Ensure economic security on validator sets (#459)

* add price oracle

* tidy up

* add todo

* bug fixes

* fix pr comments

* Use spot price, tweak some formulas

Also cleans nits.

---------

Co-authored-by: Luke Parker <lukeparker5132@gmail.com>

Cargo.lock

@@ -7716,6 +7716,7 @@ dependencies = [
  "parity-scale-codec",
  "scale-info",
  "serai-coins-pallet",
+ "serai-dex-pallet",
  "serai-primitives",
  "serai-validator-sets-primitives",
  "sp-application-crypto",

substrate/coins/pallet/src/lib.rs

@@ -1,7 +1,20 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
+use serai_primitives::{Coin, SubstrateAmount, Balance};
+
+pub trait AllowMint {
+  fn is_allowed(balance: &Balance) -> bool;
+}
+
+impl AllowMint for () {
+  fn is_allowed(_: &Balance) -> bool {
+    true
+  }
+}
+
 #[frame_support::pallet]
 pub mod pallet {
+  use super::*;
   use sp_std::{vec::Vec, any::TypeId};
   use sp_core::sr25519::Public;
   use sp_runtime::{
@@ -23,6 +36,7 @@ pub mod pallet {
   #[pallet::config]
   pub trait Config<I: 'static = ()>: frame_system::Config<AccountId = Public> {
     type RuntimeEvent: From<Event<Self, I>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
+    type AllowMint: AllowMint;
   }
 
   #[pallet::genesis_config]
@@ -43,6 +57,7 @@ pub mod pallet {
     AmountOverflowed,
     NotEnoughCoins,
     BurnWithInstructionNotAllowed,
+    MintNotAllowed,
   }
 
   #[pallet::event]
@@ -142,6 +157,10 @@ pub mod pallet {
     ///
     /// Errors if any amount overflows.
     pub fn mint(to: Public, balance: Balance) -> Result<(), Error<T, I>> {
+      if !T::AllowMint::is_allowed(&balance) {
+        Err(Error::<T, I>::MintNotAllowed)?;
+      }
+
       // update the balance
       Self::increase_balance_internal(to, balance)?;
 

substrate/dex/pallet/src/lib.rs

@@ -88,7 +88,7 @@ pub use pallet::*;
 
 use sp_runtime::{traits::TrailingZeroInput, DispatchError};
 
-use serai_primitives::{Coin, SubstrateAmount};
+use serai_primitives::{NetworkId, Coin, SubstrateAmount};
 
 use sp_std::prelude::*;
 pub use types::*;
@@ -154,11 +154,42 @@ pub mod pallet {
   #[pallet::storage]
   pub type Pools<T: Config> = StorageMap<_, Blake2_128Concat, PoolId, PoolInfo<Coin>, OptionQuery>;
 
+  #[pallet::storage]
+  #[pallet::getter(fn spot_price_for_block)]
+  pub type SpotPriceForBlock<T: Config> =
+    StorageDoubleMap<_, Identity, BlockNumberFor<T>, Identity, Coin, [u8; 8], ValueQuery>;
+
+  /// Moving window of oracle prices.
+  ///
+  /// The second [u8; 8] key is the amount's big endian bytes, and u16 is the amount of inclusions
+  /// in this multi-set.
+  #[pallet::storage]
+  #[pallet::getter(fn oracle_prices)]
+  pub type OraclePrices<T: Config> =
+    StorageDoubleMap<_, Identity, Coin, Identity, [u8; 8], u16, OptionQuery>;
+  impl<T: Config> Pallet<T> {
+    // TODO: consider an algorithm which removes outliers? This algorithm might work a good bit
+    // better if we remove the bottom n values (so some value sustained over 90% of blocks instead
+    // of all blocks in the window).
+    /// Get the highest sustained value for this window.
+    /// This is actually the lowest price observed during the windows, as it's the price
+    /// all prices are greater than or equal to.
+    pub fn highest_sustained_price(coin: &Coin) -> Option<Amount> {
+      let mut iter = OraclePrices::<T>::iter_key_prefix(coin);
+      // the first key will be the lowest price due to the keys being lexicographically ordered.
+      iter.next().map(|amount| Amount(u64::from_be_bytes(amount)))
+    }
+  }
+
+  #[pallet::storage]
+  #[pallet::getter(fn oracle_value)]
+  pub type OracleValue<T: Config> = StorageMap<_, Identity, Coin, Amount, OptionQuery>;
+
   // Pallet's events.
   #[pallet::event]
   #[pallet::generate_deposit(pub(super) fn deposit_event)]
   pub enum Event<T: Config> {
-    /// A successful call of the `CretaPool` extrinsic will create this event.
+    /// A successful call of the `CreatePool` extrinsic will create this event.
     PoolCreated {
       /// The pool id associated with the pool. Note that the order of the coins may not be
       /// the same as the order specified in the create pool extrinsic.
@@ -240,6 +271,13 @@ pub mod pallet {
   #[pallet::genesis_build]
   impl<T: Config> BuildGenesisConfig for GenesisConfig<T> {
     fn build(&self) {
+      // assert that oracle windows size can fit into u16. Otherwise number of observants
+      // for a price in the `OraclePrices` map can overflow
+      // We don't want to make this const directly a u16 because it is used the block number
+      // calculations (which are done as u32s)
+      u16::try_from(ORACLE_WINDOW_SIZE).unwrap();
+
+      // create the pools
       for coin in &self.pools {
         Pallet::<T>::create_pool(*coin).unwrap();
       }
@@ -311,8 +349,64 @@ pub mod pallet {
 
