[PATCH v2] reiserfs: add check for invalid 1st journal block

[Pavel Skripkin <paskripkin@gmail.com>]

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().

Reported-by: syzbot+0ba9909df31c6a36974d@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---

Changes in v2:
  Added necessary !SB_ONDISK_JOURNAL_DEVICE(sb) in "if"

---
 fs/reiserfs/journal.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/fs/reiserfs/journal.c b/fs/reiserfs/journal.c
index 9edc8e2b154e..0834b101c316 100644
--- a/fs/reiserfs/journal.c
+++ b/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");
-- 
2.31.1