Alternate handover batch TOCTOU fix (#397)

* Revert "Correct the prior documented TOCTOU"

This reverts commit d50fe87.

* Correct the prior documented TOCTOU

d50fe87 edited the challenge for the Batch to
fix it. This won't produce Batch n+1 until Batch n is successfully published
and verified. It's an alternative strategy able to be reviewed, with a much
smaller impact to scope.

coordinator/src/db.rs

@@ -8,7 +8,7 @@ use blake2::{
 use scale::{Encode, Decode};
 use serai_client::{
   primitives::NetworkId,
-  validator_sets::primitives::ValidatorSet,
+  validator_sets::primitives::{Session, ValidatorSet},
   in_instructions::primitives::{Batch, SignedBatch},
 };
 
@@ -142,13 +142,49 @@ impl<D: Db> MainDb<D> {
       .map(|id| u32::from_le_bytes(id.try_into().unwrap()))
   }
 
-  fn did_handover_key(set: ValidatorSet) -> Vec<u8> {
-    Self::main_key(b"did_handover", set.encode())
+  fn handover_batch_key(set: ValidatorSet) -> Vec<u8> {
+    Self::main_key(b"handover_batch", set.encode())
   }
-  pub fn set_did_handover(txn: &mut D::Transaction<'_>, set: ValidatorSet) {
-    txn.put(Self::did_handover_key(set), []);
+  fn lookup_handover_batch_key(network: NetworkId, batch: u32) -> Vec<u8> {
+    Self::main_key(b"lookup_handover_batch", (network, batch).encode())
   }
-  pub fn did_handover<G: Get>(getter: &G, set: ValidatorSet) -> bool {
-    getter.get(Self::did_handover_key(set)).is_some()
+  pub fn set_handover_batch(txn: &mut D::Transaction<'_>, set: ValidatorSet, batch: u32) {
+    txn.put(Self::handover_batch_key(set), batch.to_le_bytes());
+    txn.put(Self::lookup_handover_batch_key(set.network, batch), set.session.0.to_le_bytes());
+  }
+  pub fn handover_batch<G: Get>(getter: &G, set: ValidatorSet) -> Option<u32> {
+    getter.get(Self::handover_batch_key(set)).map(|id| u32::from_le_bytes(id.try_into().unwrap()))
+  }
+  pub fn is_handover_batch<G: Get>(
+    getter: &G,
+    network: NetworkId,
+    batch: u32,
+  ) -> Option<ValidatorSet> {
+    getter.get(Self::lookup_handover_batch_key(network, batch)).map(|session| ValidatorSet {
+      network,
+      session: Session(u32::from_le_bytes(session.try_into().unwrap())),
+    })
+  }
+
+  fn queued_batches_key(set: ValidatorSet) -> Vec<u8> {
+    Self::main_key(b"queued_batches", set.encode())
+  }
+  pub fn queue_batch(txn: &mut D::Transaction<'_>, set: ValidatorSet, batch: Transaction) {
+    let key = Self::queued_batches_key(set);
+    let mut batches = txn.get(&key).unwrap_or(vec![]);
+    batches.extend(batch.serialize());
+    txn.put(&key, batches);
+  }
+  pub fn take_queued_batches(txn: &mut D::Transaction<'_>, set: ValidatorSet) -> Vec<Transaction> {
+    let key = Self::queued_batches_key(set);
+    let batches_vec = txn.get(&key).unwrap_or(vec![]);
+    txn.del(&key);
+    let mut batches: &[u8] = &batches_vec;
+
+    let mut res = vec![];
+    while !batches.is_empty() {
+      res.push(Transaction::read(&mut batches).unwrap());
+    }
+    res
   }
 }

coordinator/src/main.rs

@@ -732,6 +732,8 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
               next += 1;
             }
 
+            let start_id = batches.front().map(|batch| batch.batch.id);
+            let last_id = batches.back().map(|batch| batch.batch.id);
             while let Some(batch) = batches.pop_front() {
               // If this Batch should no longer be published, continue
               if get_next(&serai, network).await > batch.batch.id {
@@ -768,8 +770,30 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
                 sleep(Duration::from_secs(5)).await;
               }
             }
+            // Verify the `Batch`s we just published
+            if let Some(last_id) = last_id {
+              loop {
+                let verified = verify_published_batches::<D>(&mut txn, msg.network, last_id).await;
+                if verified == Some(last_id) {
+                  break;
+                }
+              }
+            }
 
