chanfunding: extend PSBT witness input check

Fixes #5287.

The PSBT spec is a bit vague when it comes to the WitnessUtxo field of
an input as it's not strictly required for witness inputs. Therefore
Electrum for example does not include the field.
We need to make the SegWit input check a bit more elaborate by looking
at the output script of the UTXO and also the redeem script in case it's
a nested SegWit spend.

lnwallet/chanfunding/psbt_assembler_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -340,8 +341,11 @@ func TestPsbtVerify(t *testing.T) {
 			},
 		},
 		{
-			name:        "missing witness-utxo field",
-			expectedErr: "not all inputs are segwit spends",
+			name: "missing witness-utxo field",
+			expectedErr: "cannot use TX for channel funding, not " +
+				"all inputs are SegWit spends, risk of " +
+				"malleability: input 1 is non-SegWit spend " +
+				"or missing redeem script",
 			doVerify: func(amt int64, p *psbt.Packet,
 				i *PsbtIntent) error {
 
@@ -629,6 +633,98 @@ func TestPsbtFinalize(t *testing.T) {
 	}
 }
 
+func TestVerifyAllInputsSegWit(t *testing.T) {
+	testCases := []struct {
+		name       string
+		packet     string
+		shouldFail bool
+	}{{
+		// Electrum takes the spec literally and does not include the
+		// WitnessUtxo field in the PSBT.
+		name: "Electrum np2wkh spend",
+		packet: "cHNidP8BAFMCAAAAAcptbRo6Ez2R8WclfL4IAW+XEWIl8sNOHnw" +
+			"eXfeU3dTwAQAAAAD9////Ab26GQAAAAAAF6kUf+wXsa7Z4ejieW" +
+			"CSdKaGFwJmfbyHQyMeAAABAOACAAAAAAEBiS1A2NE4KW00wLQ1B" +
+			"9R05jFMINs6egYFNL/5vLseeXMBAAAAAP7///8CNl1m1AEAAAAX" +
+			"qRR/7Bexrtnh6OJ5YJJ0poYXAmZ9vIdDuxkAAAAAABepFMD8Ngx" +
+			"k/Z7M5nJM2ltpszQZ4KGahwJHMEQCIAD4EIg16hjWEnEf2pilRF" +
+			"UmWSk5UsMD7tTrR2FbFCurAiAiNvqpFQSr/FCCXknOV1HzeDVxe" +
+			"B9gXeDZ62tmZv04/AEhAxGTJ9E/ImiOp2Ti3w/l/JFHa95RvXCB" +
+			"UXFKifC/7f9dQyMeAAEEFgAU9pqV6WX0D/HEMfdw9yzU30MmkV4" +
+			"iBgP4kZAAjhAKuixOLJczugvMmlS0xxPc2eD+4IJPHVOeRgxGbt" +
+			"z0AAAAAAAAAAAAAA==",
+	}, {
+		name: "Electrum p2wkh spend",
+		packet: "cHNidP8BAFMCAAAAASaMwGRzF7QWjHMX9WaGyUp7f3jznuNigHF" +
+			"4l57hW0CgAAAAAAD9////AU+6GQAAAAAAF6kUf+wXsa7Z4ejieW" +
+			"CSdKaGFwJmfbyHRSMeAAABANYCAAAAAAEBym1tGjoTPZHxZyV8v" +
+			"ggBb5cRYiXyw04efB5d95Td1PABAAAAFxYAFPaalell9A/xxDH3" +
+			"cPcs1N9DJpFe/f///wG+uhkAAAAAABYAFPaalell9A/xxDH3cPc" +
+			"s1N9DJpFeAkcwRAIgRwfN0qpcM5DHR+YrgXFxhEi6F0qTf4dmQc" +
+			"c1cu3TnnICIH5wc0kkMbHZ2mqZUFEITRUXExR7US3wohctQK1lf" +
+			"q/oASED+JGQAI4QCrosTiyXM7oLzJpUtMcT3Nng/uCCTx1TnkZF" +
