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blue: catches KG#253 — __instancecheck__ metaclass bypass #233

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blue: catches KG#253 — __instancecheck__ metaclass bypass #233

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14 changes: 14 additions & 0 deletions kernelguard.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3025,6 +3025,14 @@ def detect_precision_downgrade(code: str | SubmissionFacts) -> list[dict]:
return matches


def detect_instancecheck_metaclass(code: str | SubmissionFacts) -> list[dict]:
facts = ensure_submission_facts(code)
code = facts.raw_code
if re.search(r'def\s+__instancecheck__\s*\(', code) and re.search(r'\bisinstance\b', code):
return [{"pattern": "INSTANCECHECK_METACLASS", "severity": "critical", "evidence": "__instancecheck__ metaclass + isinstance in entrypoint (type-check caching bypass)"}]
return []


# ---------------------------------------------------------------------------
# Score anomaly detection
# ---------------------------------------------------------------------------
Expand Down Expand Up @@ -3653,6 +3661,10 @@ class RulePolicy:
"NEAR_CLONE_SPAM", "administrative", "support", SUSPICIOUS_ONLY, (),
(), "downgrade",
),
"INSTANCECHECK_METACLASS": RulePolicy(
"INSTANCECHECK_METACLASS", "result_reuse", "hard", AUTO_FILTER, (),
(), "keep",
),
}

BASE_SCORE_CONFIG = {
Expand Down Expand Up @@ -3790,6 +3802,7 @@ def support_only_patterns(matched_patterns: list[dict]) -> bool:
detect_thread_injection,
detect_lazy_tensor,
detect_precision_downgrade,
detect_instancecheck_metaclass,
]

BASE_DETECTOR_SPECS = [
Expand Down Expand Up @@ -3827,6 +3840,7 @@ def support_only_patterns(matched_patterns: list[dict]) -> bool:
("thread_injection", detect_thread_injection),
("lazy_tensor", detect_lazy_tensor),
("precision_downgrade", detect_precision_downgrade),
("instancecheck_metaclass", detect_instancecheck_metaclass),
]

VALID_RULE_OUTCOMES = {AUTO_FILTER, SUSPICIOUS_ONLY, TELEMETRY_ONLY}
Expand Down