check previous ID is correct

p2p/cuprate-p2p/src/block_downloader.rs

@@ -39,19 +39,16 @@ use crate::{
 
 mod block_queue;
 mod chain_tracker;
-
-use crate::block_downloader::request_chain::{initial_chain_search, request_chain_entry_from_peer};
-use block_queue::{BlockQueue, ReadyQueueBatch};
-use chain_tracker::{BlocksToRetrieve, ChainEntry, ChainTracker};
-use download_batch::download_batch_task;
-
-// TODO: check first block in batch prev_id
-
 mod download_batch;
 mod request_chain;
 #[cfg(test)]
 mod tests;
 
+use block_queue::{BlockQueue, ReadyQueueBatch};
+use chain_tracker::{BlocksToRetrieve, ChainEntry, ChainTracker};
+use download_batch::download_batch_task;
+use request_chain::{initial_chain_search, request_chain_entry_from_peer};
+
 /// A downloaded batch of blocks.
 #[derive(Debug, Clone)]
 pub struct BlockBatch {
@@ -218,10 +215,6 @@ struct BlockDownloader<N: NetworkZone, S, C> {
     amount_of_empty_chain_entries: usize,
 
     /// The running block download tasks.
-    ///
-    /// Returns:
-    /// - The start height of the batch
-    /// - A result containing the batch or an error.
     block_download_tasks: JoinSet<BlockDownloadTaskResponse<N>>,
     /// The running chain entry tasks.
     ///
@@ -342,6 +335,7 @@ where
             self.block_download_tasks.spawn(download_batch_task(
                 client,
                 in_flight_batch.ids.clone(),
+                in_flight_batch.prev_id,
                 in_flight_batch.start_height,
                 in_flight_batch.requests_sent,
             ));
@@ -383,15 +377,14 @@ where
             ) {
                 tracing::debug!("Using peer to request a failed batch");
                 // They should have the blocks so send the re-request to this peer.
-                let ids = request.ids.clone();
-                let start_height = request.start_height;
 
                 request.requests_sent += 1;
 
                 self.block_download_tasks.spawn(download_batch_task(
                     client,
-                    ids,
-                    start_height,
+                    request.ids.clone(),
+                    request.prev_id,
+                    request.start_height,
                     request.requests_sent,
                 ));
 
@@ -426,6 +419,7 @@ where
         self.block_download_tasks.spawn(download_batch_task(
             client,
             block_entry_to_get.ids.clone(),
+            block_entry_to_get.prev_id,
             block_entry_to_get.start_height,
             block_entry_to_get.requests_sent,
         ));

p2p/cuprate-p2p/src/block_downloader/chain_tracker.rs

@@ -23,6 +23,8 @@ pub(crate) struct ChainEntry<N: NetworkZone> {
 pub struct BlocksToRetrieve<N: NetworkZone> {
     /// The block IDs to get.
     pub ids: ByteArrayVec<32>,
+    /// The hash of the last block before this batch.
+    pub prev_id: [u8; 32],
     /// The expected height of the first block in [`BlocksToRetrieve::ids`].
     pub start_height: u64,
     /// The peer who told us about this batch.
@@ -51,17 +53,24 @@ pub enum ChainTrackerError {
 pub struct ChainTracker<N: NetworkZone> {
     /// A list of [`ChainEntry`]s, in order.
     entries: VecDeque<ChainEntry<N>>,
-    /// The height of the first block, in the first entry in entries.
+    /// The height of the first block, in the first entry in [`Self::entries`].
     first_height: u64,
     /// The hash of the last block in the last entry.
     top_seen_hash: [u8; 32],
+    /// The hash of the block one below [`Self::first_height`].
+    previous_hash: [u8; 32],
     /// The hash of the genesis block.
     our_genesis: [u8; 32],
 }
 
 impl<N: NetworkZone> ChainTracker<N> {
     /// Creates a new chain tracker.
-    pub fn new(new_entry: ChainEntry<N>, first_height: u64, our_genesis: [u8; 32]) -> Self {
+    pub fn new(
+        new_entry: ChainEntry<N>,
+        first_height: u64,
+        our_genesis: [u8; 32],
+        previous_hash: [u8; 32],
+    ) -> Self {
         let top_seen_hash = *new_entry.ids.last().unwrap();
         let mut entries = VecDeque::with_capacity(1);
         entries.push_back(new_entry);
@@ -70,6 +79,7 @@ impl<N: NetworkZone> ChainTracker<N> {
             top_seen_hash,
             entries,
             first_height,
+            previous_hash,
             our_genesis,
         }
     }
@@ -180,6 +190,7 @@ impl<N: NetworkZone> ChainTracker<N> {
 
         let blocks = BlocksToRetrieve {
             ids: ids_to_get.into(),
+            prev_id: self.previous_hash,
             start_height: self.first_height,
             peer_who_told_us: entry.peer,
             peer_who_told_us_handle: entry.handle.clone(),
@@ -188,6 +199,8 @@ impl<N: NetworkZone> ChainTracker<N> {
         };
 
         self.first_height += u64::try_from(end_idx).unwrap();
+        // TODO: improve ByteArrayVec API.
+        self.previous_hash = blocks.ids[blocks.ids.len() - 1];
 
         if entry.ids.is_empty() {
             self.entries.pop_front();

p2p/cuprate-p2p/src/block_downloader/download_batch.rs

@@ -6,10 +6,10 @@ use tokio::time::timeout;
 use tower::{Service, ServiceExt};
 use tracing::instrument;
 
