Reject headers that build on an invalid parent

src/main.cpp

@@ -449,6 +449,10 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBl
         // are not yet downloaded and not in flight to vBlocks. In the mean time, update
         // pindexLastCommonBlock as long as all ancestors are already downloaded.
         BOOST_FOREACH(CBlockIndex* pindex, vToFetch) {
+            if (!pindex->IsValid(BLOCK_VALID_TREE)) {
+                // We consider the chain that this peer is on invalid.
+                return;
+            }
             if (pindex->nStatus & BLOCK_HAVE_DATA) {
                 if (pindex->nChainTx)
                     state->pindexLastCommonBlock = pindex;
@@ -2563,6 +2567,8 @@ bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBloc
         if (mi == mapBlockIndex.end())
             return state.DoS(10, error("%s : prev block not found", __func__), 0, "bad-prevblk");
         pindexPrev = (*mi).second;
+        if (pindexPrev->nStatus & BLOCK_FAILED_MASK)
+            return state.DoS(100, error("%s : prev block invalid", __func__), REJECT_INVALID, "bad-prevblk");
     }
 
     if (!ContextualCheckBlockHeader(block, state, pindexPrev))