SCH-27 — Verify bundlesha256 at materialization

SCH-27 — Verify bundle_sha256 at materialization

Adversarial review of the bundle_sha256 write-only residual (SCH-13 Finding 4, MINOR).

Verdict: resolved. The digest is now verified at materialization time.

Problem

StudentTaskResult.bundle_sha256 was computed by both producers (SmolCloudRunner._stage_bundle and SmolVmRunner._stage_bundle) but never recomputed by any consumer. materialize_and_verify_student_result validated task_id/repository/base_sha identity but never re-hashed the uploaded bundle. The only test asserted the producer's own returned value — a self-referential check.

Fix

1. Carry the bundle inputs in the result

Added repository_tar: bytes and task_json: bytes fields to StudentTaskResult (student_vm_runner.py:159-160). Both producers now populate them:

  • SmolCloudRunner._execute (smol_cloud_runner.py:370-371): passes the staged repository.tar and task.json bytes.
  • SmolVmRunner._execute (student_vm_runner.py:1349-1350): reads the staged files back and passes them.

2. Recompute at materialization

materialize_and_verify_student_result (student_vm_runner.py:820-826) now:

  1. Rejects results with empty repository_tar or task_json.
  2. Recomputes sha256(repository_tar + task_json).
  3. Compares to result.bundle_sha256; raises StudentVMBlocked on divergence.

3. Divergence-catching test

test_bundle_sha256_divergence_is_rejected (tests/test_student_vm_artifact.py:522-581):

  • Constructs a result with bundle_sha256="e" * 64 (wrong) and correct repository_tar/task_json.
  • Asserts materialize_and_verify_student_result raises StudentVMBlocked matching "bundle_sha256".
  • Reconstructs with the correct digest and asserts materialization succeeds.

This test FAILS if the verification is removed — it is not self-referential.

Verification

  • pytest tests/test_student_vm_artifact.py — 9 passed
  • pytest tests/test_smol_cloud_runner.py — 78 passed
  • pytest tests/test_student_vm_runner.py — 26 passed, 1 skipped
  • Combined: 113 passed, 1 skipped
  • python3 -m compileall — clean

Files changed

  • student_vm_runner.py — dataclass fields + validation + materialization check
  • smol_cloud_runner.py — populate new fields
  • tests/test_student_vm_artifact.py — updated existing tests + new divergence test

Production impact

None. Production student dispatch remains disabled. The change is purely defensive: the verifier now catches a producer/consumer mismatch that was previously invisible.

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