validate fork_version as light client (#3459)

The spec does not provide code for validating the `fork_version` field
of `LightClientUpdate`. However, we can use our own logic for additional
validation of that field. The spec's python test suite sets up states
that do not follow the fork schedule (e.g., that use Altair fork version
before Altair fork epoch), which complicates upstreaming this as code.

beacon_chain/spec/light_client_sync.nim

@@ -12,6 +12,32 @@ import
   datatypes/altair,
   helpers
 
+func period_contains_fork_version(
+    cfg: RuntimeConfig,
+    period: SyncCommitteePeriod,
+    fork_version: Version): bool =
+  ## Determine whether a given `fork_version` is used during a given `period`.
+  let
+    periodStartEpoch = period.start_epoch
+    periodEndEpoch = periodStartEpoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD - 1
+  return
+    if fork_version == cfg.SHARDING_FORK_VERSION:
+      periodEndEpoch >= cfg.SHARDING_FORK_EPOCH
+    elif fork_version == cfg.BELLATRIX_FORK_VERSION:
+      periodStartEpoch < cfg.SHARDING_FORK_EPOCH and
+      cfg.SHARDING_FORK_EPOCH != cfg.BELLATRIX_FORK_EPOCH and
+      periodEndEpoch >= cfg.BELLATRIX_FORK_EPOCH
+    elif fork_version == cfg.ALTAIR_FORK_VERSION:
+      periodStartEpoch < cfg.BELLATRIX_FORK_EPOCH and
+      cfg.BELLATRIX_FORK_EPOCH != cfg.ALTAIR_FORK_EPOCH and
+      periodEndEpoch >= cfg.ALTAIR_FORK_EPOCH
+    elif fork_version == cfg.GENESIS_FORK_VERSION:
+      # Light client sync protocol requires Altair
+      false
+    else:
+      # Unviable fork
+      false
+
 # https://github.com/ethereum/consensus-specs/blob/v1.1.10/specs/altair/sync-protocol.md#get_active_header
 func is_finality_update(update: altair.LightClientUpdate): bool =
   not update.finalized_header.isZeroMemory
@@ -38,6 +64,7 @@ proc validate_light_client_update*(
     store: LightClientStore,
     update: altair.LightClientUpdate,
     current_slot: Slot,
+    cfg: RuntimeConfig,
     genesis_validators_root: Eth2Digest): bool =
   # Verify update slot is larger than slot of current best finalized header
   let active_header = get_active_header(update)
@@ -53,6 +80,11 @@ proc validate_light_client_update*(
   if update_period notin [finalized_period, finalized_period + 1]:
     return false
 
+  # Verify fork version is acceptable
+  let fork_version = update.fork_version
+  if not cfg.period_contains_fork_version(update_period, fork_version):
+    return false
+
   # Verify that the `finalized_header`, if present, actually is the finalized
   # header saved in the state of the `attested header`
   if not update.is_finality_update:
@@ -96,13 +128,16 @@ proc validate_light_client_update*(
   for idx, bit in sync_aggregate.sync_committee_bits:
     if bit:
       participant_pubkeys.add(sync_committee.pubkeys[idx])
+  let
+    domain = compute_domain(
+      DOMAIN_SYNC_COMMITTEE, fork_version, genesis_validators_root)
+    signing_root = compute_signing_root(update.attested_header, domain)
+  if not blsFastAggregateVerify(
+      participant_pubkeys, signing_root.data,
+      sync_aggregate.sync_committee_signature):
+    return false
 
-  let domain = compute_domain(
-    DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
-  let signing_root = compute_signing_root(update.attested_header, domain)
-
-  blsFastAggregateVerify(
-    participant_pubkeys, signing_root.data, sync_aggregate.sync_committee_signature)
+  true
 
 # https://github.com/ethereum/consensus-specs/blob/v1.1.10/specs/altair/sync-protocol.md#apply_light_client_update
 func apply_light_client_update(
@@ -124,9 +159,10 @@ proc process_light_client_update*(
     store: var LightClientStore,
     update: altair.LightClientUpdate,
     current_slot: Slot,
+    cfg: RuntimeConfig,
     genesis_validators_root: Eth2Digest): bool =
   if not validate_light_client_update(
-      store, update, current_slot, genesis_validators_root):
+      store, update, current_slot, cfg, genesis_validators_root):
     return false
 
   let

tests/consensus_spec/altair/test_fixture_sync_protocol.nim

@@ -142,7 +142,7 @@ suite "EF - Altair - Unittests - Sync protocol" & preset():
         sync_aggregate: sync_aggregate,
         fork_version: state.fork.current_version)
       res = process_light_client_update(
-        store, update, signature_slot, state.genesis_validators_root)
+        store, update, signature_slot, cfg, state.genesis_validators_root)
 
     check:
       res
@@ -206,7 +206,7 @@ suite "EF - Altair - Unittests - Sync protocol" & preset():
         sync_aggregate: sync_aggregate,
         fork_version: state.fork.current_version)
       res = process_light_client_update(
-        store, update, signature_slot, state.genesis_validators_root)
+        store, update, signature_slot, cfg, state.genesis_validators_root)
 
     check:
       res
@@ -297,7 +297,7 @@ suite "EF - Altair - Unittests - Sync protocol" & preset():
         sync_aggregate: sync_aggregate,
         fork_version: state.fork.current_version)
       res = process_light_client_update(
-        store, update, signature_slot, state.genesis_validators_root)
+        store, update, signature_slot, cfg, state.genesis_validators_root)
 
     check:
       res