1 // Copyright (c) 2022-present The Bitcoin Core developers
2 // Distributed under the MIT software license, see the accompanying
3 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
4 5 #include <node/eviction.h>
6 7 #include <algorithm>
8 #include <array>
9 #include <chrono>
10 #include <cstdint>
11 #include <functional>
12 #include <map>
13 #include <vector>
14 15 16 static bool ReverseCompareNodeMinPingTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
17 {
18 return a.m_min_ping_time > b.m_min_ping_time;
19 }
20 21 static bool ReverseCompareNodeTimeConnected(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
22 {
23 return a.m_connected > b.m_connected;
24 }
25 26 static bool CompareNetGroupKeyed(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b) {
27 return a.nKeyedNetGroup < b.nKeyedNetGroup;
28 }
29 30 static bool CompareNodeBlockTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
31 {
32 // There is a fall-through here because it is common for a node to have many peers which have not yet relayed a block.
33 if (a.m_last_block_time != b.m_last_block_time) return a.m_last_block_time < b.m_last_block_time;
34 if (a.fRelevantServices != b.fRelevantServices) return b.fRelevantServices;
35 return a.m_connected > b.m_connected;
36 }
37 38 static bool CompareNodeTXTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
39 {
40 // There is a fall-through here because it is common for a node to have more than a few peers that have not yet relayed txn.
41 if (a.m_last_tx_time != b.m_last_tx_time) return a.m_last_tx_time < b.m_last_tx_time;
42 if (a.m_relay_txs != b.m_relay_txs) return b.m_relay_txs;
43 if (a.fBloomFilter != b.fBloomFilter) return a.fBloomFilter;
44 return a.m_connected > b.m_connected;
45 }
46 47 // Pick out the potential block-relay only peers, and sort them by last block time.
48 static bool CompareNodeBlockRelayOnlyTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
49 {
50 if (a.m_relay_txs != b.m_relay_txs) return a.m_relay_txs;
51 if (a.m_last_block_time != b.m_last_block_time) return a.m_last_block_time < b.m_last_block_time;
52 if (a.fRelevantServices != b.fRelevantServices) return b.fRelevantServices;
53 return a.m_connected > b.m_connected;
54 }
55 56 /**
57 * Sort eviction candidates by network/localhost and connection uptime.
58 * Candidates near the beginning are more likely to be evicted, and those
59 * near the end are more likely to be protected, e.g. less likely to be evicted.
60 * - First, nodes that are not `is_local` and that do not belong to `network`,
61 * sorted by increasing uptime (from most recently connected to connected longer).
62 * - Then, nodes that are `is_local` or belong to `network`, sorted by increasing uptime.
63 */
64 struct CompareNodeNetworkTime {
65 const bool m_is_local;
66 const Network m_network;
67 CompareNodeNetworkTime(bool is_local, Network network) : m_is_local(is_local), m_network(network) {}
68 bool operator()(const NodeEvictionCandidate& a, const NodeEvictionCandidate& b) const
69 {
70 if (m_is_local && a.m_is_local != b.m_is_local) return b.m_is_local;
71 if ((a.m_network == m_network) != (b.m_network == m_network)) return b.m_network == m_network;
72 return a.m_connected > b.m_connected;
73 };
74 };
75 76 //! Sort an array by the specified comparator, then erase the last K elements where predicate is true.
