Further DoS prevention: Verify signatures last

Loop over all inputs doing inexpensive validity checks first,
and then loop over them a second time doing expensive signature
checks. This helps prevent possible CPU exhaustion attacks
where an attacker tries to make a victim waste time checking
signatures for invalid transactions.

1 files changed, 15 insertions, 4 deletions

diff --git a/src/main.cpp b/src/main.cpp
index b7067b2092..00f0633431 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1033,15 +1033,26 @@ bool CTransaction::ConnectInputs(MapPrevTx inputs,
                     if (pindex->nBlockPos == txindex.pos.nBlockPos && pindex->nFile == txindex.pos.nFile)
                         return error("ConnectInputs() : tried to spend coinbase at depth %d", pindexBlock->nHeight - pindex->nHeight);
 
-            // Check for conflicts (double-spend)
-            if (!txindex.vSpent[prevout.n].IsNull())
-                return fMiner ? false : error("ConnectInputs() : %s prev tx already used at %s", GetHash().ToString().substr(0,10).c_str(), txindex.vSpent[prevout.n].ToString().c_str());
-
             // Check for negative or overflow input values
             nValueIn += txPrev.vout[prevout.n].nValue;
             if (!MoneyRange(txPrev.vout[prevout.n].nValue) || !MoneyRange(nValueIn))
                 return error("ConnectInputs() : txin values out of range");
 
+        }
+        // The first loop above does all the inexpensive checks.
+        // Only if ALL inputs pass do we perform expensive ECDSA signature checks.
+        // Helps prevent CPU exhaustion attacks.
+        for (unsigned int i = 0; i < vin.size(); i++)
+        {
+            COutPoint prevout = vin[i].prevout;
+            assert(inputs.count(prevout.hash) > 0);
+            CTxIndex& txindex = inputs[prevout.hash].first;
+            CTransaction& txPrev = inputs[prevout.hash].second;
+
+            // Check for conflicts (double-spend)
+            if (!txindex.vSpent[prevout.n].IsNull())
+                return fMiner ? false : error("ConnectInputs() : %s prev tx already used at %s", GetHash().ToString().substr(0,10).c_str(), txindex.vSpent[prevout.n].ToString().c_str());
+
             // Verify signature
             if (!VerifySignature(txPrev, *this, i, fStrictPayToScriptHash, 0))
             {