Slasher HTTP API to import attestations from blocks

[michaelsproul]

In light of the recent Holesky incident, it may be useful to be able to manually import blocks and attestations to the slasher.

On Holesky, many slashers were shutdown during the non-finality period, meaning they missed the detection of many surround votes made by validators. One way we could get some of these slashings would be to manually feed blocks from the invalid chain to the slasher, so it can ingest the invalid attestations contained in those blocks and detect slashings.

The complication is, attestations in blocks require their BeaconState/committee to be converted into indexed attestations. And the slasher needs indexed attestations (it needs to know which validators are attesting to the bad chain).

So I think we probably need a few new endpoints:

Import attestations

• Path: /lighthouse/slasher/import_attestations
• Method: POST
• Request body: Vec<IndexedAttestation>
• Response: status code only

This endpoint is more important. It would import all of the indexed attestations to the slasher:

lighthouse/beacon_node/beacon_chain/src/beacon_chain.rs

Lines 4305 to 4330 in bf955c7

	/// If a slasher is configured, provide the attestations from the block.
	fn import_block_update_slasher(
	&self,
	block: BeaconBlockRef<T::EthSpec>,
	state: &BeaconState<T::EthSpec>,
	ctxt: &mut ConsensusContext<T::EthSpec>,
	) {
	if let Some(slasher) = self.slasher.as_ref() {
	for attestation in block.body().attestations() {
	let indexed_attestation = match ctxt.get_indexed_attestation(state, attestation) {
	Ok(indexed) => indexed,
	Err(e) => {
	debug!(
	self.log,
	"Failed to get indexed attestation";
	"purpose" => "slasher",
	"attestation_slot" => attestation.data().slot,
	"error" => ?e,
	);
	continue;
	}
	};
	slasher.accept_attestation(indexed_attestation.clone_as_indexed_attestation());
	}
	}
	}

Export indexed attestations

• Path: /lighthouse/indexed_attestations
• Method: POST
• Request body: SignedBeaconBlock (or SignedBlindedBeaconBlock)
• Response: Vec<IndexedAttestation>

This would need to be run on a node with the dodgy chain import (possibly imported via lcli http-sync). It would convert all of the attestations in the block into indexed attestations using the committees from the parent BeaconState.

These indexed attestations could then be saved to disk and plumbed into the /lighthouse/slasher/import_attestations endpoint.

Import block

• Path: /lighthouse/slasher/import_block
• Method: POST
• Request body: SignedBeaconBlock (or SignedBlindedBeaconBlock?)
• Response: status code only

This method is not as useful as I originally hoped, it would just be able to import the block header to the slasher due to the shuffling issue described above.

The implementation should pass the block itself to the slasher, probably using accept_block_header like in block verification:

lighthouse/beacon_node/beacon_chain/src/block_verification.rs

Line 578 in bf955c7

slasher.accept_block_header(verified_header);

[eserilev]

I'd be happy to help here, i'll start with the import attestations endpoint, but should probably be able to spin up all three fairly quickly

[michaelsproul]

Process to get slashings:

1. Set up a node on the "bad chain"

• Use --invalid-block-roots "" (so that it can import the bad chain).
• Use --target-peers 0 so it doesn't sync to the correct chain.
• Manually import blocks+blobs from bad chain using POST API (might be able to reuse lcli http-sync. Fetch tarballs of blocks and blobs from PandaOps.

2. Export indexed attestations from bad node

Use the new indexed attestation API to convert all of the attestations in the bad chain blocks to indexed attestations

3. Import indexed attestations to a healthy slasher node

• Run this node with --slasher and --slasher-broadcast false so that it doesn't broadcast slashings.
• Monitor the attester slashing logs/op pool (/eth/v1/beacon/pool/attester_slashings?)