  */
-
-	if (sb->sb_fs_format == GFS2_FORMAT_FS &&
-	    sb->sb_multihost_format == GFS2_FORMAT_MULTI)
-		return 0;
+	if (sb->sb_fs_format != GFS2_FORMAT_FS ||
+	    sb->sb_multihost_format != GFS2_FORMAT_MULTI) {
+		fs_warn(sdp, "Unknown on-disk format, unable to mount\n");
+		return -EINVAL;
+	}
 
-	fs_warn(sdp, "Unknown on-disk format, unable to mount\n");
+	if (sb->sb_bsize < 512 || sb->sb_bsize > PAGE_SIZE ||
+	    (sb->sb_bsize & (sb->sb_bsize - 1))) {
+		pr_warn("Invalid superblock size\n");
+		return -EINVAL;
+	}
 
-	return -EINVAL;
+	return 0;
 }
 
 static void end_bio_io_page(struct bio *bio)