Line data    Source code

       1             : // Copyright (c) 2022 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             : #ifndef BITCOIN_HEADERSSYNC_H
       6             : #define BITCOIN_HEADERSSYNC_H
       7             : 
       8             : #include <arith_uint256.h>
       9             : #include <chain.h>
      10             : #include <consensus/params.h>
      11             : #include <net.h> // For NodeId
      12             : #include <primitives/block.h>
      13             : #include <uint256.h>
      14             : #include <util/bitdeque.h>
      15             : #include <util/hasher.h>
      16             : 
      17             : #include <deque>
      18             : #include <vector>
      19             : 
      20             : // A compressed CBlockHeader, which leaves out the prevhash
      21             : struct CompressedHeader {
      22             :     // header
      23           0 :     int32_t nVersion{0};
      24             :     uint256 hashMerkleRoot;
      25           0 :     uint32_t nTime{0};
      26           0 :     uint32_t nBits{0};
      27           0 :     uint32_t nNonce{0};
      28             : 
      29             :     CompressedHeader()
      30             :     {
      31             :         hashMerkleRoot.SetNull();
      32             :     }
      33             : 
      34           0 :     CompressedHeader(const CBlockHeader& header)
      35             :     {
      36           0 :         nVersion = header.nVersion;
      37           0 :         hashMerkleRoot = header.hashMerkleRoot;
      38           0 :         nTime = header.nTime;
      39           0 :         nBits = header.nBits;
      40           0 :         nNonce = header.nNonce;
      41           0 :     }
      42             : 
      43           0 :     CBlockHeader GetFullHeader(const uint256& hash_prev_block) {
      44           0 :         CBlockHeader ret;
      45           0 :         ret.nVersion = nVersion;
      46           0 :         ret.hashPrevBlock = hash_prev_block;
      47           0 :         ret.hashMerkleRoot = hashMerkleRoot;
      48           0 :         ret.nTime = nTime;
      49           0 :         ret.nBits = nBits;
      50           0 :         ret.nNonce = nNonce;
      51           0 :         return ret;
      52             :     };
      53             : };
      54             : 
      55             : /** HeadersSyncState:
      56             :  *
      57             :  * We wish to download a peer's headers chain in a DoS-resistant way.
      58             :  *
      59             :  * The Bitcoin protocol does not offer an easy way to determine the work on a
      60             :  * peer's chain. Currently, we can query a peer's headers by using a GETHEADERS
      61             :  * message, and our peer can return a set of up to 2000 headers that connect to
      62             :  * something we know. If a peer's chain has more than 2000 blocks, then we need
      63             :  * a way to verify that the chain actually has enough work on it to be useful to
      64             :  * us -- by being above our anti-DoS minimum-chain-work threshold -- before we
      65             :  * commit to storing those headers in memory. Otherwise, it would be cheap for
      66             :  * an attacker to waste all our memory by serving us low-work headers
      67             :  * (particularly for a new node coming online for the first time).
      68             :  *
      69             :  * To prevent memory-DoS with low-work headers, while still always being
      70             :  * able to reorg to whatever the most-work chain is, we require that a chain
      71             :  * meet a work threshold before committing it to memory. We can do this by
      72             :  * downloading a peer's headers twice, whenever we are not sure that the chain
      73             :  * has sufficient work:
      74             :  *
      75             :  * - In the first download phase, called pre-synchronization, we can calculate
      76             :  * the work on the chain as we go (just by checking the nBits value on each
      77             :  * header, and validating the proof-of-work).
      78             :  *
      79             :  * - Once we have reached a header where the cumulative chain work is
      80             :  * sufficient, we switch to downloading the headers a second time, this time
      81             :  * processing them fully, and possibly storing them in memory.
      82             :  *
      83             :  * To prevent an attacker from using (eg) the honest chain to convince us that
      84             :  * they have a high-work chain, but then feeding us an alternate set of
      85             :  * low-difficulty headers in the second phase, we store commitments to the
      86             :  * chain we see in the first download phase that we check in the second phase,
      87             :  * as follows:
      88             :  *
      89             :  * - In phase 1 (presync), store 1 bit (using a salted hash function) for every
      90             :  * N headers that we see. With a reasonable choice of N, this uses relatively
      91             :  * little memory even for a very long chain.
