openingd: floor accept_channel reserve at our own dust limit too

Changelog-Fixed: 'openingd' no longer sends an 'accept_channel' whose 'channel_reserve_satoshis' is below its own 'dust_limit_satoshis'

Author: Andezion

openingd/openingd.c

@@ -145,6 +145,8 @@ static void NORETURN negotiation_failed(struct state *state,
 
 static void set_reserve_absolute(struct state * state, const struct amount_sat dust_limit, struct amount_sat reserve_sat)
 {
+	struct amount_sat floor;
+
 	status_debug("Setting their reserve to %s",
 		     fmt_amount_sat(tmpctx, reserve_sat));
 	if (state->allowdustreserve) {
@@ -156,13 +158,22 @@ static void set_reserve_absolute(struct state * state, const struct amount_sat d
 		 * ...
 		 * - MUST set `channel_reserve_satoshis` greater than or equal
 		 *   to `dust_limit_satoshis` from the `open_channel` message.
+		 *
+		 * Our own dust_limit_satoshis must also never exceed our
+		 * channel_reserve_satoshis, so floor against whichever dust
+		 * limit (ours or the caller-supplied one) is larger
 		 */
-		if (amount_sat_greater(dust_limit, reserve_sat)) {
-			status_debug("Their reserve is too small, bumping to "
+		if (amount_sat_greater(state->localconf.dust_limit, dust_limit))
+			floor = state->localconf.dust_limit;
+		else
+			floor = dust_limit;
+
+		if (amount_sat_greater(floor, reserve_sat)) {
+			status_debug("Reserve is too small, bumping to "
 				     "dust_limit: %s < %s",
 				     fmt_amount_sat(tmpctx, reserve_sat),
-				     fmt_amount_sat(tmpctx, dust_limit));
-			state->localconf.channel_reserve = dust_limit;
+				     fmt_amount_sat(tmpctx, floor));
+			state->localconf.channel_reserve = floor;
 		} else {
 			state->localconf.channel_reserve = reserve_sat;
 		}

tests/test_connection.py

@@ -5314,16 +5314,50 @@ def send_open_channel(lconn, chain_hash, temp_chan_id, funding_sat, push_msat,
     lconn.send_message(msg)
 
 
-def read_channel_reply(lconn):
-    """Read past gossip chatter to openingd's answer to our open_channel."""
+def read_channel_reply_payload(lconn):
+    """
+    Read past gossip chatter to openingd's answer to our open_channel
+    Returns (mtype, payload), so callers can inspect accept_channel's fields
+    """
     for _ in range(20):
         msg = lconn.read_message()
         mtype = int.from_bytes(msg[0:2], 'big')
         if mtype in (WIRE_ACCEPT_CHANNEL, WIRE_WARNING, WIRE_ERROR):
-            return mtype
+            return mtype, msg[2:]
     raise AssertionError("no reply to open_channel")
 
 
+def read_channel_reply(lconn):
+    mtype, _ = read_channel_reply_payload(lconn)
+    return mtype
+
+
+def send_open_channel_reserve(lconn, chain_hash, temp_chan_id, funding_sat, push_msat,
+                              dust_limit, channel_reserve, feerate_per_kw, channel_type):
+    """Like send_open_channel, but with a caller-chosen dust_limit / channel_reserve"""
+    keys = [wire.PrivateKey(bytes([i + 1] * 32)).public_key().serializeCompressed()
+            for i in range(6)]
+
+    msg = struct.pack('>H', WIRE_OPEN_CHANNEL)
+    msg += chain_hash
+    msg += temp_chan_id
+    msg += struct.pack('>Q', funding_sat) # funding_satoshis
+    msg += struct.pack('>Q', push_msat) # push_msat
+    msg += struct.pack('>Q', dust_limit) # dust_limit_satoshis
+    msg += struct.pack('>Q', 0xFFFFFFFFFFFF) # max_htlc_value_in_flight_msat
+    msg += struct.pack('>Q', channel_reserve) # channel_reserve_satoshis
+    msg += struct.pack('>Q', 0) # htlc_minimum_msat
+    msg += struct.pack('>I', feerate_per_kw) # feerate_per_kw
+    msg += struct.pack('>H', 144) # to_self_delay
+    msg += struct.pack('>H', 483) # max_accepted_htlcs
+    for k in keys:
+        msg += k
+    msg += struct.pack('>B', 0) # channel_flags
+    msg += bytes([1, len(channel_type)]) + channel_type # TLV 1: channel_type
+
+    lconn.send_message(msg)
+
+
 def send_funding_created(lconn, temp_chan_id):
     """Drive the open to the point where we build the commitment transaction.
 
@@ -5826,3 +5860,37 @@ def test_open_channel2_tx_add_output_sum_above_max_supply(node_factory, bitcoind
 
     # node is still up
     assert l1.rpc.getinfo()["id"] == l1.info["id"]
+
+
+@pytest.mark.openchannel('v1')
+def test_accept_channel_reserve_below_our_dust_limit(node_factory, bitcoind):
+    """Our accept_channel must not set channel_reserve below our dust limit.
+
+    As fundee, openingd used to floor the 1% channel reserve at the peer's
+    dust_limit_satoshis only, never at our own.  A peer opening a small
+    channel with a low dust limit could get an accept_channel whose
+    dust_limit_satoshis (546, hardcoded) exceeded its own
+    channel_reserve_satoshis, violating BOLT #2.
+    """
+    l1 = node_factory.get_node()
+
+    chain_hash = bytes.fromhex(bitcoind.rpc.getblockhash(0))[::-1]
+    feerate = l1.rpc.feerates('perkw')['perkw']['opening']
+
+    lconn, channel_type = raw_peer_connect(l1)
+    # 1% of 30000 is 300sat, floored to our peer's 354sat dust limit -- still
+    # under our own hardcoded 546sat dust limit.  channel_reserve_satoshis=546
+    # keeps us clear of the separate issue #9439, and funding_sat is well
+    # clear of lightningd's default 10,000sat minimum effective capacity.
+    send_open_channel_reserve(lconn, chain_hash, os.urandom(32),
+                              funding_sat=30000, push_msat=0,
+                              dust_limit=354, channel_reserve=546,
+                              feerate_per_kw=feerate, channel_type=channel_type)
+    mtype, payload = read_channel_reply_payload(lconn)
+    assert mtype == WIRE_ACCEPT_CHANNEL
+
+    dust_limit = struct.unpack('>Q', payload[32:40])[0]
+    reserve = struct.unpack('>Q', payload[48:56])[0]
+    assert dust_limit <= reserve, \
+        "accept_channel dust_limit_satoshis {} exceeds its channel_reserve_satoshis {}".format(
+            dust_limit, reserve)