reiserfs: add check for invalid 1st journal block

syzbot reported divide error in reiserfs.
The problem was in incorrect journal 1st block.

Syzbot's reproducer manualy generated wrong superblock
with incorrect 1st block. In journal_init() wasn't
any checks about this particular case.

For example, if 1st journal block is before superblock
1st block, it can cause zeroing important superblock members
in do_journal_end().

Link: https://lore.kernel.org/r/20210517121545.29645-1-paskripkin@gmail.com
Reported-by: syzbot+0ba9909df31c6a36974d@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/reiserfs/journal.c

@@ -2758,6 +2758,20 @@ int journal_init(struct super_block *sb, const char *j_dev_name,
 		goto free_and_return;
 	}
 
+	/*
+	 * Sanity check to see if journal first block is correct.
+	 * If journal first block is invalid it can cause
+	 * zeroing important superblock members.
+	 */
+	if (!SB_ONDISK_JOURNAL_DEVICE(sb) &&
+	    SB_ONDISK_JOURNAL_1st_BLOCK(sb) < SB_JOURNAL_1st_RESERVED_BLOCK(sb)) {
+		reiserfs_warning(sb, "journal-1393",
+				 "journal 1st super block is invalid: 1st reserved block %d, but actual 1st block is %d",
+				 SB_JOURNAL_1st_RESERVED_BLOCK(sb),
+				 SB_ONDISK_JOURNAL_1st_BLOCK(sb));
+		goto free_and_return;
+	}
+
 	if (journal_init_dev(sb, journal, j_dev_name) != 0) {
 		reiserfs_warning(sb, "sh-462",
 				 "unable to initialize journal device");