Change in transaction pull scheduling to prevent InvBlock-related attacks

Co-authored-by: Suhas Daftuar <sdaftuar@gmail.com>

src/net.cpp

@@ -85,8 +85,6 @@ std::map<CNetAddr, LocalServiceInfo> mapLocalHost GUARDED_BY(cs_mapLocalHost);
 static bool vfLimited[NET_MAX] GUARDED_BY(cs_mapLocalHost) = {};
 std::string strSubVersion;
 
-limitedmap<uint256, int64_t> mapAlreadyAskedFor(MAX_INV_SZ);
-
 void CConnman::AddOneShot(const std::string& strDest)
 {
     LOCK(cs_vOneShots);
@@ -2644,40 +2642,6 @@ CNode::~CNode()
     CloseSocket(hSocket);
 }
 
-void CNode::AskFor(const CInv& inv)
-{
-    if (mapAskFor.size() > MAPASKFOR_MAX_SZ || setAskFor.size() > SETASKFOR_MAX_SZ)
-        return;
-    // a peer may not have multiple non-responded queue positions for a single inv item
-    if (!setAskFor.insert(inv.hash).second)
-        return;
-
-    // We're using mapAskFor as a priority queue,
-    // the key is the earliest time the request can be sent
-    int64_t nRequestTime;
-    limitedmap<uint256, int64_t>::const_iterator it = mapAlreadyAskedFor.find(inv.hash);
-    if (it != mapAlreadyAskedFor.end())
-        nRequestTime = it->second;
-    else
-        nRequestTime = 0;
-    LogPrint(BCLog::NET, "askfor %s  %d (%s) peer=%d\n", inv.ToString(), nRequestTime, FormatISO8601Time(nRequestTime/1000000), id);
-
-    // Make sure not to reuse time indexes to keep things in the same order
-    int64_t nNow = GetTimeMicros() - 1000000;
-    static int64_t nLastTime;
-    ++nLastTime;
-    nNow = std::max(nNow, nLastTime);
-    nLastTime = nNow;
-
-    // Each retry is 2 minutes after the last
-    nRequestTime = std::max(nRequestTime + 2 * 60 * 1000000, nNow);
-    if (it != mapAlreadyAskedFor.end())
-        mapAlreadyAskedFor.update(it, nRequestTime);
-    else
-        mapAlreadyAskedFor.insert(std::make_pair(inv.hash, nRequestTime));
-    mapAskFor.insert(std::make_pair(nRequestTime, inv));
-}
-
 bool CConnman::NodeFullyConnected(const CNode* pnode)
 {
     return pnode && pnode->fSuccessfullyConnected && !pnode->fDisconnect;

src/net.h

@@ -67,10 +67,6 @@ static const bool DEFAULT_UPNP = USE_UPNP;
 #else
 static const bool DEFAULT_UPNP = false;
 #endif
-/** The maximum number of entries in mapAskFor */
-static const size_t MAPASKFOR_MAX_SZ = MAX_INV_SZ;
-/** The maximum number of entries in setAskFor (larger due to getdata latency)*/
-static const size_t SETASKFOR_MAX_SZ = 2 * MAX_INV_SZ;
 /** The maximum number of peer connections to maintain. */
 static const unsigned int DEFAULT_MAX_PEER_CONNECTIONS = 125;
 /** The default for -maxuploadtarget. 0 = Unlimited */
@@ -514,8 +510,6 @@ extern bool fDiscover;
 extern bool fListen;
 extern bool fRelayTxes;
 
-extern limitedmap<uint256, int64_t> mapAlreadyAskedFor;
-
 /** Subversion as sent to the P2P network in `version` messages */
 extern std::string strSubVersion;
 
@@ -704,8 +698,6 @@ public:
     // and in the order requested.
     std::vector<uint256> vInventoryBlockToSend GUARDED_BY(cs_inventory);
     CCriticalSection cs_inventory;
-    std::set<uint256> setAskFor;
-    std::multimap<int64_t, CInv> mapAskFor;
     int64_t nNextInvSend{0};
     // Used for headers announcements - unfiltered blocks to relay
     std::vector<uint256> vBlockHashesToAnnounce GUARDED_BY(cs_inventory);
@@ -852,8 +844,6 @@ public:
         vBlockHashesToAnnounce.push_back(hash);
     }
 
