Merge bitcoin#13917: Additional safety checks in PSBT signer

5df6f08 More tests of signer checks (Andrew Chow)
7c8bffd Test that a non-witness script as witness utxo is not signed (Andrew Chow)
8254e99 Additional sanity checks in SignPSBTInput (Pieter Wuille)
c05712c Only wipe wrong UTXO type data if overwritten by wallet (Pieter Wuille)

Pull request description:

  The current PSBT signing code can end up producing a non-segwit signature, while only the UTXO being spent is provided in the PSBT (as opposed to the entire transaction being spent). This may be used to trick a user to incorrectly decide a transaction has the semantics he intends to sign.

  Fix this by refusing to sign if there is any mismatch between the provided data and what is being signed.

Tree-SHA512: b55790d79d8166e05513fc4c603a982a33710e79dc3c045060cddac6b48a1be3a28ebf8db63f988b6567b15dd27fd09bbaf48846e323c8635376ac20178956f4

src/script/sign.cpp

@@ -244,17 +244,33 @@ bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& t
     input.FillSignatureData(sigdata);
 
     // Get UTXO
+    bool require_witness_sig = false;
     CTxOut utxo;
     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];
     } 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
+        // the output being spent. This is safe in case a witness signature is produced (which includes this
+        // information directly in the hash), but not for non-witness signatures. Remember that we require
+        // a witness signature in this situation.
+        require_witness_sig = true;
     } else {
         return false;
     }
 
     MutableTransactionSignatureCreator creator(&tx, index, utxo.nValue, sighash);
+    sigdata.witness = false;
     bool sig_complete = ProduceSignature(provider, creator, utxo.scriptPubKey, sigdata);
+    // Verify that a witness signature was produced in case one was required.
+    if (require_witness_sig && !sigdata.witness) return false;
     input.FromSignatureData(sigdata);
     return sig_complete;
 }

src/script/sign.h

@@ -686,7 +686,7 @@ 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);
 
-/** Signs a PSBTInput */
+/** Signs a PSBTInput, verifying that all provided data matches what is being signed. */
 bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& tx, PSBTInput& input, SignatureData& sigdata, int index, int sighash = 1);
 
 /** Extract signature data from a transaction input, and insert it. */

src/wallet/rpcwallet.cpp

@@ -4504,10 +4504,11 @@ bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, const C
 
         // 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);
+        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;
         }
@@ -4524,11 +4525,13 @@ bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, const C
             complete &= SignPSBTInput(PublicOnlySigningProvider(pwallet), *psbtx.tx, input, sigdata, i, sighash_type);
         }
 
-        // Drop the unnecessary UTXO
-        if (sigdata.witness) {
-            input.non_witness_utxo = nullptr;
-        } else {
-            input.witness_utxo.SetNull();
+        if (it != pwallet->mapWallet.end()) {
+            // Drop the unnecessary UTXO if we added both from the wallet.
+            if (sigdata.witness) {
+                input.non_witness_utxo = nullptr;
+            } else {
+                input.witness_utxo.SetNull();
+            }
         }
 
         // Get public key paths