-use monero_p2p::{NetworkZone, PeerRequest, PeerResponse, handles::ConnectionHandle};
-use monero_wire::protocol::{GetObjectsRequest, GetObjectsResponse};
 use cuprate_helper::asynch::rayon_spawn_async;
 use fixed_bytes::ByteArrayVec;
+use monero_p2p::{handles::ConnectionHandle, NetworkZone, PeerRequest, PeerResponse};
+use monero_wire::protocol::{GetObjectsRequest, GetObjectsResponse};
 
 use crate::{
     block_downloader::{BlockBatch, BlockDownloadError, BlockDownloadTaskResponse},
@@ -17,7 +17,6 @@ use crate::{
     constants::{BLOCK_DOWNLOADER_REQUEST_TIMEOUT, MAX_TRANSACTION_BLOB_SIZE, MEDIUM_BAN},
 };
 
-
 /// Attempts to request a batch of blocks from a peer, returning [`BlockDownloadTaskResponse`].
 #[instrument(
     level = "debug",
@@ -31,12 +30,13 @@ use crate::{
 pub async fn download_batch_task<N: NetworkZone>(
     client: ClientPoolDropGuard<N>,
     ids: ByteArrayVec<32>,
+    previous_id: [u8; 32],
     expected_start_height: u64,
     _attempt: usize,
 ) -> BlockDownloadTaskResponse<N> {
     BlockDownloadTaskResponse {
         start_height: expected_start_height,
-        result: request_batch_from_peer(client, ids, expected_start_height).await,
+        result: request_batch_from_peer(client, ids, previous_id, expected_start_height).await,
     }
 }
 
@@ -47,6 +47,7 @@ pub async fn download_batch_task<N: NetworkZone>(
 async fn request_batch_from_peer<N: NetworkZone>(
     mut client: ClientPoolDropGuard<N>,
     ids: ByteArrayVec<32>,
+    previous_id: [u8; 32],
     expected_start_height: u64,
 ) -> Result<(ClientPoolDropGuard<N>, BlockBatch), BlockDownloadError> {
     // Request the blocks.
@@ -80,7 +81,13 @@ async fn request_batch_from_peer<N: NetworkZone>(
     let peer_handle = client.info.handle.clone();
 
     let blocks = rayon_spawn_async(move || {
-        deserialize_batch(blocks_response, expected_start_height, ids, peer_handle)
+        deserialize_batch(
+            blocks_response,
+            expected_start_height,
+            ids,
+            previous_id,
+            peer_handle,
+        )
     })
     .await;
 
@@ -98,6 +105,7 @@ fn deserialize_batch(
     blocks_response: GetObjectsResponse,
     expected_start_height: u64,
     requested_ids: ByteArrayVec<32>,
+    previous_id: [u8; 32],
     peer_handle: ConnectionHandle,
 ) -> Result<BlockBatch, BlockDownloadError> {
     let blocks = blocks_response
@@ -112,11 +120,26 @@ fn deserialize_batch(
             let block = Block::read(&mut block_entry.block.as_ref())
                 .map_err(|_| BlockDownloadError::PeersResponseWasInvalid)?;
 
+            let block_hash = block.hash();
+
             // Check the block matches the one requested and the peer sent enough transactions.
-            if requested_ids[i] != block.hash() || block.txs.len() != block_entry.txs.len() {
+            if requested_ids[i] != block_hash || block.txs.len() != block_entry.txs.len() {
                 return Err(BlockDownloadError::PeersResponseWasInvalid);
             }
 
+            // Check that the previous ID is correct for the first block.
+            // This is to protect use against banning the wrong peer.
+            // This must happen after the hash check.
+            if i == 0 && block.header.previous != previous_id {
+                tracing::warn!(
+                    "Invalid chain, peer told us a block follows the chain when it doesn't."
+                );
+
+                // This peer probably did nothing wrong, it was the peer who told us this blockID which
+                // is misbehaving.
+                return Err(BlockDownloadError::ChainInvalid);
+            }
+
             // Check the height lines up as expected.
             // This must happen after the hash check.
             if !block

p2p/cuprate-p2p/src/block_downloader/request_chain.rs

@@ -219,6 +219,8 @@ where
         return Err(BlockDownloadError::FailedToFindAChainToFollow);
     }
 
+    let previous_id = hashes[first_unknown - 1];
+
     let first_entry = ChainEntry {
         ids: hashes[first_unknown..].to_vec(),
         peer: peer_id,
@@ -230,7 +232,7 @@ where
         first_entry.ids.len()
     );
 
-    let tracker = ChainTracker::new(first_entry, expected_height, our_genesis);
+    let tracker = ChainTracker::new(first_entry, expected_height, our_genesis, previous_id);
 
     Ok(tracker)
 }

p2p/monero-p2p/src/client.rs

@@ -160,6 +160,8 @@ impl<Z: NetworkZone> Service<PeerRequest> for Client<Z> {
         };
 
         if let Err(e) = self.connection_tx.try_send(req) {
+            // The connection task could have closed between a call to `poll_ready` and the call to
+            // `call`, which means if we don't handle the error here the receiver would panic.
             use mpsc::error::TrySendError;
 
             match e {