Utilize anti-DoS headers download strategy

Avoid permanently storing headers from a peer, unless the headers are part of a
chain with sufficiently high work. This prevents memory attacks using low-work
headers.

Designed and co-authored with Pieter Wuille.

1 files changed, 317 insertions, 0 deletions

diff --git a/src/headerssync.cpp b/src/headerssync.cpp
new file mode 100644
index 0000000000..3eca492d81
--- /dev/null
+++ b/src/headerssync.cpp
@@ -0,0 +1,317 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <headerssync.h>
+#include <logging.h>
+#include <pow.h>
+#include <timedata.h>
+#include <util/check.h>
+
+// The two constants below are computed using the simulation script on
+// https://gist.github.com/sipa/016ae445c132cdf65a2791534dfb7ae1
+
+//! Store a commitment to a header every HEADER_COMMITMENT_PERIOD blocks.
+constexpr size_t HEADER_COMMITMENT_PERIOD{584};
+
+//! Only feed headers to validation once this many headers on top have been
+//! received and validated against commitments.
+constexpr size_t REDOWNLOAD_BUFFER_SIZE{13959}; // 13959/584 = ~23.9 commitments
+
+// Our memory analysis assumes 48 bytes for a CompressedHeader (so we should
+// re-calculate parameters if we compress further)
+static_assert(sizeof(CompressedHeader) == 48);
+
+HeadersSyncState::HeadersSyncState(NodeId id, const Consensus::Params& consensus_params,
+        const CBlockIndex* chain_start, const arith_uint256& minimum_required_work) :
+    m_id(id), m_consensus_params(consensus_params),
+    m_chain_start(chain_start),
+    m_minimum_required_work(minimum_required_work),
+    m_current_chain_work(chain_start->nChainWork),
+    m_commit_offset(GetRand<unsigned>(HEADER_COMMITMENT_PERIOD)),
+    m_last_header_received(m_chain_start->GetBlockHeader()),
+    m_current_height(chain_start->nHeight)
+{
+    // Estimate the number of blocks that could possibly exist on the peer's
+    // chain *right now* using 6 blocks/second (fastest blockrate given the MTP
+    // rule) times the number of seconds from the last allowed block until
+    // today. This serves as a memory bound on how many commitments we might
+    // store from this peer, and we can safely give up syncing if the peer
+    // exceeds this bound, because it's not possible for a consensus-valid
+    // chain to be longer than this (at the current time -- in the future we
+    // could try again, if necessary, to sync a longer chain).
+    m_max_commitments = 6*(Ticks<std::chrono::seconds>(GetAdjustedTime() - NodeSeconds{std::chrono::seconds{chain_start->GetMedianTimePast()}}) + MAX_FUTURE_BLOCK_TIME) / HEADER_COMMITMENT_PERIOD;
+
+    LogPrint(BCLog::HEADERSSYNC, "Initial headers sync started with peer=%d: height=%i, max_commitments=%i, min_work=%s\n", m_id, m_current_height, m_max_commitments, m_minimum_required_work.ToString());
+}
+
+/** Free any memory in use, and mark this object as no longer usable. This is
+ * required to guarantee that we won't reuse this object with the same
+ * SaltedTxidHasher for another sync. */
+void HeadersSyncState::Finalize()
+{
+    Assume(m_download_state != State::FINAL);
+    m_header_commitments = {};
+    m_last_header_received.SetNull();
+    m_redownloaded_headers = {};
+    m_redownload_buffer_last_hash.SetNull();
+    m_redownload_buffer_first_prev_hash.SetNull();
+    m_process_all_remaining_headers = false;
+    m_current_height = 0;
+
+    m_download_state = State::FINAL;
+}
+
+/** Process the next batch of headers received from our peer.
+ *  Validate and store commitments, and compare total chainwork to our target to
+ *  see if we can switch to REDOWNLOAD mode.  */
+HeadersSyncState::ProcessingResult HeadersSyncState::ProcessNextHeaders(const
+        std::vector<CBlockHeader>& received_headers, const bool full_headers_message)
+{
+    ProcessingResult ret;
+
+    Assume(!received_headers.empty());
+    if (received_headers.empty()) return ret;
+
+    Assume(m_download_state != State::FINAL);
+    if (m_download_state == State::FINAL) return ret;
+
+    if (m_download_state == State::PRESYNC) {
+        // During PRESYNC, we minimally validate block headers and
+        // occasionally add commitments to them, until we reach our work
+        // threshold (at which point m_download_state is updated to REDOWNLOAD).
+        ret.success = ValidateAndStoreHeadersCommitments(received_headers);
+        if (ret.success) {
+            if (full_headers_message || m_download_state == State::REDOWNLOAD) {
+                // A full headers message means the peer may have more to give us;
+                // also if we just switched to REDOWNLOAD then we need to re-request
+                // headers from the beginning.
