Reject bids with invalid prev_randao during gossip validation

specs/gloas/builder.md

@@ -115,7 +115,9 @@ to include. They produce a `SignedExecutionPayloadBid` as follows.
 04. Set `bid.block_hash` to be the block hash of the constructed payload, that
     is `payload.block_hash`.
 05. Set `bid.prev_randao` to be the previous RANDAO of the constructed payload,
-    that is `payload.prev_randao`.
+    that is `payload.prev_randao`. This value **MUST** equal
+    `get_randao_mix(parent_state, get_current_epoch(parent_state))`, where
+    `parent_state` is the post-state of `bid.parent_block_root`.
 06. Set `bid.fee_recipient` to be an execution address to receive the payment.
     The proposer's preferred fee recipient is obtained from the
     `SignedProposerPreferences` whose `message.proposal_slot` matches `bid.slot`

specs/gloas/p2p-interface.md

@@ -390,6 +390,8 @@ where `parent_state` is the post-state of `bid.parent_block_root`, and the alias
 - _[REJECT]_ The bid is for a higher slot than its parent block -- i.e. validate
   that `bid.slot` is greater than the slot of the block with root
   `bid.parent_block_root`.
+- _[REJECT]_ `bid.prev_randao` is the correct RANDAO mix -- i.e. validate that
+  `bid.prev_randao == get_randao_mix(parent_state, get_current_epoch(parent_state))`.
 - _[REJECT]_ `signed_execution_payload_bid.signature` is valid with respect to
   the `bid.builder_index`.
 

specs/gloas/validator.md

@@ -210,6 +210,8 @@ top of a `state` MUST take the following actions in order to construct the
     `should_build_on_full(store, head)` is true, otherwise
     `state.latest_execution_payload_bid.parent_block_hash`.
   - The `bid.parent_block_root` equals the current block's `parent_root`.
+  - The `bid.prev_randao` equals
+    `get_randao_mix(state, get_current_epoch(state))`.
 - Select one bid and set
   `block.body.signed_execution_payload_bid = signed_execution_payload_bid`.