Merge #14588: Refactor PSBT signing logic to enforce invariant and fix signing bug

e13fea975d Add regression test for PSBT signing bug #14473 (Glenn Willen)
565500508a Refactor PSBTInput signing to enforce invariant (Glenn Willen)
0f5bda2bd9 Simplify arguments to SignPSBTInput (Glenn Willen)
53e6fffb8f Add bool PSBTInputSigned (Glenn Willen)
65166d4cf8 New PartiallySignedTransaction constructor from CTransction (Glenn Willen)
4f3f5cb4b1 Remove redundant txConst parameter to FillPSBT (Glenn Willen)
fe5d22bc67 More concise conversion of CDataStream to string (Glenn Willen)

Pull request description:

  As discussed in the comments on #14473, I think that bug was caused primarily by failure to adhere to the invariant that a PSBTInput always has exactly one of the two utxo fields present -- an invariant that is already enforced by PSBTInput::IsSane, but which we were temporarily suspending during signing.

  This refactor repairs the invariant, also fixing the bug. It also simplifies some other code, and removes redundant parameters from some related functions.

  fixes #14473

Tree-SHA512: cbad3428175e30f9b7bac3f600668dd1a8f9acde16b915d27a940a2fa6d5149d4fbe236d5808fd590fb20a032274c99e8cac34bef17f79a53fdf69a5948c0fd0

7 files changed, 80 insertions, 68 deletions

diff --git a/src/rpc/rawtransaction.cpp b/src/rpc/rawtransaction.cpp
index 51b4e44aed..94a9ea20da 100644
--- a/src/rpc/rawtransaction.cpp
+++ b/src/rpc/rawtransaction.cpp
@@ -1538,12 +1538,13 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
     }
 
-    // Get all of the previous transactions
+    // Finalize input signatures -- in case we have partial signatures that add up to a complete
+    //   signature, but have not combined them yet (e.g. because the combiner that created this
+    //   PartiallySignedTransaction did not understand them), this will combine them into a final
+    //   script.
     bool complete = true;
     for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
-        PSBTInput& input = psbtx.inputs.at(i);
-
-        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, *psbtx.tx, input, i, 1);
+        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, psbtx, i, SIGHASH_ALL);
     }
 
     UniValue result(UniValue::VOBJ);
@@ -1556,10 +1557,10 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
             mtx.vin[i].scriptWitness = psbtx.inputs[i].final_script_witness;
         }
         ssTx << mtx;
-        result.pushKV("hex", HexStr(ssTx.begin(), ssTx.end()));
+        result.pushKV("hex", HexStr(ssTx.str()));
     } else {
         ssTx << psbtx;
-        result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
+        result.pushKV("psbt", EncodeBase64(ssTx.str()));
     }
     result.pushKV("complete", complete);
 
diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 89cc7c808c..69ee08ffd7 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -239,10 +239,17 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
     return sigdata.complete;
 }
 
-bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& tx, PSBTInput& input, int index, int sighash)
+bool PSBTInputSigned(PSBTInput& input)
 {
-    // if this input has a final scriptsig or scriptwitness, don't do anything with it
-    if (!input.final_script_sig.empty() || !input.final_script_witness.IsNull()) {
+    return !input.final_script_sig.empty() || !input.final_script_witness.IsNull();
+}
+
+bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction& psbt, int index, int sighash)
+{
+    PSBTInput& input = psbt.inputs.at(index);
+    const CMutableTransaction& tx = *psbt.tx;
+
+    if (PSBTInputSigned(input)) {
         return true;
     }
 
