Sanity check for Babe's configuration (#11385)

* Prevent div by zero in native babe code
* Additional sanity check for babe config
* Further sanity checks and postpone threshold computation
* Apply suggestions from code review
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/babe/src/lib.rs

@@ -24,6 +24,7 @@
 use codec::{Decode, Encode};
 use frame_support::{
 	dispatch::DispatchResultWithPostInfo,
+	ensure,
 	traits::{
 		ConstU32, DisabledValidators, FindAuthor, Get, KeyOwnerProofSystem, OnTimestampSet,
 		OneSessionHandler,
@@ -42,8 +43,8 @@ use sp_std::prelude::*;
 
 use sp_consensus_babe::{
 	digests::{NextConfigDescriptor, NextEpochDescriptor, PreDigest},
-	BabeAuthorityWeight, BabeEpochConfiguration, ConsensusLog, Epoch, EquivocationProof, Slot,
-	BABE_ENGINE_ID,
+	AllowedSlots, BabeAuthorityWeight, BabeEpochConfiguration, ConsensusLog, Epoch,
+	EquivocationProof, Slot, BABE_ENGINE_ID,
 };
 use sp_consensus_vrf::schnorrkel;
 
@@ -185,6 +186,8 @@ pub mod pallet {
 		InvalidKeyOwnershipProof,
 		/// A given equivocation report is valid but already previously reported.
 		DuplicateOffenceReport,
+		/// Submitted configuration is invalid.
+		InvalidConfiguration,
 	}
 
 	/// Current epoch index.
@@ -447,6 +450,14 @@ pub mod pallet {
 			config: NextConfigDescriptor,
 		) -> DispatchResult {
 			ensure_root(origin)?;
+			match config {
+				NextConfigDescriptor::V1 { c, allowed_slots } => {
+					ensure!(
+						(c.0 != 0 || allowed_slots != AllowedSlots::PrimarySlots) && c.1 != 0,
+						Error::<T>::InvalidConfiguration
+					);
+				},
+			}
 			PendingEpochConfigChange::<T>::put(config);
 			Ok(())
 		}