Merge #19109: Only allow getdata of recently announced invs

f32c408f3a0b7e597977df2bc2cdc4ae298586e5 Make sure unconfirmed parents are requestable (Pieter Wuille)
c4626bcd211af08c85b6567ef07eeae333edba47 Drop setInventoryTxToSend based filtering (Pieter Wuille)
43f02ccbff9b137d59458da7a8afdb0bf80e127f Only respond to requests for recently announced transactions (Pieter Wuille)
b24a17f03982c9cd8fd6ec665b16e022374c96f0 Introduce constant for mempool-based relay separate from mapRelay caching (Pieter Wuille)
a9bc5638031a29abaa40284273a3507b345c31e9 Swap relay pool and mempool lookup (Pieter Wuille)

Pull request description:

  This implements the follow-up suggested here: https://github.com/bitcoin/bitcoin/pull/18861#issuecomment-627630111 . Instead of checking `setInventoryTxToSend`, maintain an explicit bloom filter with the 3500 most recently announced invs, and permit fetching any of these as long as they're in the relay pool or the mempool. In addition, permit relay from the mempool after just 2 minutes instead of 15.

  This:

  * Fixes the brief opportunity an attacker has to request unannounced invs just after the connection is established (pointed out by naumenkogs, see https://github.com/bitcoin/bitcoin/pull/18861#issuecomment-627627010).
  * Guarantees that locally resubmitted invs after `filterInventoryKnown` rolls over can still be requested (pointed out by luke-jr, see https://github.com/bitcoin/bitcoin/pull/18861#discussion_r419695831).

  It adds 37 KiB of filter per peer.

  This is also a step towards dropping the relay pool entirely and always relaying from the mempool directly (see #17303), but that is still blocked by dealing properly with NOTFOUNDs (see #18238).

ACKs for top commit:
  jnewbery:
    reACK f32c408f3
  jonatack:
    re-ACK f32c408 per `git range-diff f7c19e8 2da7ee3 f32c408` and redid the following: code review, thought about motivation, DoS and privacy aspects, debug build to check for warnings after updating Clang from 6 to 11 since last review.
  ajtowns:
    re-ACK f32c408f3a0b7e597977df2bc2cdc4ae298586e5

Tree-SHA512: aa05b9fd01bad59581c4ec91836a52d7415dc933fa49d4c4adced79aa25aaad51e11166357e8c8b29fbf6021a7401b98c21b850b5d8e8ad773fdb5d6608e1e85

