Verify PSBT inputs rather than check for fields being empty

src/node/psbt.cpp

@@ -59,7 +59,7 @@ PSBTAnalysis AnalyzePSBT(PartiallySignedTransaction psbtx)
         }
 
         // Check if it is final
-        if (!utxo.IsNull() && !PSBTInputSigned(input)) {
+        if (!PSBTInputSignedAndVerified(psbtx, i, &txdata)) {
             input_analysis.is_final = false;
 
             // Figure out what is missing

src/psbt.cpp

@@ -4,6 +4,7 @@
 
 #include <psbt.h>
 
+#include <policy/policy.h>
 #include <util/check.h>
 #include <util/strencodings.h>
 
@@ -273,11 +274,41 @@ void PSBTOutput::Merge(const PSBTOutput& output)
     if (m_tap_internal_key.IsNull() && !output.m_tap_internal_key.IsNull()) m_tap_internal_key = output.m_tap_internal_key;
     if (m_tap_tree.empty() && !output.m_tap_tree.empty()) m_tap_tree = output.m_tap_tree;
 }
+
 bool PSBTInputSigned(const PSBTInput& input)
 {
     return !input.final_script_sig.empty() || !input.final_script_witness.IsNull();
 }
 
+bool PSBTInputSignedAndVerified(const PartiallySignedTransaction psbt, unsigned int input_index, const PrecomputedTransactionData* txdata)
+{
+    CTxOut utxo;
+    assert(psbt.inputs.size() >= input_index);
+    const PSBTInput& input = psbt.inputs[input_index];
+
+    if (input.non_witness_utxo) {
+        // If we're taking our information from a non-witness UTXO, verify that it matches the prevout.
+        COutPoint prevout = psbt.tx->vin[input_index].prevout;
+        if (prevout.n >= input.non_witness_utxo->vout.size()) {
+            return false;
+        }
+        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;
+    } else {
+        return false;
+    }
+
+    if (txdata) {
+        return VerifyScript(input.final_script_sig, utxo.scriptPubKey, &input.final_script_witness, STANDARD_SCRIPT_VERIFY_FLAGS, MutableTransactionSignatureChecker{&(*psbt.tx), input_index, utxo.nValue, *txdata, MissingDataBehavior::FAIL});
+    } else {
+        return VerifyScript(input.final_script_sig, utxo.scriptPubKey, &input.final_script_witness, STANDARD_SCRIPT_VERIFY_FLAGS, MutableTransactionSignatureChecker{&(*psbt.tx), input_index, utxo.nValue, MissingDataBehavior::FAIL});
+    }
+}
+
 size_t CountPSBTUnsignedInputs(const PartiallySignedTransaction& psbt) {
     size_t count = 0;
     for (const auto& input : psbt.inputs) {
@@ -331,7 +362,7 @@ bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction&
     PSBTInput& input = psbt.inputs.at(index);
     const CMutableTransaction& tx = *psbt.tx;
 
-    if (PSBTInputSigned(input)) {
+    if (PSBTInputSignedAndVerified(psbt, index, txdata)) {
         return true;
     }
 

src/psbt.h

@@ -1218,9 +1218,12 @@ std::string PSBTRoleName(PSBTRole role);
 /** Compute a PrecomputedTransactionData object from a psbt. */
 PrecomputedTransactionData PrecomputePSBTData(const PartiallySignedTransaction& psbt);
 
-/** Checks whether a PSBTInput is already signed. */
+/** Checks whether a PSBTInput is already signed by checking for non-null finalized fields. */
 bool PSBTInputSigned(const PSBTInput& input);
 
+/** Checks whether a PSBTInput is already signed by doing script verification using final fields. */
+bool PSBTInputSignedAndVerified(const PartiallySignedTransaction psbt, unsigned int input_index, const PrecomputedTransactionData* txdata);
+
 /** Signs a PSBTInput, verifying that all provided data matches what is being signed.
  *
  * txdata should be the output of PrecomputePSBTData (which can be shared across