-            None
+            // Check if any of these `Batch`s were a handover `Batch`
+            // If so, we need to publish any delayed `Batch` provided transactions
+            let mut relevant = None;
+            if let Some(start_id) = start_id {
+              let last_id = last_id.unwrap();
+              for batch in start_id .. last_id {
+                if let Some(set) = MainDb::<D>::is_handover_batch(&txn, msg.network, batch) {
+                  // TODO: relevant may malready be Some. This is a safe over-write, yet we do need
+                  // to explicitly not bother with old tributaries
+                  relevant = Some(set.session);
+                }
+              }
+            }
+            relevant
           }
         },
       };
@@ -791,11 +815,15 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
 
         let genesis = spec.genesis();
 
-        let tx = match msg.msg.clone() {
+        let txs = match msg.msg.clone() {
           ProcessorMessage::KeyGen(inner_msg) => match inner_msg {
-            key_gen::ProcessorMessage::Commitments { id, commitments } => Some(
-              Transaction::DkgCommitments(id.attempt, commitments, Transaction::empty_signed()),
-            ),
+            key_gen::ProcessorMessage::Commitments { id, commitments } => {
+              vec![Transaction::DkgCommitments(
+                id.attempt,
+                commitments,
+                Transaction::empty_signed(),
+              )]
+            }
             key_gen::ProcessorMessage::Shares { id, mut shares } => {
               // Create a MuSig-based machine to inform Substrate of this key generation
               let nonces = crate::tributary::dkg_confirmation_nonces(&key, spec, id.attempt);
@@ -815,12 +843,12 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
                 );
               }
 
-              Some(Transaction::DkgShares {
+              vec![Transaction::DkgShares {
                 attempt: id.attempt,
                 shares: tx_shares,
                 confirmation_nonces: nonces,
                 signed: Transaction::empty_signed(),
-              })
+              }]
             }
             key_gen::ProcessorMessage::GeneratedKeyPair { id, substrate_key, network_key } => {
               assert_eq!(
@@ -840,7 +868,7 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
 
               match share {
                 Ok(share) => {
-                  Some(Transaction::DkgConfirmed(id.attempt, share, Transaction::empty_signed()))
+                  vec![Transaction::DkgConfirmed(id.attempt, share, Transaction::empty_signed())]
                 }
                 Err(p) => {
                   todo!("participant {p:?} sent invalid DKG confirmation preprocesses")
@@ -853,22 +881,22 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
               if id.attempt == 0 {
                 MainDb::<D>::save_first_preprocess(&mut txn, network, id.id, preprocess);
 
-                None
+                vec![]
               } else {
-                Some(Transaction::SignPreprocess(SignData {
+                vec![Transaction::SignPreprocess(SignData {
                   plan: id.id,
                   attempt: id.attempt,
                   data: preprocess,
                   signed: Transaction::empty_signed(),
-                }))
+                })]
               }
             }
-            sign::ProcessorMessage::Share { id, share } => Some(Transaction::SignShare(SignData {
+            sign::ProcessorMessage::Share { id, share } => vec![Transaction::SignShare(SignData {
               plan: id.id,
               attempt: id.attempt,
               data: share,
               signed: Transaction::empty_signed(),
-            })),
+            })],
             sign::ProcessorMessage::Completed { key: _, id, tx } => {
               let r = Zeroizing::new(<Ristretto as Ciphersuite>::F::random(&mut OsRng));
               #[allow(non_snake_case)]
@@ -886,7 +914,7 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
                 }
                 _ => unreachable!(),
               }
-              Some(tx)
+              vec![tx]
             }
           },
           ProcessorMessage::Coordinator(inner_msg) => match inner_msg {
@@ -906,9 +934,11 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
 
                 // If this is the new key's first Batch, only create this TX once we verify all
                 // all prior published `Batch`s
-                if (spec.set().session.0 != 0) && (!MainDb::<D>::did_handover(&txn, spec.set())) {
-                  let last_received = MainDb::<D>::last_received_batch(&txn, msg.network);
-                  if let Some(last_received) = last_received {
+                let last_received = MainDb::<D>::last_received_batch(&txn, msg.network).unwrap();
+                let handover_batch = MainDb::<D>::handover_batch(&txn, spec.set());
+                if handover_batch.is_none() {
+                  MainDb::<D>::set_handover_batch(&mut txn, spec.set(), last_received);
+                  if last_received != 0 {
                     // Decrease by 1, to get the ID of the Batch prior to this Batch
                     let prior_sets_last_batch = last_received - 1;
                     loop {
@@ -921,36 +951,69 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
                       sleep(Duration::from_secs(5)).await;
                     }
                   }
-                  MainDb::<D>::set_did_handover(&mut txn, spec.set());
                 }
 
