PVF Host and Workers

The PVF host is responsible for handling requests to prepare and execute PVF code blobs, which it sends to PVF workers running in their own child processes.

This system has two high-levels goals that we will touch on here: determinism and security.

Determinism

One high-level goal is to make PVF operations as deterministic as possible, to reduce the rate of disputes. Disputes can happen due to e.g. a job timing out on one machine, but not another. While we do not have full determinism, there are some dispute reduction mechanisms in place right now.

Retrying execution requests

If the execution request fails during preparation, we will retry if it is possible that the preparation error was transient (e.g. if the error was a panic or time out). We will only retry preparation if another request comes in after 15 minutes, to ensure any potential transient conditions had time to be resolved. We will retry up to 5 times.

If the actual execution of the artifact fails, we will retry once if it was a possibly transient error, to allow the conditions that led to the error to hopefully resolve. We use a more brief delay here (1 second as opposed to 15 minutes for preparation (see above)), because a successful execution must happen in a short amount of time.

We currently know of the following specific cases that will lead to a retried execution request:

1. OOM: The host might have been temporarily low on memory due to other processes running on the same machine. NOTE: This case will lead to voting against the candidate (and possibly a dispute) if the retry is still not successful.
2. Artifact missing: The prepared artifact might have been deleted due to operator error or some bug in the system.
3. Panic: The worker thread panicked for some indeterminate reason, which may or may not be independent of the candidate or PVF.

Preparation timeouts

We use timeouts for both preparation and execution jobs to limit the amount of time they can take. As the time for a job can vary depending on the machine and load on the machine, this can potentially lead to disputes where some validators successfuly execute a PVF and others don't.

One dispute mitigation we have in place is a more lenient timeout for preparation during execution than during pre-checking. The rationale is that the PVF has already passed pre-checking, so we know it should be valid, and we allow it to take longer than expected, as this is likely due to an issue with the machine and not the PVF.

CPU clock timeouts

Another timeout-related mitigation we employ is to measure the time taken by jobs using CPU time, rather than wall clock time. This is because the CPU time of a process is less variable under different system conditions. When the overall system is under heavy load, the wall clock time of a job is affected more than the CPU time.

Internal errors

In general, for errors not raising a dispute we have to be very careful. This is only sound, if we either:

1. Ruled out that error in pre-checking. If something is not checked in pre-checking, even if independent of the candidate and PVF, we must raise a dispute.
2. We are 100% confident that it is a hardware/local issue: Like corrupted file, etc.

Reasoning: Otherwise it would be possible to register a PVF where candidates can not be checked, but we don't get a dispute - so nobody gets punished. Second, we end up with a finality stall that is not going to resolve!

There are some error conditions where we can't be sure whether the candidate is really invalid or some internal glitch occurred, e.g. panics. Whenever we are unsure, we can never treat an error as internal as we would abstain from voting. So we will first retry the candidate, and if the issue persists we are forced to vote invalid.

Security

With on-demand parachains, it is much easier to submit PVFs to the chain for preparation and execution. This makes it easier for erroneous disputes and slashing to occur, whether intentional (as a result of a malicious attacker) or not (a bug or operator error occurred).

Therefore, another goal of ours is to harden our security around PVFs, in order to protect the economic interests of validators and increase overall confidence in the system.

Possible attacks / threat model

Webassembly is already sandboxed, but there have already been reported multiple CVEs enabling remote code execution. See e.g. these two advisories from Mar 2023 and Jul 2022.

So what are we actually worried about? Things that come to mind:

1. Consensus faults - If an attacker can get some source of randomness they could vote against with 50% chance and cause unresolvable disputes.
2. Targeted slashes - An attacker can target certain validators (e.g. some validators running on vulnerable hardware) and make them vote invalid and get them slashed.
3. Mass slashes - With some source of randomness they can do an untargeted attack. I.e. a baddie can do significant economic damage by voting against with 1/3 chance, without even stealing keys or completely replacing the binary.
4. Stealing keys - That would be pretty bad. Should not be possible with sandboxing. We should at least not allow filesystem-access or network access.
5. Taking control over the validator. E.g. replacing the polkadot binary with a polkadot-evil binary. Should again not be possible with the above sandboxing in place.
6. Intercepting and manipulating packages - Effect very similar to the above, hard to do without also being able to do 4 or 5.

Restricting file-system access

A basic security mechanism is to make sure that any thread directly interfacing with untrusted code does not have access to the file-system. This provides some protection against attackers accessing sensitive data or modifying data on the host machine.

Clearing env vars

We clear environment variables before handling untrusted code, because why give attackers potentially sensitive data unnecessarily? And even if everything else is locked down, env vars can potentially provide a source of randomness (see point 1, "Consensus faults" above).