-    void AskFor(const CInv& inv);
-
     void CloseSocketDisconnect();
 
     void copyStats(CNodeStats &stats);

src/net_processing.cpp

@@ -64,6 +64,21 @@ static constexpr int STALE_RELAY_AGE_LIMIT = 30 * 24 * 60 * 60;
 /// Age after which a block is considered historical for purposes of rate
 /// limiting block relay. Set to one week, denominated in seconds.
 static constexpr int HISTORICAL_BLOCK_AGE = 7 * 24 * 60 * 60;
+/** Maximum number of in-flight transactions from a peer */
+static constexpr int32_t MAX_PEER_TX_IN_FLIGHT = 100;
+/** Maximum number of announced transactions from a peer */
+static constexpr int32_t MAX_PEER_TX_ANNOUNCEMENTS = 2 * MAX_INV_SZ;
+/** How many microseconds to delay requesting transactions from inbound peers */
+static constexpr int64_t INBOUND_PEER_TX_DELAY = 2 * 1000000;
+/** How long to wait (in microseconds) before downloading a transaction from an additional peer */
+static constexpr int64_t GETDATA_TX_INTERVAL = 60 * 1000000;
+/** Maximum delay (in microseconds) for transaction requests to avoid biasing some peers over others. */
+static constexpr int64_t MAX_GETDATA_RANDOM_DELAY = 2 * 1000000;
+static_assert(INBOUND_PEER_TX_DELAY >= MAX_GETDATA_RANDOM_DELAY,
+"To preserve security, MAX_GETDATA_RANDOM_DELAY should not exceed INBOUND_PEER_DELAY");
+/** Limit to avoid sending big packets. Not used in processing incoming GETDATA for compatibility */
+static const unsigned int MAX_GETDATA_SZ = 1000;
+
 
 struct COrphanTx {
     // When modifying, adapt the copy of this definition in tests/DoS_tests.
@@ -274,6 +289,66 @@ struct CNodeState {
     //! Time of last new block announcement
     int64_t m_last_block_announcement;
 
+    /*
+     * State associated with transaction download.
+     *
+     * Tx download algorithm:
+     *
+     *   When inv comes in, queue up (process_time, txid) inside the peer's
+     *   CNodeState (m_tx_process_time) as long as m_tx_announced for the peer
+     *   isn't too big (MAX_PEER_TX_ANNOUNCEMENTS).
+     *
+     *   The process_time for a transaction is set to nNow for outbound peers,
+     *   nNow + 2 seconds for inbound peers. This is the time at which we'll
+     *   consider trying to request the transaction from the peer in
+     *   SendMessages(). The delay for inbound peers is to allow outbound peers
+     *   a chance to announce before we request from inbound peers, to prevent
+     *   an adversary from using inbound connections to blind us to a
+     *   transaction (InvBlock).
+     *
+     *   When we call SendMessages() for a given peer,
+     *   we will loop over the transactions in m_tx_process_time, looking
+     *   at the transactions whose process_time <= nNow. We'll request each
+     *   such transaction that we don't have already and that hasn't been
+     *   requested from another peer recently, up until we hit the
+     *   MAX_PEER_TX_IN_FLIGHT limit for the peer. Then we'll update
+     *   g_already_asked_for for each requested txid, storing the time of the
+     *   GETDATA request. We use g_already_asked_for to coordinate transaction
+     *   requests amongst our peers.
+     *
+     *   For transactions that we still need but we have already recently
+     *   requested from some other peer, we'll reinsert (process_time, txid)
+     *   back into the peer's m_tx_process_time at the point in the future at
+     *   which the most recent GETDATA request would time out (ie
+     *   GETDATA_TX_INTERVAL + the request time stored in g_already_asked_for).
+     *   We add an additional delay for inbound peers, again to prefer
+     *   attempting download from outbound peers first.
+     *   We also add an extra small random delay up to 2 seconds
+     *   to avoid biasing some peers over others. (e.g., due to fixed ordering
+     *   of peer processing in ThreadMessageHandler).
+     *
+     *   When we receive a transaction from a peer, we remove the txid from the
+     *   peer's m_tx_in_flight set and from their recently announced set
+     *   (m_tx_announced).  We also clear g_already_asked_for for that entry, so
+     *   that if somehow the transaction is not accepted but also not added to
+     *   the reject filter, then we will eventually redownload from other
+     *   peers.
+     */
+    struct TxDownloadState {
+        /* Track when to attempt download of announced transactions (process
+         * time in micros -> txid)
+         */
+        std::multimap<int64_t, uint256> m_tx_process_time;
+
+        //! Store all the transactions a peer has recently announced
+        std::set<uint256> m_tx_announced;
+
+        //! Store transactions which were requested by us
+        std::set<uint256> m_tx_in_flight;
+    };
+
+    TxDownloadState m_tx_download;
+
     CNodeState(CAddress addrIn, std::string addrNameIn) : address(addrIn), name(addrNameIn) {
         fCurrentlyConnected = false;
         nMisbehavior = 0;
@@ -301,6 +376,9 @@ struct CNodeState {
     }
 };
 