-                Some(Transaction::Batch(block.0, id.id))
+                // There is a race condition here. We may verify all `Batch`s from the prior set,
+                // start signing the handover `Batch` `n`, start signing `n+1`, have `n+1` signed
+                // before `n` (or at the same time), yet then the prior set forges a malicious
+                // `Batch` `n`.
+                //
+                // The malicious `Batch` `n` would be publishable to Serai, as Serai can't
+                // distinguish what's intended to be a handover `Batch`, yet then anyone could
+                // publish the new set's `n+1`, causing their acceptance of the handover.
+                //
+                // To fix this, if this is after the handover `Batch` and we have yet to verify
+                // publication of the handover `Batch`, don't yet yield the provided.
+                let handover_batch = MainDb::<D>::handover_batch(&txn, spec.set()).unwrap();
+                let intended = Transaction::Batch(block.0, id.id);
+                let mut res = vec![intended.clone()];
+                if last_received > handover_batch {
+                  if let Some(last_verified) = MainDb::<D>::last_verified_batch(&txn, msg.network) {
+                    if last_verified < handover_batch {
+                      res = vec![];
+                    }
+                  } else {
+                    res = vec![];
+                  }
+                }
+
+                if res.is_empty() {
+                  MainDb::<D>::queue_batch(&mut txn, spec.set(), intended);
+                }
+
+                res
               } else {
-                Some(Transaction::BatchPreprocess(SignData {
+                vec![Transaction::BatchPreprocess(SignData {
                   plan: id.id,
                   attempt: id.attempt,
                   data: preprocess,
                   signed: Transaction::empty_signed(),
-                }))
+                })]
               }
             }
             coordinator::ProcessorMessage::BatchShare { id, share } => {
-              Some(Transaction::BatchShare(SignData {
+              vec![Transaction::BatchShare(SignData {
                 plan: id.id,
                 attempt: id.attempt,
                 data: share.to_vec(),
                 signed: Transaction::empty_signed(),
-              }))
+              })]
             }
           },
           ProcessorMessage::Substrate(inner_msg) => match inner_msg {
             processor_messages::substrate::ProcessorMessage::Batch { .. } => unreachable!(),
-            processor_messages::substrate::ProcessorMessage::SignedBatch { .. } => unreachable!(),
+            processor_messages::substrate::ProcessorMessage::SignedBatch { .. } => {
+              // We only reach here if this SignedBatch triggered the publication of a handover
+              // Batch
+              // Since the handover `Batch` was successfully published and verified, we no longer
+              // have to worry about the above n+1 attack
+              MainDb::<D>::take_queued_batches(&mut txn, spec.set())
+            }
           },
         };
 
-        // If this created a transaction, publish it
-        if let Some(mut tx) = tx {
+        // If this created transactions, publish them
+        for mut tx in txs {
           log::trace!("processor message effected transaction {}", hex::encode(tx.hash()));
 
           match tx.kind() {

processor/src/substrate_signer.rs

@@ -52,18 +52,6 @@ impl<D: Db> SubstrateSignerDb<D> {
     D::key(b"SUBSTRATE_SIGNER", dst, key)
   }
 
-  fn first_batch_key(key: [u8; 32]) -> Vec<u8> {
-    Self::sign_key(b"first_batch", key)
-  }
-  fn save_first_batch(txn: &mut D::Transaction<'_>, key: [u8; 32], id: u32) {
-    txn.put(Self::first_batch_key(key), id.to_le_bytes());
-  }
-  fn first_batch<G: Get>(getter: &G, key: [u8; 32]) -> Option<u32> {
-    getter
-      .get(Self::first_batch_key(key))
-      .map(|bytes| u32::from_le_bytes(bytes.try_into().unwrap()))
-  }
-
   fn completed_key(id: [u8; 32]) -> Vec<u8> {
     Self::sign_key(b"completed", id)
   }
@@ -241,11 +229,6 @@ impl<D: Db> SubstrateSigner<D> {
       return;
     }
 
