Don't process unrequested, low-work blocks

sdaftuar · sdaftuar · commit ce8cd7a7da91 · 2017-10-19T20:33:45.000-04:00
A peer could try to waste our resources by sending us unrequested blocks with low work, eg to fill up our disk. Since e265200 we no longer request blocks until we know we're on a chain with more than nMinimumChainWork (our anti-DoS threshold), but we would still process unrequested blocks that had more work than our tip. This commit fixes that behavior.
diff --git a/src/validation.cpp b/src/validation.cpp
@@ -3135,6 +3135,12 @@ static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidation
         if (pindex->nTx != 0) return true;  // This is a previously-processed block that was pruned
         if (!fHasMoreWork) return true;     // Don't process less-work chains
         if (fTooFarAhead) return true;      // Block height is too high
+
+        // Protect against DoS attacks from low-work chains.
+        // If our tip is behind, a peer could try to send us
+        // low-work blocks on a fake chain that we would never
+        // request; don't process these.
+        if (pindex->nChainWork < nMinimumChainWork) return true;
     }
     if (fNewBlock) *fNewBlock = true;
 
