Add txids with non-standard inputs to reject filter

Our policy checks for non-standard inputs depend only on the non-witness
portion of a transaction: we look up the scriptPubKey of the input being
spent from our UTXO set (which is covered by the input txid), and the p2sh
checks only rely on the scriptSig portion of the input.

Consequently it's safe to add txids of transactions that fail these checks to
the reject filter, as the witness is irrelevant to the failure. This is helpful
for any situation where we might request the transaction again via txid (either
from txid-relay peers, or if we might fetch the transaction via txid due to
parent-fetching of orphans).

Further, in preparation for future witness versions being deployed on the
network, ensure that WITNESS_UNKNOWN transactions are rejected in
AreInputsStandard(), so that transactions spending v1 (or greater) witness
outputs will fall into this category of having their txid added to the reject
filter.

Github-Pull: #19620
Rebased-From: 7989901c7eb62ca28b3d1e5d5831041a7267e495

4 files changed, 25 insertions, 6 deletions

diff --git a/src/consensus/validation.h b/src/consensus/validation.h
index a79e7b9d12..3a90cd69b3 100644
--- a/src/consensus/validation.h
+++ b/src/consensus/validation.h
@@ -26,7 +26,8 @@ enum class TxValidationResult {
      * is uninteresting.
      */
     TX_RECENT_CONSENSUS_CHANGE,
-    TX_NOT_STANDARD,          //!< didn't meet our local policy rules
+    TX_INPUTS_NOT_STANDARD,   //!< inputs (covered by txid) failed policy rules
+    TX_NOT_STANDARD,          //!< otherwise didn't meet our local policy rules
     TX_MISSING_INPUTS,        //!< transaction was missing some of its inputs
     TX_PREMATURE_SPEND,       //!< transaction spends a coinbase too early, or violates locktime/sequence locks
     /**
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index ddc516ce0f..8572ebb9f7 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -1069,6 +1069,7 @@ static bool MaybePunishNodeForTx(NodeId nodeid, const TxValidationState& state,
         }
     // Conflicting (but not necessarily invalid) data or different policy:
     case TxValidationResult::TX_RECENT_CONSENSUS_CHANGE:
+    case TxValidationResult::TX_INPUTS_NOT_STANDARD:
     case TxValidationResult::TX_NOT_STANDARD:
     case TxValidationResult::TX_MISSING_INPUTS:
     case TxValidationResult::TX_PREMATURE_SPEND:
@@ -1901,10 +1902,15 @@ void static ProcessOrphanTx(CConnman* connman, CTxMemPool& mempool, std::set<uin
             // Has inputs but not accepted to mempool
             // Probably non-standard or insufficient fee
             LogPrint(BCLog::MEMPOOL, "   removed orphan tx %s\n", orphanHash.ToString());
-            if (!orphanTx.HasWitness() && orphan_state.GetResult() != TxValidationResult::TX_WITNESS_MUTATED) {
+            if ((!orphanTx.HasWitness() && orphan_state.GetResult() != TxValidationResult::TX_WITNESS_MUTATED) ||
+                    orphan_state.GetResult() == TxValidationResult::TX_INPUTS_NOT_STANDARD) {
                 // Do not use rejection cache for witness transactions or
                 // witness-stripped transactions, as they can have been malleated.
                 // See https://github.com/bitcoin/bitcoin/issues/8279 for details.
+                // However, if the transaction failed for TX_INPUTS_NOT_STANDARD,
+                // then we know that the witness was irrelevant to the policy
+                // failure, since this check depends only on the txid
+                // (the scriptPubKey being spent is covered by the txid).
                 assert(recentRejects);
                 recentRejects->insert(orphanHash);
             }
@@ -2596,10 +2602,15 @@ bool ProcessMessage(CNode* pfrom, const std::string& msg_type, CDataStream& vRec
                 recentRejects->insert(tx.GetHash());
             }
         } else {
-            if (!tx.HasWitness() && state.GetResult() != TxValidationResult::TX_WITNESS_MUTATED) {
+            if ((!tx.HasWitness() && state.GetResult() != TxValidationResult::TX_WITNESS_MUTATED) ||
+                    state.GetResult() == TxValidationResult::TX_INPUTS_NOT_STANDARD) {
                 // Do not use rejection cache for witness transactions or
                 // witness-stripped transactions, as they can have been malleated.
                 // See https://github.com/bitcoin/bitcoin/issues/8279 for details.
+                // However, if the transaction failed for TX_INPUTS_NOT_STANDARD,
+                // then we know that the witness was irrelevant to the policy
+                // failure, since this check depends only on the txid
+                // (the scriptPubKey being spent is covered by the txid).
                 assert(recentRejects);
                 recentRejects->insert(tx.GetHash());
                 if (RecursiveDynamicUsage(*ptx) < 100000) {
diff --git a/src/policy/policy.cpp b/src/policy/policy.cpp
index 07d51c0088..a0ac6bdd83 100644
--- a/src/policy/policy.cpp
+++ b/src/policy/policy.cpp
@@ -152,6 +152,8 @@ bool IsStandardTx(const CTransaction& tx, bool permit_bare_multisig, const CFeeR
  * script can be anything; an attacker could use a very
  * expensive-to-check-upon-redemption script like:
  *   DUP CHECKSIG DROP ... repeated 100 times... OP_1
+ *
+ * Note that only the non-witness portion of the transaction is checked here.
  */
 bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 {
@@ -164,7 +166,11 @@ bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 
         std::vector<std::vector<unsigned char> > vSolutions;
         txnouttype whichType = Solver(prev.scriptPubKey, vSolutions);
-        if (whichType == TX_NONSTANDARD) {
+        if (whichType == TX_NONSTANDARD || whichType == TX_WITNESS_UNKNOWN) {
+            // WITNESS_UNKNOWN failures are typically also caught with a policy
+            // flag in the script interpreter, but it can be helpful to catch
+            // this type of NONSTANDARD transaction earlier in transaction
+            // validation.
             return false;
         } else if (whichType == TX_SCRIPTHASH) {
             std::vector<std::vector<unsigned char> > stack;
diff --git a/src/validation.cpp b/src/validation.cpp
index 7ee94f8657..5a98b2cb92 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -665,8 +665,9 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     }
 
     // Check for non-standard pay-to-script-hash in inputs
-    if (fRequireStandard && !AreInputsStandard(tx, m_view))
-        return state.Invalid(TxValidationResult::TX_NOT_STANDARD, "bad-txns-nonstandard-inputs");
+    if (fRequireStandard && !AreInputsStandard(tx, m_view)) {
+        return state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs");
+    }
 
     // Check for non-standard witness in P2WSH
     if (tx.HasWitness() && fRequireStandard && !IsWitnessStandard(tx, m_view))