      92             :  *
      93             :  * - In phase 2 (redownload), keep a lookahead buffer and only accept headers
      94             :  * from that buffer into the block index (permanent memory usage) once they
      95             :  * have some target number of verified commitments on top of them. With this
      96             :  * parametrization, we can achieve a given security target for potential
      97             :  * permanent memory usage, while choosing N to minimize memory use during the
      98             :  * sync (temporary, per-peer storage).
      99             :  */
     100             : 
     101             : class HeadersSyncState {
     102             : public:
     103           0 :     ~HeadersSyncState() {}
     104             : 
     105             :     enum class State {
     106             :         /** PRESYNC means the peer has not yet demonstrated their chain has
     107             :          * sufficient work and we're only building commitments to the chain they
     108             :          * serve us. */
     109             :         PRESYNC,
     110             :         /** REDOWNLOAD means the peer has given us a high-enough-work chain,
     111             :          * and now we're redownloading the headers we saw before and trying to
     112             :          * accept them */
     113             :         REDOWNLOAD,
     114             :         /** We're done syncing with this peer and can discard any remaining state */
     115             :         FINAL
     116             :     };
     117             : 
     118             :     /** Return the current state of our download */
     119           0 :     State GetState() const { return m_download_state; }
     120             : 
     121             :     /** Return the height reached during the PRESYNC phase */
     122           0 :     int64_t GetPresyncHeight() const { return m_current_height; }
     123             : 
     124             :     /** Return the block timestamp of the last header received during the PRESYNC phase. */
     125           0 :     uint32_t GetPresyncTime() const { return m_last_header_received.nTime; }
     126             : 
     127             :     /** Return the amount of work in the chain received during the PRESYNC phase. */
     128           0 :     arith_uint256 GetPresyncWork() const { return m_current_chain_work; }
     129             : 
     130             :     /** Construct a HeadersSyncState object representing a headers sync via this
     131             :      *  download-twice mechanism).
     132             :      *
     133             :      * id: node id (for logging)
     134             :      * consensus_params: parameters needed for difficulty adjustment validation
     135             :      * chain_start: best known fork point that the peer's headers branch from
     136             :      * minimum_required_work: amount of chain work required to accept the chain
     137             :      */
     138             :     HeadersSyncState(NodeId id, const Consensus::Params& consensus_params,
     139             :             const CBlockIndex* chain_start, const arith_uint256& minimum_required_work);
     140             : 
     141             :     /** Result data structure for ProcessNextHeaders. */
     142           0 :     struct ProcessingResult {
     143             :         std::vector<CBlockHeader> pow_validated_headers;
     144           0 :         bool success{false};
     145           0 :         bool request_more{false};
     146             :     };
     147             : 
     148             :     /** Process a batch of headers, once a sync via this mechanism has started
     149             :      *
     150             :      * received_headers: headers that were received over the network for processing.
     151             :      *                   Assumes the caller has already verified the headers
     152             :      *                   are continuous, and has checked that each header
     153             :      *                   satisfies the proof-of-work target included in the
     154             :      *                   header (but not necessarily verified that the
     155             :      *                   proof-of-work target is correct and passes consensus
     156             :      *                   rules).
     157             :      * full_headers_message: true if the message was at max capacity,
     158             :      *                       indicating more headers may be available
     159             :      * ProcessingResult.pow_validated_headers: will be filled in with any
     160             :      *                       headers that the caller can fully process and
     161             :      *                       validate now (because these returned headers are
     162             :      *                       on a chain with sufficient work)
     163             :      * ProcessingResult.success: set to false if an error is detected and the sync is
     164             :      *                       aborted; true otherwise.
     165             :      * ProcessingResult.request_more: if true, the caller is suggested to call
     166             :      *                       NextHeadersRequestLocator and send a getheaders message using it.
     167             :      */
     168             :     ProcessingResult ProcessNextHeaders(const std::vector<CBlockHeader>&
     169             :             received_headers, bool full_headers_message);
     170             : 
     171             :     /** Issue the next GETHEADERS message to our peer.
     172             :      *
     173             :      * This will return a locator appropriate for the current sync object, to continue the
     174             :      * synchronization phase it is in.
     175             :      */
     176             :     CBlockLocator NextHeadersRequestLocator() const;
     177             : 
     178             : protected:
     179             :     /** The (secret) offset on the heights for which to create commitments.
     180             :      *
     181             :      * m_header_commitments entries are created at any height h for which
     182             :      * (h % HEADER_COMMITMENT_PERIOD) == m_commit_offset. */
     183             :     const unsigned m_commit_offset;
     184             : 
     185             : private:
     186             :     /** Clear out all download state that might be in progress (freeing any used
     187             :      * memory), and mark this object as no longer usable.