+// Keeps track of the time (in microseconds) when transactions were requested last time
+limitedmap<uint256, int64_t> g_already_asked_for GUARDED_BY(cs_main)(MAX_INV_SZ);
+
 /** Map maintaining per-node state. */
 static std::map<NodeId, CNodeState> mapNodeState GUARDED_BY(cs_main);
 
@@ -591,6 +669,58 @@ static void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vec
     }
 }
 
+void EraseTxRequest(const uint256& txid) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+{
+    g_already_asked_for.erase(txid);
+}
+
+int64_t GetTxRequestTime(const uint256& txid) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+{
+    auto it = g_already_asked_for.find(txid);
+    if (it != g_already_asked_for.end()) {
+        return it->second;
+    }
+    return 0;
+}
+
+void UpdateTxRequestTime(const uint256& txid, int64_t request_time) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+{
+    auto it = g_already_asked_for.find(txid);
+    if (it == g_already_asked_for.end()) {
+        g_already_asked_for.insert(std::make_pair(txid, request_time));
+    } else {
+        g_already_asked_for.update(it, request_time);
+    }
+}
+
+
+void RequestTx(CNodeState* state, const uint256& txid, int64_t nNow) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+{
+    CNodeState::TxDownloadState& peer_download_state = state->m_tx_download;
+    if (peer_download_state.m_tx_announced.size() >= MAX_PEER_TX_ANNOUNCEMENTS || peer_download_state.m_tx_announced.count(txid)) {
+        // Too many queued announcements from this peer, or we already have
+        // this announcement
+        return;
+    }
+    peer_download_state.m_tx_announced.insert(txid);
+
+    int64_t process_time;
+    int64_t last_request_time = GetTxRequestTime(txid);
+    // First time requesting this tx
+    if (last_request_time == 0) {
+        process_time = nNow;
+    } else {
+        // Randomize the delay to avoid biasing some peers over others (such as due to
+        // fixed ordering of peer processing in ThreadMessageHandler)
+        process_time = last_request_time + GETDATA_TX_INTERVAL + GetRand(MAX_GETDATA_RANDOM_DELAY);
+    }
+
+    // We delay processing announcements from non-preferred (eg inbound) peers
+    if (!state->fPreferredDownload) process_time += INBOUND_PEER_TX_DELAY;
+
+    peer_download_state.m_tx_process_time.emplace(process_time, txid);
+}
+
 } // namespace
 
 // This function is used for testing the stale tip eviction logic, see
@@ -1945,6 +2075,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         LOCK(cs_main);
 
         uint32_t nFetchFlags = GetFetchFlags(pfrom);
+        int64_t nNow = GetTimeMicros();
 
         for (CInv &inv : vInv)
         {
@@ -1976,7 +2107,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
                 if (fBlocksOnly) {
                     LogPrint(BCLog::NET, "transaction (%s) inv sent in violation of protocol peer=%d\n", inv.hash.ToString(), pfrom->GetId());
                 } else if (!fAlreadyHave && !fImporting && !fReindex && !IsInitialBlockDownload()) {
-                    pfrom->AskFor(inv);
+                    RequestTx(State(pfrom->GetId()), inv.hash, nNow);
                 }
             }
         }
@@ -2211,8 +2342,10 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         bool fMissingInputs = false;
         CValidationState state;
 
-        pfrom->setAskFor.erase(inv.hash);
+        CNodeState* nodestate = State(pfrom->GetId());
-        mapAlreadyAskedFor.erase(inv.hash);
+        nodestate->m_tx_download.m_tx_announced.erase(inv.hash);
+        nodestate->m_tx_download.m_tx_in_flight.erase(inv.hash);
+        EraseTxRequest(inv.hash);
 
         std::list<CTransactionRef> lRemovedTxn;
 
@@ -2303,10 +2436,12 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
             }
             if (!fRejectedParents) {
                 uint32_t nFetchFlags = GetFetchFlags(pfrom);
+                int64_t nNow = GetTimeMicros();
+
                 for (const CTxIn& txin : tx.vin) {
                     CInv _inv(MSG_TX | nFetchFlags, txin.prevout.hash);
                     pfrom->AddInventoryKnown(_inv);
-                    if (!AlreadyHave(_inv)) pfrom->AskFor(_inv);
+                    if (!AlreadyHave(_inv)) RequestTx(State(pfrom->GetId()), _inv.hash, nNow);
                 }
                 AddOrphanTx(ptx, pfrom->GetId());
 
