Fix spurious panic on bogus funding txn that confirm and are spent

lightning/src/chain/channelmonitor.rs

@@ -1729,6 +1729,26 @@ macro_rules! fail_unbroadcast_htlcs {
 	} }
 }
 
+// In the `test_invalid_funding_tx` test, we need a bogus script which matches the HTLC-Accepted
+// witness length match (ie is 136 bytes long). We generate one here which we also use in some
+// in-line tests later.
+
+#[cfg(test)]
+pub fn deliberately_bogus_accepted_htlc_witness_program() -> Vec<u8> {
+	let mut ret = [opcodes::all::OP_NOP.into_u8(); 136];
+	ret[131] = opcodes::all::OP_DROP.into_u8();
+	ret[132] = opcodes::all::OP_DROP.into_u8();
+	ret[133] = opcodes::all::OP_DROP.into_u8();
+	ret[134] = opcodes::all::OP_DROP.into_u8();
+	ret[135] = opcodes::OP_TRUE.into_u8();
+	Vec::from(&ret[..])
+}
+
+#[cfg(test)]
+pub fn deliberately_bogus_accepted_htlc_witness() -> Vec<Vec<u8>> {
+	vec![Vec::new(), Vec::new(), Vec::new(), Vec::new(), deliberately_bogus_accepted_htlc_witness_program().into()].into()
+}
+
 impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 	/// Inserts a revocation secret into this channel monitor. Prunes old preimages if neither
 	/// needed by holder commitment transactions HTCLs nor by counterparty ones. Unless we haven't already seen
@@ -2701,14 +2721,21 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 					if *idx == input.previous_output.vout {
 						#[cfg(test)]
 						{
-						        // If the expected script is a known type, check that the witness
-						        // appears to be spending the correct type (ie that the match would
-						        // actually succeed in BIP 158/159-style filters).
-						        if _script_pubkey.is_v0_p2wsh() {
-						                assert_eq!(&bitcoin::Address::p2wsh(&Script::from(input.witness.last().unwrap().to_vec()), bitcoin::Network::Bitcoin).script_pubkey(), _script_pubkey);
-						        } else if _script_pubkey.is_v0_p2wpkh() {
-						                assert_eq!(&bitcoin::Address::p2wpkh(&bitcoin::PublicKey::from_slice(&input.witness.last().unwrap()).unwrap(), bitcoin::Network::Bitcoin).unwrap().script_pubkey(), _script_pubkey);
-						        } else { panic!(); }
+							// If the expected script is a known type, check that the witness
+							// appears to be spending the correct type (ie that the match would
+							// actually succeed in BIP 158/159-style filters).
+							if _script_pubkey.is_v0_p2wsh() {
+								if input.witness.last().unwrap().to_vec() == deliberately_bogus_accepted_htlc_witness_program() {
+									// In at least one test we use a deliberately bogus witness
+									// script which hit an old panic. Thus, we check for that here
+									// and avoid the assert if its the expected bogus script.
+									return true;
+								}
+
+								assert_eq!(&bitcoin::Address::p2wsh(&Script::from(input.witness.last().unwrap().to_vec()), bitcoin::Network::Bitcoin).script_pubkey(), _script_pubkey);
+							} else if _script_pubkey.is_v0_p2wpkh() {
+								assert_eq!(&bitcoin::Address::p2wpkh(&bitcoin::PublicKey::from_slice(&input.witness.last().unwrap()).unwrap(), bitcoin::Network::Bitcoin).unwrap().script_pubkey(), _script_pubkey);
+							} else { panic!(); }
 						}
 						return true;
 					}
@@ -2793,10 +2820,13 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 			let prev_last_witness_len = input.witness.second_to_last().map(|w| w.len()).unwrap_or(0);
 			let revocation_sig_claim = (witness_items == 3 && htlctype == Some(HTLCType::OfferedHTLC) && prev_last_witness_len == 33)
 				|| (witness_items == 3 && htlctype == Some(HTLCType::AcceptedHTLC) && prev_last_witness_len == 33);
-			let accepted_preimage_claim = witness_items == 5 && htlctype == Some(HTLCType::AcceptedHTLC);
+			let accepted_preimage_claim = witness_items == 5 && htlctype == Some(HTLCType::AcceptedHTLC)
+				&& input.witness.second_to_last().unwrap().len() == 32;
 			#[cfg(not(fuzzing))]
 			let accepted_timeout_claim = witness_items == 3 && htlctype == Some(HTLCType::AcceptedHTLC) && !revocation_sig_claim;
-			let offered_preimage_claim = witness_items == 3 && htlctype == Some(HTLCType::OfferedHTLC) && !revocation_sig_claim;
+			let offered_preimage_claim = witness_items == 3 && htlctype == Some(HTLCType::OfferedHTLC) &&
+				!revocation_sig_claim && input.witness.second_to_last().unwrap().len() == 32;
+
 			#[cfg(not(fuzzing))]
 			let offered_timeout_claim = witness_items == 5 && htlctype == Some(HTLCType::OfferedHTLC);
 