-    let group_key = self.keys.group_key().to_bytes();
-    if SubstrateSignerDb::<D>::first_batch(txn, group_key).is_none() {
-      SubstrateSignerDb::<D>::save_first_batch(txn, group_key, batch.id);
-    }
-
     self.signable.insert(id, batch);
     self.attempt(txn, id, 0).await;
   }
@@ -287,13 +270,8 @@ impl<D: Db> SubstrateSigner<D> {
           Err(e) => todo!("malicious signer: {:?}", e),
         };
 
-        let batch = &self.signable[&id.id];
-        let is_first_batch =
-          SubstrateSignerDb::<D>::first_batch(txn, self.keys.group_key().to_bytes()).unwrap() ==
-            batch.id;
-
         let (machine, share) =
-          match machine.sign(preprocesses, &batch_message(is_first_batch, batch)) {
+          match machine.sign(preprocesses, &batch_message(&self.signable[&id.id])) {
             Ok(res) => res,
             Err(e) => todo!("malicious signer: {:?}", e),
           };

processor/src/tests/substrate_signer.rs

@@ -146,9 +146,8 @@ async fn test_substrate_signer() {
       signers.get_mut(i).unwrap().events.pop_front().unwrap()
     {
       assert_eq!(signed_batch.batch, batch);
-      // SubstrateSigner will believe this is the first batch for this set, hence `true`
       assert!(Public::from_raw(keys[&participant_one].group_key().to_bytes())
-        .verify(&batch_message(true, &batch), &signed_batch.signature));
+        .verify(&batch_message(&batch), &signed_batch.signature));
     } else {
       panic!("didn't get signed batch back");
     }

substrate/client/tests/common/in_instructions.rs

@@ -39,8 +39,7 @@ pub async fn provide_batch(batch: Batch) -> [u8; 32] {
 
   let block = publish_tx(&Serai::execute_batch(SignedBatch {
     batch: batch.clone(),
-    // TODO: This `batch.id == 0` line only works when session == 0
-    signature: pair.sign(&batch_message(batch.id == 0, &batch)),
+    signature: pair.sign(&batch_message(&batch)),
   }))
   .await;
 

substrate/in-instructions/pallet/src/lib.rs

@@ -155,21 +155,14 @@ pub mod pallet {
       // verify the signature
       let network = batch.batch.network;
       let (current_session, prior, current) = keys_for_network::<T>(network)?;
+      let batch_message = batch_message(&batch.batch);
       // Check the prior key first since only a single `Batch` (the last one) will be when prior is
       // Some yet prior wasn't the signing key
-      let valid_by_prior = if let Some(key) = prior {
-        key.verify(&batch_message(false, &batch.batch), &batch.signature)
-      } else {
-        false
-      };
+      let valid_by_prior =
+        if let Some(key) = prior { key.verify(&batch_message, &batch.signature) } else { false };
       let valid = valid_by_prior ||
         (if let Some(key) = current {
-          key.verify(
-            // This `== 0` is valid as either it'll be the first Batch for the first set, or if
-            // they never had a Batch, the first Batch for the next set
-            &batch_message((batch.batch.id == 0) || prior.is_some(), &batch.batch),
-            &batch.signature,
-          )
+          key.verify(&batch_message, &batch.signature)
         } else {
           false
         });

substrate/in-instructions/primitives/src/lib.rs

@@ -84,6 +84,6 @@ impl Zeroize for SignedBatch {
 
 // TODO: Make this an associated method?
 /// The message for the batch signature.
-pub fn batch_message(is_first_batch_of_set: bool, batch: &Batch) -> Vec<u8> {
-  [b"InInstructions-batch".as_ref(), &(is_first_batch_of_set, batch).encode()].concat()
+pub fn batch_message(batch: &Batch) -> Vec<u8> {
+  [b"InInstructions-batch".as_ref(), &batch.encode()].concat()
 }

tests/coordinator/src/tests/batch.rs

@@ -165,7 +165,7 @@ pub async fn batch(
   let signature = Signature(
     schnorrkel::keys::Keypair::from_bytes(&schnorrkel_key_pair)
       .unwrap()
-      .sign_simple(b"substrate", &batch_message(batch.id == 0, &batch))
+      .sign_simple(b"substrate", &batch_message(&batch))
       .to_bytes(),
   );