PVF: add landlock sandboxing

node/core/pvf/common/src/worker/security.rs

@@ -35,18 +35,36 @@ pub mod landlock {
 
  use landlock::{Access, AccessFs, Ruleset, RulesetAttr, RulesetError, ABI};
 
- /// Version of landlock ABI. Use the latest version supported by our reference kernel version.
+ /// Landlock ABI version. Use the latest version supported by our reference kernel version.
+ ///
+ /// # Versions (June 2023)
  ///
  /// - Reference kernel version: 5.16+
  /// - ABI V1: 5.13
  /// - ABI V2: 5.19
  ///
- /// Please update these docs if they are out-of-date, and bump the ABI version to the minimum
+ /// Please update these values if they are out-of-date, and bump the ABI version to the minimum
  /// ABI supported by the reference kernel version.
+ ///
+ /// # Determinism
+ ///
+ /// You may wonder whether we could always use the latest ABI instead of the ABI supported by
+ /// the reference kernel version. It seems plausible, since landlock provides a best-effort
+ /// approach to enabling sandboxing. For example, if the reference version only supported V1 and
+ /// we were on V2, then landlock would use V2 if it was supported on the current machine, and
+ /// just fall back to V1 if not.
+ ///
+ /// The issue with this is indeterminacy. If half of validators were on V2 and half were on V1,
+ /// they may have different semantics on some PVFs. So a malicious PVF now has a new attack
+ /// vector: they can exploit this indeterminism between landlock ABIs! But this is exactly the
+ /// kind of thing we want to prevent! So, we have to stick only to the latest ABI supported by
+ /// our reference kernel version (in this example ABI V1). And if a validator's machine does not
+ /// *fully* support it, we can't let them run as a secure validator.
  pub const LANDLOCK_ABI: ABI = ABI::V1;
 
  // TODO: <https://github.com/landlock-lsm/rust-landlock/issues/36>
- /// Returns to what degree landlock is enabled on the current Linux environment.
+ /// Returns to what degree landlock is enabled with the given ABI on the current Linux
+ /// environment.
  pub fn get_status() -> Result<RulesetStatus, Box<dyn std::error::Error>> {
  match std::thread::spawn(|| try_restrict_thread()).join() {
  Ok(Ok(status)) => Ok(status),
@@ -55,8 +73,12 @@ pub mod landlock {
  }
  }
 
- /// Returns a single bool indicating whether landlock is fully enabled on the current Linux
- /// environment.
+ /// Returns a single bool indicating whether the given landlock ABI is fully enabled on the
+ /// current Linux environment.
+ ///
+ /// NOTE: Secure validators *should* have this *fully* enabled for the reference ABI. Even having
+ /// this partially enabled (which landlock does in a best-effort capacity) may lead to
+ /// indeterminism. See "Determinism" under [`LANDLOCK_ABI`].
  pub fn is_fully_enabled() -> bool {
  matches!(get_status(), Ok(RulesetStatus::FullyEnforced))
  }