Merge #11580: Do not send (potentially) invalid headers in response to getheaders

725b79a [test] Verify node doesn't send headers that haven't been fully validated (Russell Yanofsky)
3788a84 Do not send (potentially) invalid headers in response to getheaders (Matt Corallo)

Pull request description:

  Nowhere else in the protocol do we send headers which are for
  blocks we have not fully validated except in response to getheaders
  messages with a null locator. On my public node I have not seen any
  such request (whether for an invalid block or not) in at least two
  years of debug.log output, indicating that this should have minimal
  impact.

Tree-SHA512: c1f6e0cdcdfb78ea577d555f9b3ceb1b4b60eff4f6cf313bfd8b576c9562d797bea73abc23f7011f249ae36dd539c715f3d20487ac03ace60e84e1b77c0c1e1a

src/net_processing.cpp

@@ -781,11 +781,13 @@ void Misbehaving(NodeId pnode, int howmuch)
 
 // To prevent fingerprinting attacks, only send blocks/headers outside of the
 // active chain if they are no more than a month older (both in time, and in
-// best equivalent proof of work) than the best header chain we know about.
+// best equivalent proof of work) than the best header chain we know about and
-static bool StaleBlockRequestAllowed(const CBlockIndex* pindex, const Consensus::Params& consensusParams)
+// we fully-validated them at some point.
+static bool BlockRequestAllowed(const CBlockIndex* pindex, const Consensus::Params& consensusParams)
 {
     AssertLockHeld(cs_main);
-    return (pindexBestHeader != nullptr) &&
+    if (chainActive.Contains(pindex)) return true;
+    return pindex->IsValid(BLOCK_VALID_SCRIPTS) && (pindexBestHeader != nullptr) &&
         (pindexBestHeader->GetBlockTime() - pindex->GetBlockTime() < STALE_RELAY_AGE_LIMIT) &&
         (GetBlockProofEquivalentTime(*pindexBestHeader, *pindex, *pindexBestHeader, consensusParams) < STALE_RELAY_AGE_LIMIT);
 }
@@ -1074,14 +1076,9 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
                         CValidationState dummy;
                         ActivateBestChain(dummy, Params(), a_recent_block);
                     }
-                    if (chainActive.Contains(mi->second)) {
+                    send = BlockRequestAllowed(mi->second, consensusParams);
-                        send = true;
+                    if (!send) {
-                    } else {
+                        LogPrintf("%s: ignoring request from peer=%i for old block that isn't in the main chain\n", __func__, pfrom->GetId());
-                        send = mi->second->IsValid(BLOCK_VALID_SCRIPTS) &&
-                            StaleBlockRequestAllowed(mi->second, consensusParams);
-                        if (!send) {
-                            LogPrintf("%s: ignoring request from peer=%i for old block that isn't in the main chain\n", __func__, pfrom->GetId());
-                        }
                     }
                 }
                 // disconnect node in case we have reached the outbound limit for serving historical blocks
@@ -2034,8 +2031,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
                 return true;
             pindex = (*mi).second;
 
-            if (!chainActive.Contains(pindex) &&
+            if (!BlockRequestAllowed(pindex, chainparams.GetConsensus())) {
-                !StaleBlockRequestAllowed(pindex, chainparams.GetConsensus())) {
                 LogPrintf("%s: ignoring request from peer=%i for old block header that isn't in the main chain\n", __func__, pfrom->GetId());
                 return true;
             }

test/functional/sendheaders.py

@@ -10,6 +10,17 @@ Setup:
   receive inv's (omitted from testing description below, this is our control).
   Second node is used for creating reorgs.
 
+test_null_locators
+==================
+
+Sends two getheaders requests with null locator values. First request's hashstop
+value refers to validated block, while second request's hashstop value refers to
+a block which hasn't been validated. Verifies only the first request returns
+headers.
+
+test_nonnull_locators
+=====================
+
 Part 1: No headers announcements before "sendheaders"
 a. node mines a block [expect: inv]
    send getdata for the block [expect: block]
@@ -221,6 +232,29 @@ class SendHeadersTest(BitcoinTestFramework):
         inv_node.sync_with_ping()
         test_node.sync_with_ping()
 
+        self.test_null_locators(test_node)
+        self.test_nonnull_locators(test_node, inv_node)
+
+    def test_null_locators(self, test_node):
+        tip = self.nodes[0].getblockheader(self.nodes[0].generate(1)[0])
+        tip_hash = int(tip["hash"], 16)
+
+        self.log.info("Verify getheaders with null locator and valid hashstop returns headers.")
+        test_node.clear_last_announcement()
+        test_node.get_headers(locator=[], hashstop=tip_hash)
+        assert_equal(test_node.check_last_announcement(headers=[tip_hash]), True)
+
+        self.log.info("Verify getheaders with null locator and invalid hashstop does not return headers.")
+        block = create_block(int(tip["hash"], 16), create_coinbase(tip["height"] + 1), tip["mediantime"] + 1)
+        block.solve()
+        test_node.send_header_for_blocks([block])
+        test_node.clear_last_announcement()
+        test_node.get_headers(locator=[], hashstop=int(block.hash, 16))
+        test_node.sync_with_ping()
+        assert_equal(test_node.block_announced, False)
+        test_node.send_message(msg_block(block))
+
+    def test_nonnull_locators(self, test_node, inv_node):
         tip = int(self.nodes[0].getbestblockhash(), 16)
 
         # PART 1