fix(tpm): reject trailing quote bytes, bound AIA HTTP fetches, and verify report.platform in verify_tpm_report (#282)

src/ca2a_runtime/tee/tpm.py

@@ -79,6 +79,7 @@
 # party verify offline later: the chain travels with the evidence.
 _AIA_FETCH_TIMEOUT_SECONDS = 5
 _AIA_MAX_DEPTH = 4
+_AIA_MAX_CERT_BYTES = 64 * 1024
 _AIA_CA_ISSUERS_OID = "1.3.6.1.5.5.7.48.2"
 
 
@@ -123,6 +124,9 @@ class TpmQuote:
 
     @classmethod
     def parse(cls, blob: bytes) -> TpmQuote:
+        if not isinstance(blob, (bytes, bytearray, memoryview)):
+            raise AttestationFailed("TPM quote must be bytes", detail=f"got {type(blob).__name__}")
+        blob = bytes(blob)
         if len(blob) < 6:
             raise AttestationFailed("TPM quote too short")
         magic = int.from_bytes(blob[0:4], "big")
@@ -141,12 +145,17 @@ def parse(cls, blob: bytes) -> TpmQuote:
             size_of_select = blob[pos + 2]
             pos += 3 + size_of_select
         pcr_digest, pos = _read_2b(blob, pos)
+        if pos != len(blob):
+            raise AttestationFailed(
+                "TPM quote carries trailing bytes after PCR digest",
+                detail=f"parsed {pos} bytes, got {len(blob)} bytes",
+            )
         return cls(
             magic=magic,
             attest_type=attest_type,
             qualifying_data=qualifying_data,
             pcr_digest=pcr_digest,
-            raw=bytes(blob),
+            raw=blob,
         )
 
 
@@ -495,7 +504,13 @@ def load_any(data: bytes) -> list[x509.Certificate]:
                     with urllib.request.urlopen(  # noqa: S310  # nosec B310
                         url, timeout=_AIA_FETCH_TIMEOUT_SECONDS
                     ) as response:
-                        found = load_any(response.read())
+                        raw = response.read(_AIA_MAX_CERT_BYTES + 1)
+                    if len(raw) > _AIA_MAX_CERT_BYTES:
+                        logger.debug(
+                            "AIA response exceeded %d bytes for %s", _AIA_MAX_CERT_BYTES, url
+                        )
+                        continue
+                    found = load_any(raw)
                 except Exception as exc:  # noqa: BLE001
                     logger.debug("AIA fetch failed for %s: %s", url, exc)
                     continue

src/ca2a_verify/tpm.py

@@ -197,6 +197,11 @@ def verify_tpm_report(
     report's ``measurement`` field, so what the caller acts on is the value the TPM
     signed. A mismatch between the two is itself a failure.
     """
+    if report.platform != "tpm":
+        raise AttestationFailed(
+            "expected a 'tpm' attestation report",
+            detail=f"got platform={report.platform!r}",
+        )
     if report.nonce != expected_nonce:
         raise AttestationFailed(
             "the report nonce does not match the expected nonce",

tests/unit/test_tpm.py

@@ -188,3 +188,34 @@ def test_real_tpm_quote_parses_and_verifies() -> None:
     tampered[100] ^= 0x01
     with pytest.raises(InvalidSignature):
         ak_pub.verify(signature, bytes(tampered), padding.PKCS1v15(), hashes.SHA256())
+
+
+@pytest.mark.parametrize(
+    "bad_blob",
+    [
+        build_attest(qualifying_data=b"n", pcr_digest=b"\x11" * 32) + b"\x00",
+        build_attest(qualifying_data=b"n", pcr_digest=b"\x11" * 32) + b"trailing",
+        "not-bytes",  # type: ignore[list-item]
+    ],
+)
+def test_tpm_quote_parse_rejects_trailing_bytes_and_non_bytes(bad_blob: bytes) -> None:
+    with pytest.raises(AttestationFailed):
+        TpmQuote.parse(bad_blob)
+
+
+@pytest.mark.parametrize("wrong_platform", ["tdx", "sev-snp", "software-only", ""])
+def test_verify_tpm_report_rejects_non_tpm_platform(wrong_platform: str) -> None:
+    from ca2a_runtime.tee.base import AttestationReport
+    from ca2a_verify.tpm import verify_tpm_report
+
+    report = AttestationReport(
+        platform=wrong_platform,
+        measurement="sha256:" + "11" * 32,
+        public_key="aa" * 32,
+        nonce="fresh",
+        raw_evidence=build_attest(qualifying_data=b"n", pcr_digest=b"\x11" * 32),
+        quote_signature=b"\x00" * 64,
+        attestation_key_chain_pem=b"-----BEGIN CERTIFICATE-----\n",
+    )
+    with pytest.raises(AttestationFailed, match="expected a 'tpm' attestation report"):
+        verify_tpm_report(report, "fresh", trusted_roots_pem=b"-----BEGIN CERTIFICATE-----\n")