77 template <typename T, typename Comparator>
78 static void EraseLastKElements(
79 std::vector<T>& elements, Comparator comparator, size_t k,
80 std::function<bool(const NodeEvictionCandidate&)> predicate = [](const NodeEvictionCandidate& n) { return true; })
81 {
82 std::sort(elements.begin(), elements.end(), comparator);
83 size_t eraseSize = std::min(k, elements.size());
84 elements.erase(std::remove_if(elements.end() - eraseSize, elements.end(), predicate), elements.end());
85 }
86 87 void ProtectNoBanConnections(std::vector<NodeEvictionCandidate>& eviction_candidates)
88 {
89 eviction_candidates.erase(std::remove_if(eviction_candidates.begin(), eviction_candidates.end(),
90 [](NodeEvictionCandidate const& n) {
91 return n.m_noban;
92 }),
93 eviction_candidates.end());
94 }
95 96 void ProtectOutboundConnections(std::vector<NodeEvictionCandidate>& eviction_candidates)
97 {
98 eviction_candidates.erase(std::remove_if(eviction_candidates.begin(), eviction_candidates.end(),
99 [](NodeEvictionCandidate const& n) {
100 return n.m_conn_type != ConnectionType::INBOUND;
101 }),
102 eviction_candidates.end());
103 }
104 105 void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& eviction_candidates)
106 {
107 // Protect the half of the remaining nodes which have been connected the longest.
108 // This replicates the non-eviction implicit behavior, and precludes attacks that start later.
109 // To favorise the diversity of our peer connections, reserve up to half of these protected
110 // spots for Tor/onion, localhost, I2P, and CJDNS peers, even if they're not longest uptime
111 // overall. This helps protect these higher-latency peers that tend to be otherwise
112 // disadvantaged under our eviction criteria.
113 const size_t initial_size = eviction_candidates.size();
114 const size_t total_protect_size{initial_size / 2};
115 116 // Disadvantaged networks to protect. In the case of equal counts, earlier array members
117 // have the first opportunity to recover unused slots from the previous iteration.
118 struct Net { bool is_local; Network id; size_t count; };
119 std::array<Net, 4> networks{
120 {{false, NET_CJDNS, 0}, {false, NET_I2P, 0}, {/*localhost=*/true, NET_MAX, 0}, {false, NET_ONION, 0}}};
121 122 // Count and store the number of eviction candidates per network.
123 for (Net& n : networks) {
124 n.count = std::count_if(eviction_candidates.cbegin(), eviction_candidates.cend(),
125 [&n](const NodeEvictionCandidate& c) {
126 return n.is_local ? c.m_is_local : c.m_network == n.id;
127 });
128 }
129 // Sort `networks` by ascending candidate count, to give networks having fewer candidates
130 // the first opportunity to recover unused protected slots from the previous iteration.
131 std::stable_sort(networks.begin(), networks.end(), [](Net a, Net b) { return a.count < b.count; });
132 133 // Protect up to 25% of the eviction candidates by disadvantaged network.
134 const size_t max_protect_by_network{total_protect_size / 2};
135 size_t num_protected{0};
136 137 while (num_protected < max_protect_by_network) {
138 // Count the number of disadvantaged networks from which we have peers to protect.
139 auto num_networks = std::count_if(networks.begin(), networks.end(), [](const Net& n) { return n.count; });
140 if (num_networks == 0) {
141 break;
142 }
143 const size_t disadvantaged_to_protect{max_protect_by_network - num_protected};
144 const size_t protect_per_network{std::max(disadvantaged_to_protect / num_networks, static_cast<size_t>(1))};
145 // Early exit flag if there are no remaining candidates by disadvantaged network.
146 bool protected_at_least_one{false};
147 148 for (Net& n : networks) {
149 if (n.count == 0) continue;
150 const size_t before = eviction_candidates.size();
151 EraseLastKElements(eviction_candidates, CompareNodeNetworkTime(n.is_local, n.id),
152 protect_per_network, [&n](const NodeEvictionCandidate& c) {
153 return n.is_local ? c.m_is_local : c.m_network == n.id;
154 });
155 const size_t after = eviction_candidates.size();
156 if (before > after) {
157 protected_at_least_one = true;
158 const size_t delta{before - after};
159 num_protected += delta;
160 if (num_protected >= max_protect_by_network) {
161 break;
162 }
163 n.count -= delta;
164 }
165 }
166 if (!protected_at_least_one) {
167 break;
168 }
169 }
170 171 // Calculate how many we removed, and update our total number of peers that
172 // we want to protect based on uptime accordingly.