@@ -3731,24 +3866,39 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
         //
         // Message: getdata (non-blocks)
         //
-        while (!pto->mapAskFor.empty() && (*pto->mapAskFor.begin()).first <= nNow)
+        auto& tx_process_time = state.m_tx_download.m_tx_process_time;
-        {
+        while (!tx_process_time.empty() && tx_process_time.begin()->first <= nNow && state.m_tx_download.m_tx_in_flight.size() < MAX_PEER_TX_IN_FLIGHT) {
-            const CInv& inv = (*pto->mapAskFor.begin()).second;
+            const uint256& txid = tx_process_time.begin()->second;
-            if (!AlreadyHave(inv))
+            CInv inv(MSG_TX | GetFetchFlags(pto), txid);
-            {
+            if (!AlreadyHave(inv)) {
-                LogPrint(BCLog::NET, "Requesting %s peer=%d\n", inv.ToString(), pto->GetId());
+                // If this transaction was last requested more than 1 minute ago,
-                vGetData.push_back(inv);
+                // then request.
-                if (vGetData.size() >= 1000)
+                int64_t last_request_time = GetTxRequestTime(inv.hash);
-                {
+                if (last_request_time <= nNow - GETDATA_TX_INTERVAL) {
-                    connman->PushMessage(pto, msgMaker.Make(NetMsgType::GETDATA, vGetData));
+                    LogPrint(BCLog::NET, "Requesting %s peer=%d\n", inv.ToString(), pto->GetId());
-                    vGetData.clear();
+                    vGetData.push_back(inv);
+                    if (vGetData.size() >= MAX_GETDATA_SZ) {
+                        connman->PushMessage(pto, msgMaker.Make(NetMsgType::GETDATA, vGetData));
+                        vGetData.clear();
+                    }
+                    UpdateTxRequestTime(inv.hash, nNow);
+                    state.m_tx_download.m_tx_in_flight.insert(inv.hash);
+                } else {
+                    // This transaction is in flight from someone else; queue
+                    // up processing to happen after the download times out
+                    // (with a slight delay for inbound peers, to prefer
+                    // requests to outbound peers).
+                    RequestTx(&state, txid, nNow);
                 }
             } else {
-                //If we're not going to ask, don't expect a response.
+                // We have already seen this transaction, no need to download.
-                pto->setAskFor.erase(inv.hash);
+                state.m_tx_download.m_tx_announced.erase(inv.hash);
+                state.m_tx_download.m_tx_in_flight.erase(inv.hash);
             }
-            pto->mapAskFor.erase(pto->mapAskFor.begin());
+            tx_process_time.erase(tx_process_time.begin());
         }
+
+
         if (!vGetData.empty())
             connman->PushMessage(pto, msgMaker.Make(NetMsgType::GETDATA, vGetData));
 

src/test/util_tests.cpp

@@ -169,11 +169,6 @@ BOOST_AUTO_TEST_CASE(util_FormatISO8601Date)
     BOOST_CHECK_EQUAL(FormatISO8601Date(1317425777), "2011-09-30");
 }
 
-BOOST_AUTO_TEST_CASE(util_FormatISO8601Time)
-{
-    BOOST_CHECK_EQUAL(FormatISO8601Time(1317425777), "23:36:17Z");
-}
-
 struct TestArgsManager : public ArgsManager
 {
     TestArgsManager() { m_network_only_args.clear(); }

src/util/time.cpp

@@ -97,14 +97,3 @@ std::string FormatISO8601Date(int64_t nTime) {
 #endif
     return strprintf("%04i-%02i-%02i", ts.tm_year + 1900, ts.tm_mon + 1, ts.tm_mday);
 }
-
-std::string FormatISO8601Time(int64_t nTime) {
-    struct tm ts;
-    time_t time_val = nTime;
-#ifdef _MSC_VER
-    gmtime_s(&ts, &time_val);
-#else
-    gmtime_r(&time_val, &ts);
-#endif
-    return strprintf("%02i:%02i:%02iZ", ts.tm_hour, ts.tm_min, ts.tm_sec);
-}

src/util/time.h

@@ -33,6 +33,5 @@ void MilliSleep(int64_t n);
  */
 std::string FormatISO8601DateTime(int64_t nTime);
 std::string FormatISO8601Date(int64_t nTime);
-std::string FormatISO8601Time(int64_t nTime);
 
 #endif // BITCOIN_UTIL_TIME_H