@@ -253,15 +260,19 @@ bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& t
     // Get UTXO
     bool require_witness_sig = false;
     CTxOut utxo;
+
+    // Verify input sanity, which checks that at most one of witness or non-witness utxos is provided.
+    if (!input.IsSane()) {
+        return false;
+    }
+
     if (input.non_witness_utxo) {
         // If we're taking our information from a non-witness UTXO, verify that it matches the prevout.
-        if (input.non_witness_utxo->GetHash() != tx.vin[index].prevout.hash) return false;
-        // If both witness and non-witness UTXO are provided, verify that they match. This check shouldn't
-        // matter, as the PSBT deserializer enforces only one of both is provided, and the only way both
-        // can be present is when they're added simultaneously by FillPSBT (in which case they always match).
-        // Still, check in order to not rely on callers to enforce this.
-        if (!input.witness_utxo.IsNull() && input.non_witness_utxo->vout[tx.vin[index].prevout.n] != input.witness_utxo) return false;
-        utxo = input.non_witness_utxo->vout[tx.vin[index].prevout.n];
+        COutPoint prevout = tx.vin[index].prevout;
+        if (input.non_witness_utxo->GetHash() != prevout.hash) {
+            return false;
+        }
+        utxo = input.non_witness_utxo->vout[prevout.n];
     } else if (!input.witness_utxo.IsNull()) {
         utxo = input.witness_utxo;
         // When we're taking our information from a witness UTXO, we can't verify it is actually data from
@@ -280,18 +291,10 @@ bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& t
     if (require_witness_sig && !sigdata.witness) return false;
     input.FromSignatureData(sigdata);
 
+    // If we have a witness signature, use the smaller witness UTXO.
     if (sigdata.witness) {
-        assert(!utxo.IsNull());
         input.witness_utxo = utxo;
-    }
-
-    // If both UTXO types are present, drop the unnecessary one.
-    if (input.non_witness_utxo && !input.witness_utxo.IsNull()) {
-        if (sigdata.witness) {
-            input.non_witness_utxo = nullptr;
-        } else {
-            input.witness_utxo.SetNull();
-        }
+        input.non_witness_utxo = nullptr;
     }
 
     return sig_complete;
@@ -513,6 +516,12 @@ bool IsSolvable(const SigningProvider& provider, const CScript& script)
     return false;
 }
 
+PartiallySignedTransaction::PartiallySignedTransaction(const CTransaction& tx) : tx(tx)
+{
+    inputs.resize(tx.vin.size());
+    outputs.resize(tx.vout.size());
+}
+
 bool PartiallySignedTransaction::IsNull() const
 {
     return !tx && inputs.empty() && outputs.empty() && unknown.empty();
diff --git a/src/script/sign.h b/src/script/sign.h
index d47aada17d..e71f43f96d 100644
--- a/src/script/sign.h
+++ b/src/script/sign.h
@@ -566,6 +566,7 @@ struct PartiallySignedTransaction
     bool IsSane() const;
     PartiallySignedTransaction() {}
     PartiallySignedTransaction(const PartiallySignedTransaction& psbt_in) : tx(psbt_in.tx), inputs(psbt_in.inputs), outputs(psbt_in.outputs), unknown(psbt_in.unknown) {}
+    explicit PartiallySignedTransaction(const CTransaction& tx);
 
     // Only checks if they refer to the same transaction
     friend bool operator==(const PartiallySignedTransaction& a, const PartiallySignedTransaction &b)
@@ -729,8 +730,11 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
 bool SignSignature(const SigningProvider &provider, const CScript& fromPubKey, CMutableTransaction& txTo, unsigned int nIn, const CAmount& amount, int nHashType);
 bool SignSignature(const SigningProvider &provider, const CTransaction& txFrom, CMutableTransaction& txTo, unsigned int nIn, int nHashType);
 
+/** Checks whether a PSBTInput is already signed. */
+bool PSBTInputSigned(PSBTInput& input);
+
 /** Signs a PSBTInput, verifying that all provided data matches what is being signed. */
-bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& tx, PSBTInput& input, int index, int sighash = SIGHASH_ALL);
+bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction& psbt, int index, int sighash = SIGHASH_ALL);
 
 /** Extract signature data from a transaction input, and insert it. */
 SignatureData DataFromTransaction(const CMutableTransaction& tx, unsigned int nIn, const CTxOut& txout);
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 5a89448e02..bfac990639 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -3772,24 +3772,34 @@ void AddKeypathToMap(const CWallet* pwallet, const CKeyID& keyID, std::map<CPubK
     hd_keypaths.emplace(vchPubKey, std::move(info));
 }
 
-bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, const CTransaction* txConst, int sighash_type, bool sign, bool bip32derivs)
+bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, int sighash_type, bool sign, bool bip32derivs)
 {
     LOCK(pwallet->cs_wallet);
     // Get all of the previous transactions
     bool complete = true;
-    for (unsigned int i = 0; i < txConst->vin.size(); ++i) {
-        const CTxIn& txin = txConst->vin[i];
+    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
+        const CTxIn& txin = psbtx.tx->vin[i];
         PSBTInput& input = psbtx.inputs.at(i);
 
-        // If we don't know about this input, skip it and let someone else deal with it
-        const uint256& txhash = txin.prevout.hash;
-        const auto it = pwallet->mapWallet.find(txhash);
-        if (it != pwallet->mapWallet.end()) {
-            const CWalletTx& wtx = it->second;
-            CTxOut utxo = wtx.tx->vout[txin.prevout.n];
-            // Update both UTXOs from the wallet.
-            input.non_witness_utxo = wtx.tx;
-            input.witness_utxo = utxo;
+        if (PSBTInputSigned(input)) {
+            continue;
+        }
+
+        // Verify input looks sane. This will check that we have at most one uxto, witness or non-witness.
+        if (!input.IsSane()) {
+            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "PSBT input is not sane.");
+        }
+
+        // If we have no utxo, grab it from the wallet.
+        if (!input.non_witness_utxo && input.witness_utxo.IsNull()) {
+            const uint256& txhash = txin.prevout.hash;
+            const auto it = pwallet->mapWallet.find(txhash);
+            if (it != pwallet->mapWallet.end()) {
+                const CWalletTx& wtx = it->second;
+                // We only need the non_witness_utxo, which is a superset of the witness_utxo.
+                //   The signing code will switch to the smaller witness_utxo if this is ok.
+                input.non_witness_utxo = wtx.tx;
+            }
         }
 
         // Get the Sighash type
@@ -3797,12 +3807,12 @@ bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, const C
             throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Specified Sighash and sighash in PSBT do not match.");
         }
 
-        complete &= SignPSBTInput(HidingSigningProvider(pwallet, !sign, !bip32derivs), *psbtx.tx, input, i, sighash_type);
+        complete &= SignPSBTInput(HidingSigningProvider(pwallet, !sign, !bip32derivs), psbtx, i, sighash_type);
     }
 
     // Fill in the bip32 keypaths and redeemscripts for the outputs so that hardware wallets can identify change
-    for (unsigned int i = 0; i < txConst->vout.size(); ++i) {
-        const CTxOut& out = txConst->vout.at(i);
+    for (unsigned int i = 0; i < psbtx.tx->vout.size(); ++i) {
+        const CTxOut& out = psbtx.tx->vout.at(i);
         PSBTOutput& psbt_out = psbtx.outputs.at(i);
 
         // Fill a SignatureData with output info
@@ -3867,19 +3877,15 @@ UniValue walletprocesspsbt(const JSONRPCRequest& request)
     // Get the sighash type
     int nHashType = ParseSighashString(request.params[2]);
 
-    // Use CTransaction for the constant parts of the
-    // transaction to avoid rehashing.
-    const CTransaction txConst(*psbtx.tx);
-
     // Fill transaction with our data and also sign
     bool sign = request.params[1].isNull() ? true : request.params[1].get_bool();
     bool bip32derivs = request.params[3].isNull() ? false : request.params[3].get_bool();
-    bool complete = FillPSBT(pwallet, psbtx, &txConst, nHashType, sign, bip32derivs);
+    bool complete = FillPSBT(pwallet, psbtx, nHashType, sign, bip32derivs);
 
     UniValue result(UniValue::VOBJ);
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
     ssTx << psbtx;
-    result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
+    result.pushKV("psbt", EncodeBase64(ssTx.str()));
     result.pushKV("complete", complete);
 
     return result;
@@ -3971,29 +3977,18 @@ UniValue walletcreatefundedpsbt(const JSONRPCRequest& request)
     FundTransaction(pwallet, rawTx, fee, change_position, request.params[3]);
 
     // Make a blank psbt
-    PartiallySignedTransaction psbtx;
-    psbtx.tx = rawTx;
-    for (unsigned int i = 0; i < rawTx.vin.size(); ++i) {
-        psbtx.inputs.push_back(PSBTInput());
-    }
-    for (unsigned int i = 0; i < rawTx.vout.size(); ++i) {
-        psbtx.outputs.push_back(PSBTOutput());
-    }
-
-    // Use CTransaction for the constant parts of the
-    // transaction to avoid rehashing.
-    const CTransaction txConst(*psbtx.tx);
+    PartiallySignedTransaction psbtx(rawTx);
 