lightning/src/ln/functional_tests.rs

@@ -37,7 +37,7 @@ use util::config::UserConfig;
 
 use bitcoin::hash_types::BlockHash;
 use bitcoin::blockdata::block::{Block, BlockHeader};
-use bitcoin::blockdata::script::Builder;
+use bitcoin::blockdata::script::{Builder, Script};
 use bitcoin::blockdata::opcodes;
 use bitcoin::blockdata::constants::genesis_block;
 use bitcoin::network::constants::Network;
@@ -9424,6 +9424,10 @@ fn test_invalid_funding_tx() {
 	// funding transactions from their counterparties, leading to a multi-implementation critical
 	// security vulnerability (though we always sanitized properly, we've previously had
 	// un-released crashes in the sanitization process).
+	//
+	// Further, if the funding transaction is consensus-valid, confirms, and is later spent, we'd
+	// previously have crashed in `ChannelMonitor` even though we closed the channel as bogus and
+	// gave up on it. We test this here by generating such a transaction.
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
@@ -9434,9 +9438,19 @@ fn test_invalid_funding_tx() {
 	nodes[0].node.handle_accept_channel(&nodes[1].node.get_our_node_id(), InitFeatures::known(), &get_event_msg!(nodes[1], MessageSendEvent::SendAcceptChannel, nodes[0].node.get_our_node_id()));
 
 	let (temporary_channel_id, mut tx, _) = create_funding_transaction(&nodes[0], &nodes[1].node.get_our_node_id(), 100_000, 42);
+
+	// Create a witness program which can be spent by a 4-empty-stack-elements witness and which is
+	// 136 bytes long. This matches our "accepted HTLC preimage spend" matching, previously causing
+	// a panic as we'd try to extract a 32 byte preimage from a witness element without checking
+	// its length.
+	let mut wit_program: Vec<u8> = channelmonitor::deliberately_bogus_accepted_htlc_witness_program();
+	assert!(chan_utils::HTLCType::scriptlen_to_htlctype(wit_program.len()).unwrap() ==
+		chan_utils::HTLCType::AcceptedHTLC);
+
+	let wit_program_script: Script = wit_program.clone().into();
 	for output in tx.output.iter_mut() {
 		// Make the confirmed funding transaction have a bogus script_pubkey
-		output.script_pubkey = bitcoin::Script::new();
+		output.script_pubkey = Script::new_v0_p2wsh(&wit_program_script.wscript_hash());
 	}
 
 	nodes[0].node.funding_transaction_generated_unchecked(&temporary_channel_id, &nodes[1].node.get_our_node_id(), tx.clone(), 0).unwrap();
@@ -9466,6 +9480,28 @@ fn test_invalid_funding_tx() {
 		} else { panic!(); }
 	} else { panic!(); }
 	assert_eq!(nodes[1].node.list_channels().len(), 0);
+
+	// Now confirm a spend of the (bogus) funding transaction. As long as the witness is 5 elements
+	// long the ChannelMonitor will try to read 32 bytes from the second-to-last element, panicing
+	// as its not 32 bytes long.
+	let mut spend_tx = Transaction {
+		version: 2i32, lock_time: 0,
+		input: tx.output.iter().enumerate().map(|(idx, _)| TxIn {
+			previous_output: BitcoinOutPoint {
+				txid: tx.txid(),
+				vout: idx as u32,
+			},
+			script_sig: Script::new(),
+			sequence: 0xfffffffd,
+			witness: Witness::from_vec(channelmonitor::deliberately_bogus_accepted_htlc_witness())
+		}).collect(),
+		output: vec![TxOut {
+			value: 1000,
+			script_pubkey: Script::new(),
+		}]
+	};
+	check_spends!(spend_tx, tx);
+	mine_transaction(&nodes[1], &spend_tx);
 }
 
 fn do_test_tx_confirmed_skipping_blocks_immediate_broadcast(test_height_before_timelock: bool) {