Summary

The Linearizer can double-commit equivocating blocks at the same (Round, Author) slot due to incorrect commit status checking.

Problem Description

In linearize_sub_dag(), the commit status check uses is_committed(BlockRef) which includes the Digest:

.filter(|ancestor| {
    ancestor.round > gc_round && !dag_state.is_committed(ancestor)
})

Since two equivocating blocks have different digests, they are treated as different blocks. If Block A is committed first, is_committed(Block B) still returns false, allowing Block B to be committed later through a different DAG path.

Attack Scenario

1. Byzantine validator V1 creates Block A (Round 1) and broadcasts to 60% of nodes
2. Block A is committed via main chain
3. V1 creates Block B (Round 1, same slot, different digest) and broadcasts to remaining 40%
4. Some honest node (only received B) creates a block referencing B
5. Block B propagates and passes BlockVerifier (signature is valid)
6. A later Leader references Block B through a side chain
7. When that Leader is committed, Linearizer traverses to Block B
8. is_committed(Block B) returns false (different digest)
9. Block B is committed even though Block A was already committed!

Production Impact

• consensus_gc_depth = 60 in production (from sui-protocol-config)
• This gives attackers a ~30-60 second window to execute the attack
• Violates consensus safety: same (Round, Author) slot committed twice
• Requires only 1 Byzantine validator (within f tolerance)

Proposed Fix

Add is_any_block_at_slot_committed(Slot) check to filter out ancestors where any block at that slot has already been committed:

.filter(|ancestor| {
    ancestor.round > gc_round
        && !dag_state.is_committed(ancestor)
        && !dag_state.is_any_block_at_slot_committed((*ancestor).into())
})

Why This Is Not a False Positive

Unlike issue #24475 which used invalid AuthorityIndex that BlockVerifier rejects:

1. Equivocating blocks are valid VerifiedBlocks - BlockVerifier does NOT check for equivocation
2. DagState intentionally accepts equivocating blocks - Only self-equivocation is blocked
3. Production config allows attack - gc_depth=60 provides ample time window

Security Impact

• Severity: High (Safety violation)
• Attack Vector: Network (1 Byzantine validator)
• Impact: Same slot committed twice, violating consensus uniqueness