+                ret.request_more = true;
+            } else {
+                Assume(m_download_state == State::PRESYNC);
+                // If we're in PRESYNC and we get a non-full headers
+                // message, then the peer's chain has ended and definitely doesn't
+                // have enough work, so we can stop our sync.
+                LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (presync phase)\n", m_id, m_current_height);
+            }
+        }
+    } else if (m_download_state == State::REDOWNLOAD) {
+        // During REDOWNLOAD, we compare our stored commitments to what we
+        // receive, and add headers to our redownload buffer. When the buffer
+        // gets big enough (meaning that we've checked enough commitments),
+        // we'll return a batch of headers to the caller for processing.
+        ret.success = true;
+        for (const auto& hdr : received_headers) {
+            if (!ValidateAndStoreRedownloadedHeader(hdr)) {
+                // Something went wrong -- the peer gave us an unexpected chain.
+                // We could consider looking at the reason for failure and
+                // punishing the peer, but for now just give up on sync.
+                ret.success = false;
+                break;
+            }
+        }
+
+        if (ret.success) {
+            // Return any headers that are ready for acceptance.
+            ret.pow_validated_headers = PopHeadersReadyForAcceptance();
+
+            // If we hit our target blockhash, then all remaining headers will be
+            // returned and we can clear any leftover internal state.
+            if (m_redownloaded_headers.empty() && m_process_all_remaining_headers) {
+                LogPrint(BCLog::HEADERSSYNC, "Initial headers sync complete with peer=%d: releasing all at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height);
+            } else if (full_headers_message) {
+                // If the headers message is full, we need to request more.
+                ret.request_more = true;
+            } else {
+                // For some reason our peer gave us a high-work chain, but is now
+                // declining to serve us that full chain again. Give up.
+                // Note that there's no more processing to be done with these
+                // headers, so we can still return success.
+                LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height);
+            }
+        }
+    }
+
+    if (!(ret.success && ret.request_more)) Finalize();
+    return ret;
+}
+
+bool HeadersSyncState::ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers)
+{
+    // The caller should not give us an empty set of headers.
+    Assume(headers.size() > 0);
+    if (headers.size() == 0) return true;
+
+    Assume(m_download_state == State::PRESYNC);
+    if (m_download_state != State::PRESYNC) return false;
+
+    if (headers[0].hashPrevBlock != m_last_header_received.GetHash()) {
+        // Somehow our peer gave us a header that doesn't connect.
+        // This might be benign -- perhaps our peer reorged away from the chain
+        // they were on. Give up on this sync for now (likely we will start a
+        // new sync with a new starting point).
+        LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (presync phase)\n", m_id, m_current_height);
+        return false;
+    }
+
+    // If it does connect, (minimally) validate and occasionally store
+    // commitments.
+    for (const auto& hdr : headers) {
+        if (!ValidateAndProcessSingleHeader(hdr)) {
+            return false;
+        }
+    }
+
+    if (m_current_chain_work >= m_minimum_required_work) {
+        m_redownloaded_headers.clear();
+        m_redownload_buffer_last_height = m_chain_start->nHeight;
+        m_redownload_buffer_first_prev_hash = m_chain_start->GetBlockHash();
+        m_redownload_buffer_last_hash = m_chain_start->GetBlockHash();
+        m_redownload_chain_work = m_chain_start->nChainWork;
+        m_download_state = State::REDOWNLOAD;
+        LogPrint(BCLog::HEADERSSYNC, "Initial headers sync transition with peer=%d: reached sufficient work at height=%i, redownloading from height=%i\n", m_id, m_current_height, m_redownload_buffer_last_height);
+    }
+    return true;
+}
+
+bool HeadersSyncState::ValidateAndProcessSingleHeader(const CBlockHeader& current)
+{
+    Assume(m_download_state == State::PRESYNC);
+    if (m_download_state != State::PRESYNC) return false;
+
+    int next_height = m_current_height + 1;
+
+    // Verify that the difficulty isn't growing too fast; an adversary with
+    // limited hashing capability has a greater chance of producing a high
+    // work chain if they compress the work into as few blocks as possible,
+    // so don't let anyone give a chain that would violate the difficulty
+    // adjustment maximum.
+    if (!PermittedDifficultyTransition(m_consensus_params, next_height,
+                m_last_header_received.nBits, current.nBits)) {
+        LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (presync phase)\n", m_id, next_height);
+        return false;
+    }
+
+    if (next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) {
+        // Add a commitment.
+        m_header_commitments.push_back(m_hasher(current.GetHash()) & 1);
+        if (m_header_commitments.size() > m_max_commitments) {
+            // The peer's chain is too long; give up.