     // Fill transaction with out data but don't sign
     bool bip32derivs = request.params[4].isNull() ? false : request.params[4].get_bool();
-    FillPSBT(pwallet, psbtx, &txConst, 1, false, bip32derivs);
+    FillPSBT(pwallet, psbtx, 1, false, bip32derivs);
 
     // Serialize the PSBT
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
     ssTx << psbtx;
 
     UniValue result(UniValue::VOBJ);
-    result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
+    result.pushKV("psbt", EncodeBase64(ssTx.str()));
     result.pushKV("fee", ValueFromAmount(fee));
     result.pushKV("changepos", change_position);
     return result;
diff --git a/src/wallet/rpcwallet.h b/src/wallet/rpcwallet.h
index 9b9a159b86..abd7750874 100644
--- a/src/wallet/rpcwallet.h
+++ b/src/wallet/rpcwallet.h
@@ -30,5 +30,5 @@ bool EnsureWalletIsAvailable(CWallet *, bool avoidException);
 
 UniValue getaddressinfo(const JSONRPCRequest& request);
 UniValue signrawtransactionwithwallet(const JSONRPCRequest& request);
-bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, const CTransaction* txConst, int sighash_type = 1, bool sign = true, bool bip32derivs = false);
+bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, int sighash_type = 1 /* SIGHASH_ALL */, bool sign = true, bool bip32derivs = false);
 #endif //BITCOIN_WALLET_RPCWALLET_H
diff --git a/src/wallet/test/psbt_wallet_tests.cpp b/src/wallet/test/psbt_wallet_tests.cpp
index cb1ad25461..9918eeb89f 100644
--- a/src/wallet/test/psbt_wallet_tests.cpp
+++ b/src/wallet/test/psbt_wallet_tests.cpp
@@ -59,12 +59,8 @@ BOOST_AUTO_TEST_CASE(psbt_updater_test)
     CDataStream ssData(ParseHex("70736274ff01009a020000000258e87a21b56daf0c23be8e7070456c336f7cbaa5c8757924f545887bb2abdd750000000000ffffffff838d0427d0ec650a68aa46bb0b098aea4422c071b2ca78352a077959d07cea1d0100000000ffffffff0270aaf00800000000160014d85c2b71d0060b09c9886aeb815e50991dda124d00e1f5050000000016001400aea9a2e5f0f876a588df5546e8742d1d87008f000000000000000000"), SER_NETWORK, PROTOCOL_VERSION);
     ssData >> psbtx;
 
-    // Use CTransaction for the constant parts of the
-    // transaction to avoid rehashing.
-    const CTransaction txConst(*psbtx.tx);
-
     // Fill transaction with our data
-    FillPSBT(&m_wallet, psbtx, &txConst, 1, false, true);
+    FillPSBT(&m_wallet, psbtx, SIGHASH_ALL, false, true);
 
     // Get the final tx
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index fca910bf64..86b043176c 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -207,6 +207,13 @@ class PSBTTest(BitcoinTestFramework):
             assert tx_in["sequence"] > MAX_BIP125_RBF_SEQUENCE
         assert_equal(decoded_psbt["tx"]["locktime"], 0)
 
+        # Regression test for 14473 (mishandling of already-signed witness transaction):
+        psbtx_info = self.nodes[0].walletcreatefundedpsbt([{"txid":unspent["txid"], "vout":unspent["vout"]}], [{self.nodes[2].getnewaddress():unspent["amount"]+1}])
+        complete_psbt = self.nodes[0].walletprocesspsbt(psbtx_info["psbt"])
+        double_processed_psbt = self.nodes[0].walletprocesspsbt(complete_psbt["psbt"])
+        assert_equal(complete_psbt, double_processed_psbt)
+        # We don't care about the decode result, but decoding must succeed.
+        self.nodes[0].decodepsbt(double_processed_psbt["psbt"])
 
         # BIP 174 Test Vectors