173 assert(num_protected == initial_size - eviction_candidates.size());
174 const size_t remaining_to_protect{total_protect_size - num_protected};
175 EraseLastKElements(eviction_candidates, ReverseCompareNodeTimeConnected, remaining_to_protect);
176 }
177 178 [[nodiscard]] std::optional<NodeId> SelectNodeToEvict(std::vector<NodeEvictionCandidate>&& vEvictionCandidates)
179 {
180 // Protect connections with certain characteristics
181 182 ProtectNoBanConnections(vEvictionCandidates);
183 184 ProtectOutboundConnections(vEvictionCandidates);
185 186 // Deterministically select 4 peers to protect by netgroup.
187 // An attacker cannot predict which netgroups will be protected
188 EraseLastKElements(vEvictionCandidates, CompareNetGroupKeyed, 4);
189 // Protect the 8 nodes with the lowest minimum ping time.
190 // An attacker cannot manipulate this metric without physically moving nodes closer to the target.
191 EraseLastKElements(vEvictionCandidates, ReverseCompareNodeMinPingTime, 8);
192 // Protect 4 nodes that most recently sent us novel transactions accepted into our mempool.
193 // An attacker cannot manipulate this metric without performing useful work.
194 EraseLastKElements(vEvictionCandidates, CompareNodeTXTime, 4);
195 // Protect up to 8 non-tx-relay peers that have sent us novel blocks.
196 EraseLastKElements(vEvictionCandidates, CompareNodeBlockRelayOnlyTime, 8,
197 [](const NodeEvictionCandidate& n) { return !n.m_relay_txs && n.fRelevantServices; });
198 199 // Protect 4 nodes that most recently sent us novel blocks.
200 // An attacker cannot manipulate this metric without performing useful work.
201 EraseLastKElements(vEvictionCandidates, CompareNodeBlockTime, 4);
202 203 // Protect some of the remaining eviction candidates by ratios of desirable
204 // or disadvantaged characteristics.
205 ProtectEvictionCandidatesByRatio(vEvictionCandidates);
206 207 if (vEvictionCandidates.empty()) return std::nullopt;
208 209 // If any remaining peers are preferred for eviction consider only them.
210 // This happens after the other preferences since if a peer is really the best by other criteria (esp relaying blocks)
211 // then we probably don't want to evict it no matter what.
212 if (std::any_of(vEvictionCandidates.begin(),vEvictionCandidates.end(),[](NodeEvictionCandidate const &n){return n.prefer_evict;})) {
213 vEvictionCandidates.erase(std::remove_if(vEvictionCandidates.begin(),vEvictionCandidates.end(),
214 [](NodeEvictionCandidate const &n){return !n.prefer_evict;}),vEvictionCandidates.end());
215 }
216 217 // Identify the network group with the most connections and youngest member.
218 // (vEvictionCandidates is already sorted by reverse connect time)
219 uint64_t naMostConnections;
220 unsigned int nMostConnections = 0;
221 NodeClock::time_point nMostConnectionsTime{NodeClock::epoch};
222 std::map<uint64_t, std::vector<NodeEvictionCandidate> > mapNetGroupNodes;
223 for (const NodeEvictionCandidate &node : vEvictionCandidates) {
224 std::vector<NodeEvictionCandidate> &group = mapNetGroupNodes[node.nKeyedNetGroup];
225 group.push_back(node);
226 const auto grouptime{group[0].m_connected};
227 228 if (group.size() > nMostConnections || (group.size() == nMostConnections && grouptime > nMostConnectionsTime)) {
229 nMostConnections = group.size();
230 nMostConnectionsTime = grouptime;
231 naMostConnections = node.nKeyedNetGroup;
232 }
233 }
234 235 // Reduce to the network group with the most connections
236 vEvictionCandidates = std::move(mapNetGroupNodes[naMostConnections]);
237 238 // Disconnect from the network group with the most connections
239 return vEvictionCandidates.front().id;
240 }
241