Refactor PSBTInput signing to enforce invariant

Refactor the process of PSBTInput signing to enforce the invariant that
a PSBTInput always has _either_ a witness_utxo or a non_witness_utxo,
never both.

This simplifies the logic of SignPSBTInput slightly, since it no longer
has to deal with the "both" case. When calling it, we now give it, in
order of preference: (1) whichever of the utxo fields was already
present in the PSBT we received, or (2) if neither, the
non_witness_utxo field, which is just a copy of the input transaction,
which we get from the wallet.

SignPSBTInput no longer has to remove one of the two fields; instead, it
will check if we have a witness signature, and if so, it will replace
the non_witness_utxo with the witness_utxo (which is smaller, as it is
just a copy of the output being spent.)

Add PSBTInput::IsSane checks in two more places, which checks for
both utxo fields being present; we will now give an RPC error early on
if we are supplied such a malformed PSBT to fill in.

Also add a check to FillPSBT, to avoid touching any input that is
already signed. (This is now redundant, since we should no longer
potentially harm an already-signed input, but it's harmless.)

fixes #14473

Github-Pull: #14588

1 files changed, 18 insertions, 7 deletions

diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 8bc142f6e1..f97d6a2533 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -253,15 +253,19 @@ bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction&
     // 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
@@ -279,6 +283,13 @@ bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction&
     // Verify that a witness signature was produced in case one was required.
     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) {
+        input.witness_utxo = utxo;
+        input.non_witness_utxo = nullptr;
+    }
+
     return sig_complete;
 }