   #[pallet::hooks]
   impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {
-    fn integrity_test() {
-      assert!(T::MaxSwapPathLength::get() > 1, "the `MaxSwapPathLength` should be greater than 1",);
+    fn on_finalize(n: BlockNumberFor<T>) {
+      // we run this on on_finalize because we want to use the last price of the block for a coin.
+      // This prevents the exploit where a malicious block proposer spikes the price in either
+      // direction, then includes a swap in the other direction (ensuring they don't get arbitraged
+      // against)
+      // Since they'll have to leave the spike present at the end of the block, making the next
+      // block the one to include any arbitrage transactions (which there's no guarantee they'll
+      // produce), this cannot be done in a way without significant risk
+      for coin in Pools::<T>::iter_keys() {
+        // insert the new price to our oracle window
+        // The spot price for 1 coin, in atomic units, to SRI is used
+        let pool_id = Self::get_pool_id(coin, Coin::native()).ok().unwrap();
+        let pool_account = Self::get_pool_account(pool_id);
+        let sri_balance = Self::get_balance(&pool_account, Coin::native());
+        let coin_balance = Self::get_balance(&pool_account, coin);
+        // We use 1 coin to handle rounding errors which may occur with atomic units
+        // If we used atomic units, any coin whose atomic unit is worth less than SRI's atomic unit
+        // would cause a 'price' of 0
+        // If the decimals aren't large enough to provide sufficient buffer, use 10,000
+        let coin_decimals = coin.decimals().max(5);
+        let accuracy_increase =
+          HigherPrecisionBalance::from(SubstrateAmount::pow(10, coin_decimals));
+        let sri_per_coin = u64::try_from(
+          accuracy_increase * HigherPrecisionBalance::from(sri_balance) /
+            HigherPrecisionBalance::from(coin_balance),
+        )
+        .unwrap_or(u64::MAX);
+        let sri_per_coin = sri_per_coin.to_be_bytes();
+
+        SpotPriceForBlock::<T>::set(n, coin, sri_per_coin);
+
+        // Include this spot price into the multiset
+        {
+          let observed = OraclePrices::<T>::get(coin, sri_per_coin).unwrap_or(0);
+          OraclePrices::<T>::set(coin, sri_per_coin, Some(observed + 1));
+        }
+
+        // pop the earliest key from the window once we reach its full size.
+        if n >= ORACLE_WINDOW_SIZE.into() {
+          let start_of_window = n - ORACLE_WINDOW_SIZE.into();
+          let start_spot_price = Self::spot_price_for_block(start_of_window, coin);
+          SpotPriceForBlock::<T>::remove(start_of_window, coin);
+          // Remove this price from the multiset
+          OraclePrices::<T>::mutate_exists(coin, start_spot_price, |v| {
+            *v = Some(v.unwrap() - 1);
+            if *v == Some(0) {
+              *v = None;
+            }
+          });
+        }
+
+        // update the oracle value
+        let highest_sustained = Self::highest_sustained_price(&coin).unwrap_or(Amount(0));
+        let oracle_value = Self::oracle_value(coin).unwrap_or(Amount(0));
+        if highest_sustained > oracle_value {
+          OracleValue::<T>::set(coin, Some(highest_sustained));
+        }
+      }
     }
   }
 
