aura, babe: don't allow disabled validators to build blocks (#9414)

* frame-support: add trait for checking disabled validators

* pallet-session: implement DisabledValidators trait

* pallet-babe: check for disabled validators

* pallet-babe: add test for disabled validators

* pallet-aura: check for disabled validators

* pallet-aura: add test for disabled validators

* runtime: fix missing DisableValidator

* test-runtime: add missing DisabledValidators

* frame-support: clean up doc

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/babe/src/lib.rs

@@ -24,7 +24,9 @@
 use codec::{Decode, Encode};
 use frame_support::{
 	dispatch::DispatchResultWithPostInfo,
-	traits::{FindAuthor, Get, KeyOwnerProofSystem, OnTimestampSet, OneSessionHandler},
+	traits::{
+		DisabledValidators, FindAuthor, Get, KeyOwnerProofSystem, OnTimestampSet, OneSessionHandler,
+	},
 	weights::{Pays, Weight},
 };
 use sp_application_crypto::Public;
@@ -137,6 +139,11 @@ pub mod pallet {
 		/// when no other module is responsible for changing authority set.
 		type EpochChangeTrigger: EpochChangeTrigger;
 
+		/// A way to check whether a given validator is disabled and should not be authoring blocks.
+		/// Blocks authored by a disabled validator will lead to a panic as part of this module's
+		/// initialization.
+		type DisabledValidators: DisabledValidators;
+
 		/// The proof of key ownership, used for validating equivocation reports.
 		/// The proof must include the session index and validator count of the
 		/// session at which the equivocation occurred.
@@ -678,6 +685,13 @@ impl<T: Config> Pallet<T> {
 
 			let authority_index = digest.authority_index();
 
+			if T::DisabledValidators::is_disabled(authority_index) {
+				panic!(
+					"Validator with index {:?} is disabled and should not be attempting to author blocks.",
+					authority_index,
+				);
+			}
+
 			// Extract out the VRF output if we have it
 			digest.vrf_output().and_then(|vrf_output| {
 				// Reconstruct the bytes of VRFInOut using the authority id.

substrate/frame/babe/src/mock.rs

@@ -238,6 +238,7 @@ impl Config for Test {
 	type EpochDuration = EpochDuration;
 	type ExpectedBlockTime = ExpectedBlockTime;
 	type EpochChangeTrigger = crate::ExternalTrigger;
+	type DisabledValidators = Session;
 
 	type KeyOwnerProofSystem = Historical;
 

substrate/frame/babe/src/tests.rs

@@ -373,6 +373,31 @@ fn tracks_block_numbers_when_current_and_previous_epoch_started() {
 	});
 }
 
+#[test]
+#[should_panic(
+	expected = "Validator with index 0 is disabled and should not be attempting to author blocks."
+)]
+fn disabled_validators_cannot_author_blocks() {
+	new_test_ext(4).execute_with(|| {
+		start_era(1);
+
+		// let's disable the validator at index 1
+		Session::disable_index(1);
+
+		// the mocking infrastructure always authors all blocks using authority index 0,
+		// so we should still be able to author blocks
+		start_era(2);
+
+		assert_eq!(Staking::current_era().unwrap(), 2);
+
+		// let's disable the validator at index 0
+		Session::disable_index(0);
+
+		// this should now panic as the validator authoring blocks is disabled
+		start_era(3);
+	});
+}
+
 #[test]
 fn report_equivocation_current_session_works() {
 	let (pairs, mut ext) = new_test_ext_with_pairs(3);
@@ -394,8 +419,8 @@ fn report_equivocation_current_session_works() {
 			);
 		}
 
-		// we will use the validator at index 0 as the offending authority
-		let offending_validator_index = 0;
+		// we will use the validator at index 1 as the offending authority
+		let offending_validator_index = 1;
 		let offending_validator_id = Session::validators()[offending_validator_index];
 		let offending_authority_pair = pairs
 			.into_iter()
@@ -456,7 +481,7 @@ fn report_equivocation_old_session_works() {
 		let authorities = Babe::authorities();
 
 		// we will use the validator at index 0 as the offending authority
-		let offending_validator_index = 0;
+		let offending_validator_index = 1;
 		let offending_validator_id = Session::validators()[offending_validator_index];
 		let offending_authority_pair = pairs
 			.into_iter()