# 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](https://github.com/orgs/paritytech/projects/67), 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](https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-ff4p-7xrq-q5r8)
and [Jul 2022](https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-7f6x-jwh5-m9r4).

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 process directly interfacing
with untrusted code does not have unnecessary 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).
