jbd2: fix r_count overflows leading to buffer overflow in journal recovery

commit e531d0bc upstream.

The journal revoke block recovery code does not check r_count for
sanity, which means that an evil value of r_count could result in
the kernel reading off the end of the revoke table and into whatever
garbage lies beyond.  This could crash the kernel, so fix that.

However, in testing this fix, I discovered that the code to write
out the revoke tables also was not correctly checking to see if the
block was full -- the current offset check is fine so long as the
revoke table space size is a multiple of the record size, but this
is not true when either journal_csum_v[23] are set.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
[bwh: Backported to 3.2: journal checksumming is not supported, so only
 the first fix is needed]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/jbd2/recovery.c

@@ -711,11 +711,16 @@ static int scan_revoke_records(journal_t *journal, struct buffer_head *bh,
 {
 	jbd2_journal_revoke_header_t *header;
 	int offset, max;
+	__u32 rcount;
 	int record_len = 4;
 
 	header = (jbd2_journal_revoke_header_t *) bh->b_data;
 	offset = sizeof(jbd2_journal_revoke_header_t);
-	max = be32_to_cpu(header->r_count);
+	rcount = be32_to_cpu(header->r_count);
+
+	if (rcount > journal->j_blocksize)
+		return -EINVAL;
+	max = rcount;
 
 	if (JBD2_HAS_INCOMPAT_FEATURE(journal, JBD2_FEATURE_INCOMPAT_64BIT))
 		record_len = 8;