     188             :      */
     189             :     void Finalize();
     190             : 
     191             :     /**
     192             :      *  Only called in PRESYNC.
     193             :      *  Validate the work on the headers we received from the network, and
     194             :      *  store commitments for later. Update overall state with successfully
     195             :      *  processed headers.
     196             :      *  On failure, this invokes Finalize() and returns false.
     197             :      */
     198             :     bool ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers);
     199             : 
     200             :     /** In PRESYNC, process and update state for a single header */
     201             :     bool ValidateAndProcessSingleHeader(const CBlockHeader& current);
     202             : 
     203             :     /** In REDOWNLOAD, check a header's commitment (if applicable) and add to
     204             :      * buffer for later processing */
     205             :     bool ValidateAndStoreRedownloadedHeader(const CBlockHeader& header);
     206             : 
     207             :     /** Return a set of headers that satisfy our proof-of-work threshold */
     208             :     std::vector<CBlockHeader> PopHeadersReadyForAcceptance();
     209             : 
     210             : private:
     211             :     /** NodeId of the peer (used for log messages) **/
     212             :     const NodeId m_id;
     213             : 
     214             :     /** We use the consensus params in our anti-DoS calculations */
     215             :     const Consensus::Params& m_consensus_params;
     216             : 
     217             :     /** Store the last block in our block index that the peer's chain builds from */
     218             :     const CBlockIndex* m_chain_start{nullptr};
     219             : 
     220             :     /** Minimum work that we're looking for on this chain. */
     221             :     const arith_uint256 m_minimum_required_work;
     222             : 
     223             :     /** Work that we've seen so far on the peer's chain */
     224             :     arith_uint256 m_current_chain_work;
     225             : 
     226             :     /** m_hasher is a salted hasher for making our 1-bit commitments to headers we've seen. */
     227             :     const SaltedTxidHasher m_hasher;
     228             : 
     229             :     /** A queue of commitment bits, created during the 1st phase, and verified during the 2nd. */
     230             :     bitdeque<> m_header_commitments;
     231             : 
     232             :     /** m_max_commitments is a bound we calculate on how long an honest peer's chain could be,
     233             :      * given the MTP rule.
     234             :      *
     235             :      * Any peer giving us more headers than this will have its sync aborted. This serves as a
     236             :      * memory bound on m_header_commitments. */
     237             :     uint64_t m_max_commitments{0};
     238             : 
     239             :     /** Store the latest header received while in PRESYNC (initialized to m_chain_start) */
     240             :     CBlockHeader m_last_header_received;
     241             : 
     242             :     /** Height of m_last_header_received */
     243             :     int64_t m_current_height{0};
     244             : 
     245             :     /** During phase 2 (REDOWNLOAD), we buffer redownloaded headers in memory
     246             :      *  until enough commitments have been verified; those are stored in
     247             :      *  m_redownloaded_headers */
     248             :     std::deque<CompressedHeader> m_redownloaded_headers;
     249             : 
     250             :     /** Height of last header in m_redownloaded_headers */
     251             :     int64_t m_redownload_buffer_last_height{0};
     252             : 
     253             :     /** Hash of last header in m_redownloaded_headers (initialized to
     254             :      * m_chain_start). We have to cache it because we don't have hashPrevBlock
     255             :      * available in a CompressedHeader.
     256             :      */
     257             :     uint256 m_redownload_buffer_last_hash;
     258             : 
     259             :     /** The hashPrevBlock entry for the first header in m_redownloaded_headers
     260             :      * We need this to reconstruct the full header when it's time for
     261             :      * processing.
     262             :      */
     263             :     uint256 m_redownload_buffer_first_prev_hash;
     264             : 
     265             :     /** The accumulated work on the redownloaded chain. */
     266             :     arith_uint256 m_redownload_chain_work;
     267             : 
     268             :     /** Set this to true once we encounter the target blockheader during phase
     269             :      * 2 (REDOWNLOAD). At this point, we can process and store all remaining
     270             :      * headers still in m_redownloaded_headers.
     271             :      */
     272             :     bool m_process_all_remaining_headers{false};
     273             : 
     274             :     /** Current state of our headers sync. */
     275             :     State m_download_state{State::PRESYNC};
     276             : };
     277             : 
     278             : #endif // BITCOIN_HEADERSSYNC_H