+            // It's possible the chain grew since we started the sync; so
+            // potentially we could succeed in syncing the peer's chain if we
+            // try again later.
+            LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: exceeded max commitments at height=%i (presync phase)\n", m_id, next_height);
+            return false;
+        }
+    }
+
+    m_current_chain_work += GetBlockProof(CBlockIndex(current));
+    m_last_header_received = current;
+    m_current_height = next_height;
+
+    return true;
+}
+
+bool HeadersSyncState::ValidateAndStoreRedownloadedHeader(const CBlockHeader& header)
+{
+    Assume(m_download_state == State::REDOWNLOAD);
+    if (m_download_state != State::REDOWNLOAD) return false;
+
+    int64_t next_height = m_redownload_buffer_last_height + 1;
+
+    // Ensure that we're working on a header that connects to the chain we're
+    // downloading.
+    if (header.hashPrevBlock != m_redownload_buffer_last_hash) {
+        LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (redownload phase)\n", m_id, next_height);
+        return false;
+    }
+
+    // Check that the difficulty adjustments are within our tolerance:
+    uint32_t previous_nBits{0};
+    if (!m_redownloaded_headers.empty()) {
+        previous_nBits = m_redownloaded_headers.back().nBits;
+    } else {
+        previous_nBits = m_chain_start->nBits;
+    }
+
+    if (!PermittedDifficultyTransition(m_consensus_params, next_height,
+                previous_nBits, header.nBits)) {
+        LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (redownload phase)\n", m_id, next_height);
+        return false;
+    }
+
+    // Track work on the redownloaded chain
+    m_redownload_chain_work += GetBlockProof(CBlockIndex(header));
+
+    if (m_redownload_chain_work >= m_minimum_required_work) {
+        m_process_all_remaining_headers = true;
+    }
+
+    // If we're at a header for which we previously stored a commitment, verify
+    // it is correct. Failure will result in aborting download.
+    // Also, don't check commitments once we've gotten to our target blockhash;
+    // it's possible our peer has extended its chain between our first sync and
+    // our second, and we don't want to return failure after we've seen our
+    // target blockhash just because we ran out of commitments.
+    if (!m_process_all_remaining_headers && next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) {
+        if (m_header_commitments.size() == 0) {
+            LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: commitment overrun at height=%i (redownload phase)\n", m_id, next_height);
+            // Somehow our peer managed to feed us a different chain and
+            // we've run out of commitments.
+            return false;
+        }
+        bool commitment = m_hasher(header.GetHash()) & 1;
+        bool expected_commitment = m_header_commitments.front();
+        m_header_commitments.pop_front();
+        if (commitment != expected_commitment) {
+            LogPrint(BCLog::HEADERSSYNC, "Initial headers sync aborted with peer=%d: commitment mismatch at height=%i (redownload phase)\n", m_id, next_height);
+            return false;
+        }
+    }
+
+    // Store this header for later processing.
+    m_redownloaded_headers.push_back(header);
+    m_redownload_buffer_last_height = next_height;
+    m_redownload_buffer_last_hash = header.GetHash();
+
+    return true;
+}
+
+std::vector<CBlockHeader> HeadersSyncState::PopHeadersReadyForAcceptance()
+{
+    std::vector<CBlockHeader> ret;
+
+    Assume(m_download_state == State::REDOWNLOAD);
+    if (m_download_state != State::REDOWNLOAD) return ret;
+
+    while (m_redownloaded_headers.size() > REDOWNLOAD_BUFFER_SIZE ||
+            (m_redownloaded_headers.size() > 0 && m_process_all_remaining_headers)) {
+        ret.emplace_back(m_redownloaded_headers.front().GetFullHeader(m_redownload_buffer_first_prev_hash));
+        m_redownloaded_headers.pop_front();
+        m_redownload_buffer_first_prev_hash = ret.back().GetHash();
+    }
+    return ret;
+}
+
+CBlockLocator HeadersSyncState::NextHeadersRequestLocator() const
+{
+    Assume(m_download_state != State::FINAL);
+    if (m_download_state == State::FINAL) return {};
+
+    auto chain_start_locator = LocatorEntries(m_chain_start);
+    std::vector<uint256> locator;
+
+    if (m_download_state == State::PRESYNC) {
+        // During pre-synchronization, we continue from the last header received.
+        locator.push_back(m_last_header_received.GetHash());
+    }
+
+    if (m_download_state == State::REDOWNLOAD) {
+        // During redownload, we will download from the last received header that we stored.
+        locator.push_back(m_redownload_buffer_last_hash);
+    }
+
+    locator.insert(locator.end(), chain_start_locator.begin(), chain_start_locator.end());
+
+    return CBlockLocator{std::move(locator)};
+}