ext4: validate external journal superblock checksum

If the external journal device has metadata_csum enabled, verify
that the superblock checksum matches the block before we try to
mount.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -4414,6 +4414,15 @@ static journal_t *ext4_get_dev_journal(struct super_block *sb,
 		goto out_bdev;
 	}
 
+	if ((le32_to_cpu(es->s_feature_ro_compat) &
+	     EXT4_FEATURE_RO_COMPAT_METADATA_CSUM) &&
+	    es->s_checksum != ext4_superblock_csum(sb, es)) {
+		ext4_msg(sb, KERN_ERR, "external journal has "
+				       "corrupt superblock");
+		brelse(bh);
+		goto out_bdev;
+	}
+
 	if (memcmp(EXT4_SB(sb)->s_es->s_journal_uuid, es->s_uuid, 16)) {
 		ext4_msg(sb, KERN_ERR, "journal UUID does not match");
 		brelse(bh);