1 files changed, 49 insertions, 24 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index bfc60b18f9..1fb6accb82 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -38,7 +38,9 @@ static constexpr int64_t ORPHAN_TX_EXPIRE_TIME = 20 * 60;
 /** Minimum time between orphan transactions expire time checks in seconds */
 static constexpr int64_t ORPHAN_TX_EXPIRE_INTERVAL = 5 * 60;
 /** How long to cache transactions in mapRelay for normal relay */
-static constexpr std::chrono::seconds RELAY_TX_CACHE_TIME{15 * 60};
+static constexpr std::chrono::seconds RELAY_TX_CACHE_TIME = std::chrono::minutes{15};
+/** How long a transaction has to be in the mempool before it can unconditionally be relayed (even when not in mapRelay). */
+static constexpr std::chrono::seconds UNCONDITIONAL_RELAY_DELAY = std::chrono::minutes{2};
 /** Headers download timeout expressed in microseconds
  *  Timeout = base + per_header * (expected number of headers) */
 static constexpr int64_t HEADERS_DOWNLOAD_TIMEOUT_BASE = 15 * 60 * 1000000; // 15 minutes
@@ -119,9 +121,18 @@ static constexpr std::chrono::seconds AVG_ADDRESS_BROADCAST_INTERVAL{30};
 /** Average delay between trickled inventory transmissions in seconds.
  *  Blocks and peers with noban permission bypass this, outbound peers get half this delay. */
 static const unsigned int INVENTORY_BROADCAST_INTERVAL = 5;
-/** Maximum number of inventory items to send per transmission.
+/** Maximum rate of inventory items to send per second.
  *  Limits the impact of low-fee transaction floods. */
-static constexpr unsigned int INVENTORY_BROADCAST_MAX = 7 * INVENTORY_BROADCAST_INTERVAL;
+static constexpr unsigned int INVENTORY_BROADCAST_PER_SECOND = 7;
+/** Maximum number of inventory items to send per transmission. */
+static constexpr unsigned int INVENTORY_BROADCAST_MAX = INVENTORY_BROADCAST_PER_SECOND * INVENTORY_BROADCAST_INTERVAL;
+/** The number of most recently announced transactions a peer can request. */
+static constexpr unsigned int INVENTORY_MAX_RECENT_RELAY = 3500;
+/** Verify that INVENTORY_MAX_RECENT_RELAY is enough to cache everything typically
+ *  relayed before unconditional relay from the mempool kicks in. This is only a
+ *  lower bound, and it should be larger to account for higher inv rate to outbound
+ *  peers, and random variations in the broadcast mechanism. */
+static_assert(INVENTORY_MAX_RECENT_RELAY >= INVENTORY_BROADCAST_PER_SECOND * UNCONDITIONAL_RELAY_DELAY / std::chrono::seconds{1}, "INVENTORY_RELAY_MAX too low");
 /** Average delay between feefilter broadcasts in seconds. */
 static constexpr unsigned int AVG_FEEFILTER_BROADCAST_INTERVAL = 10 * 60;
 /** Maximum feefilter broadcast delay after significant change. */
@@ -395,6 +406,9 @@ struct CNodeState {
     //! Whether this peer is a manual connection
     bool m_is_manual_connection;
 
+    //! A rolling bloom filter of all announced tx CInvs to this peer.
+    CRollingBloomFilter m_recently_announced_invs = CRollingBloomFilter{INVENTORY_MAX_RECENT_RELAY, 0.000001};
+
     CNodeState(CAddress addrIn, std::string addrNameIn, bool is_inbound, bool is_manual) :
         address(addrIn), name(std::move(addrNameIn)), m_is_inbound(is_inbound),
         m_is_manual_connection (is_manual)
@@ -422,6 +436,7 @@ struct CNodeState {
         fSupportsDesiredCmpctVersion = false;
         m_chain_sync = { 0, nullptr, false, false };
         m_last_block_announcement = 0;
+        m_recently_announced_invs.reset();
     }
 };
 
@@ -1617,30 +1632,28 @@ void static ProcessGetBlockData(CNode& pfrom, const CChainParams& chainparams, c
 }
 
 //! Determine whether or not a peer can request a transaction, and return it (or nullptr if not found or not allowed).
-CTransactionRef static FindTxForGetData(CNode& peer, const uint256& txid, const std::chrono::seconds mempool_req, const std::chrono::seconds longlived_mempool_time) LOCKS_EXCLUDED(cs_main)
+CTransactionRef static FindTxForGetData(const CNode& peer, const uint256& txid, const std::chrono::seconds mempool_req, const std::chrono::seconds now) LOCKS_EXCLUDED(cs_main)
 {
-    // Check if the requested transaction is so recent that we're just
-    // about to announce it to the peer; if so, they certainly shouldn't
-    // know we already have it.
-    {
-        LOCK(peer.m_tx_relay->cs_tx_inventory);
-        if (peer.m_tx_relay->setInventoryTxToSend.count(txid)) return {};
+    auto txinfo = mempool.info(txid);
+    if (txinfo.tx) {
+        // If a TX could have been INVed in reply to a MEMPOOL request,
+        // or is older than UNCONDITIONAL_RELAY_DELAY, permit the request
+        // unconditionally.
+        if ((mempool_req.count() && txinfo.m_time <= mempool_req) || txinfo.m_time <= now - UNCONDITIONAL_RELAY_DELAY) {
+            return std::move(txinfo.tx);
+        }
     }
 
     {
         LOCK(cs_main);
-        // Look up transaction in relay pool
-        auto mi = mapRelay.find(txid);
-        if (mi != mapRelay.end()) return mi->second;
-    }
 
