Fork choice: checking FFG source/target

[paulhauner]

In fork choice attestation processing, the attestation.data.target and attestation.data.source fields are not checked to ensure they are ancestors/descendants of each other and attestation.data.beacon_block_root.

What does this mean?

1. Faulty actors influence fork choice

An attestation where the source/target/beacon_block_root are on different chains is produced by a faulty validator.

I would expect that good practice would be to drop any message that is provably invalid and ban the peer.

2. You can pick-and-choose your shuffling/validator registry

Say we have 3 chains:

   2    4    8     <- slot 36 (epoch 1)
   |    |    |
   .    .    .
   .    .    .
   .    .    .
   |    |    |
   1    3    5     <- slot 32 (epoch 1)
    \   |   /
        |
        .
        .
        .

Consider that I want to set attestation.data.beacon_block_root to block 4. I am able to choose whether to set attestation.data.target to blocks 1, 3 or 5.

The honest choice is 3, but perhaps in that state I am scheduled to vote at slot 63 and instead I want to vote earlier. Now I can pick between the other two states to find the earliest assignment and have my vote included in fork choice earlier than naturally intended.

[mkalinin]

When there is no partitions in the network it doesn't matter whether vote was dropped or not. As fork choice will not be abused unless there is a 51% attacker. However, it could be exploited if adversary were able to partition the network to eclipse one of those parts with invalid votes.

A discrepancy between on-chain and fork choice checks has been discussed in #1408 and #1456. I and I believe @ericsson49 are on the same page with @paulhauner regarding that:

I would expect that good practice would be to drop any message that is provably invalid and ban the peer.

In addition we advocate for making on-chain checks explicitly run by the fork choice and get rid of dependency from validation on the network layer. Is that what you implied @paulhauner ?