jbd2: don't mark block as modified if the handle is out of credits

Do not set the b_modified flag in block's journal head should not
until after we're sure that jbd2_journal_dirty_metadat() will not
abort with an error due to there not being enough space reserved in
the jbd2 handle.

Otherwise, future attempts to modify the buffer may lead a large
number of spurious errors and warnings.

This addresses CVE-2018-10883.

https://bugzilla.kernel.org/show_bug.cgi?id=200071

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/jbd2/transaction.c

@@ -1361,6 +1361,13 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
 		if (jh->b_transaction == transaction &&
 		    jh->b_jlist != BJ_Metadata) {
 			jbd_lock_bh_state(bh);
+			if (jh->b_transaction == transaction &&
+			    jh->b_jlist != BJ_Metadata)
+				pr_err("JBD2: assertion failure: h_type=%u "
+				       "h_line_no=%u block_no=%llu jlist=%u\n",
+				       handle->h_type, handle->h_line_no,
+				       (unsigned long long) bh->b_blocknr,
+				       jh->b_jlist);
 			J_ASSERT_JH(jh, jh->b_transaction != transaction ||
 					jh->b_jlist == BJ_Metadata);
 			jbd_unlock_bh_state(bh);
@@ -1380,11 +1387,11 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
 		 * of the transaction. This needs to be done
 		 * once a transaction -bzzz
 		 */
-		jh->b_modified = 1;
 		if (handle->h_buffer_credits <= 0) {
 			ret = -ENOSPC;
 			goto out_unlock_bh;
 		}
+		jh->b_modified = 1;
 		handle->h_buffer_credits--;
 	}