-    auto txinfo = mempool.info(txid);
-    if (txinfo.tx) {
-        // To protect privacy, do not answer getdata using the mempool when
-        // that TX couldn't have been INVed in reply to a MEMPOOL request,
-        // or when it's too recent to have expired from mapRelay.
-        if ((mempool_req.count() && txinfo.m_time <= mempool_req) || txinfo.m_time <= longlived_mempool_time) {
-            return txinfo.tx;
+        // Otherwise, the transaction must have been announced recently.
+        if (State(peer.GetId())->m_recently_announced_invs.contains(txid)) {
+            // If it was, it can be relayed from either the mempool...
+            if (txinfo.tx) return std::move(txinfo.tx);
+            // ... or the relay pool.
+            auto mi = mapRelay.find(txid);
+            if (mi != mapRelay.end()) return mi->second;
         }
     }
 
@@ -1655,8 +1668,7 @@ void static ProcessGetData(CNode& pfrom, const CChainParams& chainparams, CConnm
     std::vector<CInv> vNotFound;
     const CNetMsgMaker msgMaker(pfrom.GetSendVersion());
 
-    // mempool entries added before this time have likely expired from mapRelay
-    const std::chrono::seconds longlived_mempool_time = GetTime<std::chrono::seconds>() - RELAY_TX_CACHE_TIME;
+    const std::chrono::seconds now = GetTime<std::chrono::seconds>();
     // Get last mempool request time
     const std::chrono::seconds mempool_req = pfrom.m_tx_relay != nullptr ? pfrom.m_tx_relay->m_last_mempool_req.load()
                                                                           : std::chrono::seconds::min();
@@ -1677,11 +1689,22 @@ void static ProcessGetData(CNode& pfrom, const CChainParams& chainparams, CConnm
             continue;
         }
 
-        CTransactionRef tx = FindTxForGetData(pfrom, inv.hash, mempool_req, longlived_mempool_time);
+        CTransactionRef tx = FindTxForGetData(pfrom, inv.hash, mempool_req, now);
         if (tx) {
             int nSendFlags = (inv.type == MSG_TX ? SERIALIZE_TRANSACTION_NO_WITNESS : 0);
             connman->PushMessage(&pfrom, msgMaker.Make(nSendFlags, NetMsgType::TX, *tx));
             mempool.RemoveUnbroadcastTx(inv.hash);
+            // As we're going to send tx, make sure its unconfirmed parents are made requestable.
+            for (const auto& txin : tx->vin) {
+                auto txinfo = mempool.info(txin.prevout.hash);
+                if (txinfo.tx && txinfo.m_time > now - UNCONDITIONAL_RELAY_DELAY) {
+                    // Relaying a transaction with a recent but unconfirmed parent.
+                    if (WITH_LOCK(pfrom.m_tx_relay->cs_tx_inventory, return !pfrom.m_tx_relay->filterInventoryKnown.contains(txin.prevout.hash))) {
+                        LOCK(cs_main);
+                        State(pfrom.GetId())->m_recently_announced_invs.insert(txin.prevout.hash);
+                    }
+                }
+            }
         } else {
             vNotFound.push_back(inv);
         }
@@ -4149,6 +4172,7 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                             if (!pto->m_tx_relay->pfilter->IsRelevantAndUpdate(*txinfo.tx)) continue;
                         }
                         pto->m_tx_relay->filterInventoryKnown.insert(hash);
+                        // Responses to MEMPOOL requests bypass the m_recently_announced_invs filter.
                         vInv.push_back(inv);
                         if (vInv.size() == MAX_INV_SZ) {
                             connman->PushMessage(pto, msgMaker.Make(NetMsgType::INV, vInv));
@@ -4202,6 +4226,7 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                         }
                         if (pto->m_tx_relay->pfilter && !pto->m_tx_relay->pfilter->IsRelevantAndUpdate(*txinfo.tx)) continue;
                         // Send
+                        State(pto->GetId())->m_recently_announced_invs.insert(hash);
                         vInv.push_back(CInv(MSG_TX, hash));
                         nRelayedTransactions++;
                         {