Allow overshooting final cltv_expiry

Final nodes previously had stricter requirements on HTLC contents
matching onion value compared to intermediate nodes. This allowed
for probing, i.e. the last intermediate node could overshoot the
value by a small amount and conclude from the acceptance or rejection
of the HTLC whether the next node was the destination. This also
applies to the msat amount, however this change was already present.

Author: alecchendev

lightning/src/ln/channelmanager.rs

@@ -2095,9 +2095,9 @@ where
 		payment_hash: PaymentHash, amt_msat: u64, cltv_expiry: u32, phantom_shared_secret: Option<[u8; 32]>) -> Result<PendingHTLCInfo, ReceiveError>
 	{
 		// final_incorrect_cltv_expiry
-		if hop_data.outgoing_cltv_value != cltv_expiry {
+		if hop_data.outgoing_cltv_value > cltv_expiry {
 			return Err(ReceiveError {
-				msg: "Upstream node set CLTV to the wrong value",
+				msg: "Upstream node set CLTV to less than the CLTV set by the sender",
 				err_code: 18,
 				err_data: cltv_expiry.to_be_bytes().to_vec()
 			})

lightning/src/ln/onion_route_tests.rs

@@ -552,7 +552,7 @@ fn test_onion_failure() {
 			for f in pending_forwards.iter_mut() {
 				match f {
 					&mut HTLCForwardInfo::AddHTLC(PendingAddHTLCInfo { ref mut forward_info, .. }) =>
-						forward_info.outgoing_cltv_value += 1,
+						forward_info.outgoing_cltv_value -= 1,
 					_ => {},
 				}
 			}
@@ -602,6 +602,48 @@ fn test_onion_failure() {
 	}, true, Some(23), None, None);
 }
 
+#[test]
+fn test_overshoot_final_cltv() {
+	let chanmon_cfgs = create_chanmon_cfgs(3);
+	let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None; 3]);
+	let mut nodes = create_network(3, &node_cfgs, &node_chanmgrs);
+	create_announced_chan_between_nodes(&nodes, 0, 1);
+	create_announced_chan_between_nodes(&nodes, 1, 2);
+	let (route, payment_hash, payment_preimage, payment_secret) = get_route_and_payment_hash!(nodes[0], nodes[2], 40000);
+
+	let payment_id = PaymentId(nodes[0].keys_manager.backing.get_secure_random_bytes());
+	nodes[0].node.send_payment(&route, payment_hash, &Some(payment_secret), payment_id).unwrap();
+
+	check_added_monitors!(nodes[0], 1);
+	let update_0 = get_htlc_update_msgs!(nodes[0], nodes[1].node.get_our_node_id());
+	let mut update_add_0 = update_0.update_add_htlcs[0].clone();
+	nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &update_add_0);
+	commitment_signed_dance!(nodes[1], nodes[0], &update_0.commitment_signed, false, true);
+
+	assert!(nodes[1].node.get_and_clear_pending_msg_events().is_empty());
+	for (_, pending_forwards) in nodes[1].node.forward_htlcs.lock().unwrap().iter_mut() {
+		for f in pending_forwards.iter_mut() {
+			match f {
+				&mut HTLCForwardInfo::AddHTLC(PendingAddHTLCInfo { ref mut forward_info, .. }) =>
+					forward_info.outgoing_cltv_value += 1,
+				_ => {},
+			}
+		}
+	}
+	expect_pending_htlcs_forwardable!(nodes[1]);
+
+	check_added_monitors!(&nodes[1], 1);
+	let update_1 = get_htlc_update_msgs!(nodes[1], nodes[2].node.get_our_node_id());
+	let mut update_add_1 = update_1.update_add_htlcs[0].clone();
+	nodes[2].node.handle_update_add_htlc(&nodes[1].node.get_our_node_id(), &update_add_1);
+	commitment_signed_dance!(nodes[2], nodes[1], update_1.commitment_signed, false, true);
+
+	expect_pending_htlcs_forwardable!(nodes[2]);
+	expect_payment_claimable!(nodes[2], payment_hash, payment_secret, 40_000);
+	claim_payment(&nodes[0], &[&nodes[1], &nodes[2]], payment_preimage);
+}
+
 fn do_test_onion_failure_stale_channel_update(announced_channel: bool) {
 	// Create a network of three nodes and two channels connecting them. We'll be updating the
 	// HTLC relay policy of the second channel, causing forwarding failures at the first hop.
@@ -1096,7 +1138,7 @@ fn test_phantom_final_incorrect_cltv_expiry() {
 				&mut HTLCForwardInfo::AddHTLC(PendingAddHTLCInfo {
 					forward_info: PendingHTLCInfo { ref mut outgoing_cltv_value, .. }, ..
 				}) => {
-					*outgoing_cltv_value += 1;
+					*outgoing_cltv_value -= 1;
 				},
 				_ => panic!("Unexpected forward"),
 			}
@@ -1114,7 +1156,7 @@ fn test_phantom_final_incorrect_cltv_expiry() {
 	commitment_signed_dance!(nodes[0], nodes[1], update_1.commitment_signed, false);
 
 	// Ensure the payment fails with the expected error.
-	let expected_cltv: u32 = 82;
+	let expected_cltv: u32 = 80;
 	let error_data = expected_cltv.to_be_bytes().to_vec();
 	let mut fail_conditions = PaymentFailedConditions::new()
 		.blamed_scid(phantom_scid)