@@ -338,6 +432,14 @@ pub mod pallet {
 
       Ok(())
     }
+
+    /// A hook to be called whenever a network's session is rotated.
+    pub fn on_new_session(network: NetworkId) {
+      // reset the oracle value
+      for coin in network.coins() {
+        OracleValue::<T>::set(*coin, Self::highest_sustained_price(coin));
+      }
+    }
   }
 
   /// Pallet's callable functions.

substrate/dex/pallet/src/mock.rs

@@ -78,10 +78,12 @@ impl frame_system::Config for Test {
 
 impl coins::Config for Test {
   type RuntimeEvent = RuntimeEvent;
+  type AllowMint = ();
 }
 
 impl coins::Config<coins::Instance1> for Test {
   type RuntimeEvent = RuntimeEvent;
+  type AllowMint = ();
 }
 
 impl Config for Test {

substrate/dex/pallet/src/types.rs

@@ -23,6 +23,10 @@ use super::*;
 use codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;
 
+/// This needs to be long enough for arbitrage to occur and make holding
+/// any fake price up sufficiently unprofitable.
+pub const ORACLE_WINDOW_SIZE: u32 = 1000;
+
 /// Stores the lp_token coin id a particular pool has been assigned.
 #[derive(Decode, Encode, Default, PartialEq, Eq, MaxEncodedLen, TypeInfo)]
 pub struct PoolInfo<PoolCoinId> {

substrate/primitives/src/networks.rs

@@ -25,6 +25,16 @@ pub enum NetworkId {
   Ethereum,
   Monero,
 }
+impl NetworkId {
+  pub fn coins(&self) -> &'static [Coin] {
+    match self {
+      Self::Serai => &[Coin::Serai],
+      Self::Bitcoin => &[Coin::Bitcoin],
+      Self::Ethereum => &[Coin::Ether, Coin::Dai],
+      Self::Monero => &[Coin::Monero],
+    }
+  }
+}
 
 pub const NETWORKS: [NetworkId; 4] =
   [NetworkId::Serai, NetworkId::Bitcoin, NetworkId::Ethereum, NetworkId::Monero];

substrate/runtime/src/lib.rs

@@ -224,10 +224,12 @@ impl transaction_payment::Config for Runtime {
 
 impl coins::Config for Runtime {
   type RuntimeEvent = RuntimeEvent;
+  type AllowMint = ValidatorSets;
 }
 
 impl coins::Config<coins::Instance1> for Runtime {
   type RuntimeEvent = RuntimeEvent;
+  type AllowMint = ();
 }
 
 impl dex::Config for Runtime {

substrate/validator-sets/pallet/Cargo.toml

@@ -33,6 +33,7 @@ serai-primitives = { path = "../../primitives", default-features = false }
 validator-sets-primitives = { package = "serai-validator-sets-primitives", path = "../primitives", default-features = false }
 
 coins-pallet = { package = "serai-coins-pallet", path = "../../coins/pallet", default-features = false }
+dex-pallet = { package = "serai-dex-pallet", path = "../../dex/pallet", default-features = false }
 
 [features]
 std = [
@@ -56,6 +57,7 @@ std = [
   "validator-sets-primitives/std",
 
   "coins-pallet/std",
+  "dex-pallet/std",
 ]
 
 runtime-benchmarks = [

substrate/validator-sets/pallet/src/lib.rs

@@ -22,7 +22,8 @@ pub mod pallet {
   pub use validator_sets_primitives as primitives;
   use primitives::*;
 
-  use coins_pallet::Pallet as Coins;
+  use coins_pallet::{Pallet as Coins, AllowMint};
+  use dex_pallet::Pallet as Dex;
 
   use pallet_babe::{Pallet as Babe, AuthorityId as BabeAuthorityId};
   use pallet_grandpa::{Pallet as Grandpa, AuthorityId as GrandpaAuthorityId};
@@ -31,6 +32,7 @@ pub mod pallet {
   pub trait Config:
     frame_system::Config<AccountId = Public>
     + coins_pallet::Config
+    + dex_pallet::Config
     + pallet_babe::Config
     + pallet_grandpa::Config
     + TypeInfo
@@ -333,6 +335,8 @@ pub mod pallet {
 
   impl<T: Config> Pallet<T> {
     fn new_set(network: NetworkId) {
+      // TODO: prevent new set if it doesn't have enough stake for economic security.
+
       // Update CurrentSession
       let session = {
         let new_session = CurrentSession::<T>::get(network)
@@ -411,6 +415,8 @@ pub mod pallet {
     BadSignature,
     /// Validator wasn't registered or active.
     NonExistentValidator,
+    /// Deallocation would take the stake below what is required.
+    DeallocationWouldRemoveEconomicSecurity,
   }
 
