1 files changed, 45 insertions, 5 deletions

diff --git a/src/main.cpp b/src/main.cpp
index d9fe713..8926d57 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -3293,16 +3293,56 @@ bool ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
         CInv inv(MSG_BLOCK, pblock->GetHash());
         pfrom->AddInventoryKnown(inv);
 
-        // The body is here, so the fetch is over whatever validation says
-        // next -- unlike the mapAlreadyAskedFor erase below, which stays
-        // conditional on acceptance so a peer feeding us garbage does not
-        // cancel the scheduled retries for the real thing.
-        mapBlockFetch.erase(inv.hash);
+        // GetHash() covers the 80-byte header and nothing else, so the hash
+        // coming out right does not make this the block we asked for. Every
+        // peer that saw X announced holds X's header, and can answer our
+        // getdata with that header bolted onto a transaction list of its own
+        // invention -- the reply still hashes to X. hashMerkleRoot is the only
+        // thing binding the header to the body, so check it here, before the
+        // mapBlockFetch entry is touched and before CheckBlock spends a
+        // RandomX hash on a body already known to be forged.
+        //
+        // There is no honest way to arrive here. Peer traffic rides the .btf
+        // channel, which is XChaCha20-Poly1305, so a body that arrives at all
+        // arrived intact: a mismatch is the peer's own construction, not line
+        // noise. Drop the peer and leave the fetch outstanding so the failover
+        // sweep asks somebody else for the real X, rather than recording X as
+        // delivered on the strength of a header anyone could copy.
+        if (pblock->hashMerkleRoot != pblock->BuildMerkleTree())
+        {
+            if (LogAcceptsCategory("net"))
+                printf("peer %s served a forged body for %s\n",
+                       pfrom->addr.ToString().c_str(), inv.ToString().c_str());
+            // One line per connection: ProcessMessages keeps draining the
+            // receive buffer after fDisconnect is set, so a peer that packs
+            // many forged bodies into one buffer must not get one log line
+            // for each of them.
+            if (!pfrom->fDisconnect)
+                printf("block fetch: a peer answered the request for %s with a body that is not "
+                       "that block -- dropping it\n", inv.hash.ToString().substr(0,14).c_str());
+            pfrom->fDisconnect = true;
+            return error("ProcessMessage() : forged block body for %s",
+                         inv.hash.ToString().substr(0,14).c_str());
+        }
 
         if (ProcessBlock(pfrom, pblock.release()))
         {
             mapAlreadyAskedFor.erase(inv);
 
+            // The body arrived and validated, so the fetch really is over:
+            // the block is now in mapBlockIndex, or parked in mapOrphanBlocks,
+            // and either way AlreadyHave answers for it from here on.
+            //
+            // This has to stay inside the success branch, next to the erase
+            // above. When validation rejects a body nothing is stored,
+            // AlreadyHave stays false, and the mapAskFor entry was already
+            // consumed when the getdata went out -- so erasing the tracking
+            // here would leave the block requested from nobody at all until
+            // some peer happened to re-announce it. That is exactly the stall
+            // of #33, and erasing before ProcessBlock made it reachable on
+            // demand.
+            mapBlockFetch.erase(inv.hash);
+
             // A peer that just handed us a block is demonstrably at least at
             // that block's height. Without this, nStartingHeight is a snapshot
             // taken once during the version handshake and never touched again,