+			"Ix4AIgYD+JGQAI4QCrosTiyXM7oLzJpUtMcT3Nng/uCCTx1TnkY" +
+			"MRm7c9AAAAAAAAAAAAAA=",
+	}, {
+		name: "Bitcoind p2wkh spend",
+		packet: "cHNidP8BAHICAAAAAeEw64k3k+YEk5EgNTytvkGlpiF6iMSsm0o" +
+			"dCSm03dt7AAAAAAD+////AsSvp8oAAAAAFgAUzWRW/Ccwf/Cosy" +
+			"dy0uYe5fa/Gb0A4fUFAAAAABepFMD8Ngxk/Z7M5nJM2ltpszQZ4" +
+			"KGahwAAAAAAAQBxAgAAAAEtMrUZuqjRoaSEMU9sc7pS2zKlS8X3" +
+			"/CUaENX1IHOICgEAAAAA/v///wLcm53QAAAAABYAFB1zJOaGOpk" +
+			"NHVkIguiZ2Zh46dlmAGXNHQAAAAAWABTbjTIbrBMD+sWxE6mahK" +
+			"tKGa//O1wAAAABAR/cm53QAAAAABYAFB1zJOaGOpkNHVkIguiZ2" +
+			"Zh46dlmIgYDjVRSrQ7d5Rld1nDBTY/uA7Xp+SKUfaFpB0SQiM8O" +
+			"3usQBp+HNQAAAIABAACADwAAgAAiAgJ9LOXb4MwHRoq31B9fQ2U" +
+			"0K+Uj8BBJzy+MWc0OOOVuQxAGn4c1AAAAgAEAAIARAACAAAA=",
+	}, {
+		name: "Bitcoind p2pkh spend",
+		packet: "cHNidP8BAHICAAAAAdp3QE/zv3Q7RhkAR0JyzBWLttUqRHJ" +
+			"pAYmHLIqJzGXOAQAAAAD/////AoDw+gIAAAAAF6kUwPw2DGT9ns" +
+			"zmckzaW2mzNBngoZqHUN/6AgAAAAAWABRLzlCRi8BozkTClLcIN" +
+			"tnz8zYTtAAAAAAAAQB0AgAAAAHhMOuJN5PmBJORIDU8rb5BpaYh" +
+			"eojErJtKHQkptN3bewAAAAAA/v///wKcr6fKAAAAABYAFOvePfh" +
+			"lztKeDuQRgcGsHLS5BPhTAOH1BQAAAAAZdqkUt9QziR33rxhwcg" +
+			"ipzwrDti93vTWIrAAAAAAiBgNKzHC4j7KaSKeH9RYx3Ur3dHL5w" +
+			"YRjXpOAi3SI12WNFxAGn4c1AAAAgAAAAIAGAACAAAAiAgK8Fq3O" +
+			"5nnASvhn9LJMIJOkGBMYQFd5DcbvOCbBCX/VBRAGn4c1AAAAgAE" +
+			"AAIAVAACAAA==",
+		shouldFail: true,
+	}, {
+		name: "Bitcoind p2sh multisig spend",
+		packet: "cHNidP8BAHICAAAAAQvO6z5f4wghsQ2c5+Zcw2qdZ4FOYkyWBFe" +
+			"U/jiIKcwdAAAAAAD/////AkTc+gIAAAAAFgAU2kWEYfMLfgwVQ0" +
+			"2wJwFNsOmorBWA8PoCAAAAABepFMD8Ngxk/Z7M5nJM2ltpszQZ4" +
+			"KGahwAAAAAAAQByAgAAAAHad0BP8790O0YZAEdCcswVi7bVKkRy" +
+			"aQGJhyyKicxlzgAAAAAA/v///wIA4fUFAAAAABepFJdSg2Xdeo3" +
+			"mYbTqbcZnZIH8oWbPh4TDscQAAAAAFgAUs5d3GhxrF5Zdi8fQHy" +
+			"A05BSZr4t3AAAAAQRHUSEDdy41G190ATg/VnhXHE4dufESLLl53" +
+			"RewoYB2ZYRJ/4AhA6MR4qMgHUkIyqhXW0jEECV8cHg/DCiuLgUk" +
+			"YvQeub1zUq4AAAA=",
+		shouldFail: true,
+	}}
+
+	for _, tc := range testCases {
+		tc := tc
+
+		t.Run(tc.name, func(t *testing.T) {
+			r := strings.NewReader(tc.packet)
+			packet, err := psbt.NewFromRawBytes(r, true)
+			require.NoError(t, err)
+
+			txIns := packet.UnsignedTx.TxIn
+			ins := packet.Inputs
+
+			err = verifyAllInputsSegWit(txIns, ins)
+			if tc.shouldFail {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}
+
 // clonePsbt creates a clone of a PSBT packet by serializing then de-serializing
 // it.
 func clonePsbt(t *testing.T, p *psbt.Packet) *psbt.Packet {