   #[pallet::hooks]
@@ -560,7 +566,16 @@ pub mod pallet {
       account: T::AccountId,
       amount: Amount,
     ) -> Result<bool, DispatchError> {
-      // TODO: Check it's safe to decrease this set's stake by this amount
+      // Check it's safe to decrease this set's stake by this amount
+      let new_total_staked = Self::total_allocated_stake(network)
+        .unwrap()
+        .0
+        .checked_sub(amount.0)
+        .ok_or(Error::<T>::NotEnoughAllocated)?;
+      let required_stake = Self::required_stake_for_network(network);
+      if new_total_staked < required_stake {
+        Err(Error::<T>::DeallocationWouldRemoveEconomicSecurity)?;
+      }
 
       let old_allocation =
         Self::allocation((network, account)).ok_or(Error::<T>::NonExistentValidator)?.0;
@@ -680,6 +695,8 @@ pub mod pallet {
         // - The current set was actually established with a completed handover protocol
         if (network == NetworkId::Serai) || Self::handover_completed(network, current_session) {
           Pallet::<T>::new_set(network);
+          // let the Dex know session is rotated.
+          Dex::<T>::on_new_session(network);
         }
       }
     }
@@ -739,6 +756,39 @@ pub mod pallet {
         validators.into_iter().map(|(id, w)| (GrandpaAuthorityId::from(id), w)).collect(),
       );
     }
+
+    /// Returns the required stake in terms SRI for a given `Balance`.
+    pub fn required_stake(balance: &Balance) -> SubstrateAmount {
+      use dex_pallet::HigherPrecisionBalance;
+
+      // This is inclusive to an increase in accuracy
+      let sri_per_coin = Dex::<T>::oracle_value(balance.coin).unwrap_or(Amount(0));
+
+      // See dex-pallet for the reasoning on these
+      let coin_decimals = balance.coin.decimals().max(5);
+      let accuracy_increase = HigherPrecisionBalance::from(SubstrateAmount::pow(10, coin_decimals));
+
+      let total_coin_value = u64::try_from(
+        HigherPrecisionBalance::from(balance.amount.0) *
+          HigherPrecisionBalance::from(sri_per_coin.0) /
+          accuracy_increase,
+      )
+      .unwrap_or(u64::MAX);
+
+      // required stake formula (COIN_VALUE * 1.5) + margin(20%)
+      let required_stake = total_coin_value.saturating_mul(3).saturating_div(2);
+      required_stake.saturating_add(total_coin_value.saturating_div(5))
+    }
+
+    /// Returns the current total required stake for a given `network`.
+    pub fn required_stake_for_network(network: NetworkId) -> SubstrateAmount {
+      let mut total_required = 0;
+      for coin in network.coins() {
+        let supply = Coins::<T>::supply(coin);
+        total_required += Self::required_stake(&Balance { coin: *coin, amount: Amount(supply) });
+      }
+      total_required
+    }
   }
 
   #[pallet::call]
@@ -866,6 +916,7 @@ pub mod pallet {
             Err(Error::DeallocationWouldRemoveFaultTolerance) |
             Err(Error::NonExistentDeallocation) |
             Err(Error::NonExistentValidator) |
+            Err(Error::DeallocationWouldRemoveEconomicSecurity) |
             Err(Error::BadSignature) => Err(InvalidTransaction::BadProof)?,
             Err(Error::__Ignore(_, _)) => unreachable!(),
             Ok(()) => (),
@@ -982,6 +1033,18 @@ pub mod pallet {
     }
   }
 
+  impl<T: Config> AllowMint for Pallet<T> {
+    fn is_allowed(balance: &Balance) -> bool {
+      // get the required stake
+      let current_required = Self::required_stake_for_network(balance.coin.network());
+      let new_required = current_required + Self::required_stake(balance);
+
+      // get the total stake for the network & compare.
+      let staked = Self::total_allocated_stake(balance.coin.network()).unwrap_or(Amount(0));
+      staked.0 >= new_required
+    }
+  }
+
   impl<T: Config> DisabledValidators for Pallet<T> {
     fn is_disabled(_: u32) -> bool {
       // TODO