Don't process unrequested, low-work blocks

A peer could try to waste our resources by sending us unrequested blocks with
low work, eg to fill up our disk.  Since
e2652002b6011f793185d473f87f1730c625593b 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.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index d19521bd3c..866e0c9fba 100644
--- 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;