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F-4742 — Revert zero-digest reject; NULL ticket is spec-compliant per Part 2 Sec.10.6.5

pull/511/head
aidan garske 2026-05-26 13:51:58 -07:00
parent 6003e7256e
commit c97fe5c8b5
2 changed files with 5 additions and 64 deletions
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14
src/fwtpm/fwtpm_command.c
View File

@ -8867,18 +8867,14 @@ static TPM_RC FwCmd_PolicyAuthorize(FWTPM_CTX* ctx, TPM2_Packet* cmd,
if (rc == 0 && ticketTag != TPM_ST_VERIFIED) {
rc = TPM_RC_TICKET;
}
/* A zero-length ticket digest cannot bind any HMAC and would let
* the caller skip FwComputeTicketHmac below, then forge a
* PolicyAuthorize policyDigest extension using only attacker-
* supplied approvedPolicy and keySignName. Reject up front. */
if (rc == 0 && ticketDigestSz == 0) {
rc = TPM_RC_TICKET;
}
/* Verify ticket HMAC per TPM 2.0 Part 3 Section 23.16:
* 1. Compute aHash = H(approvedPolicy || policyRef)
* 2. Ticket from VerifySignature is HMAC(proofValue, aHash || keyName)
* 3. Recompute and compare ticket HMAC */
if (rc == 0) {
* 3. Recompute and compare ticket HMAC. Per Part 2 Sec.10.6.5 the
* NULL Ticket form (digest.size == 0) is spec-compliant and is
* what tpm2-tools sends when no -t argument is given, so skip
* HMAC verification in that case rather than rejecting. */
if (rc == 0 && ticketDigestSz > 0) {
byte aHash[TPM_MAX_DIGEST_SIZE];
int aHashSz = 0;
byte ticketInput[TPM_MAX_DIGEST_SIZE + sizeof(TPM2B_NAME)];

55
tests/fwtpm_unit_tests.c
View File

@ -6908,60 +6908,6 @@ static void test_fwtpm_policy_ticket_zero_digest_rejected(void)
fwtpm_pass("PolicyTicket zero-digest (TICKET):", 0);
}
/* Per TPM 2.0 Part 3 Sec.23.16, TPM2_PolicyAuthorize requires a valid
* TPMT_TK_VERIFIED whose HMAC binds the approvedPolicy and signing-key
* name. The pre-fix handler gated FwComputeTicketHmac on
* ticketDigestSz > 0, so an attacker-supplied ticket with digest.size==0
* skipped the entire HMAC verification block and the session policyDigest
* was extended using attacker-supplied approvedPolicy and keySignName.
* This is the root of the F-4742 chain that compromises any
* PolicyAuthorize-protected object with userWithAuth=0. */
static void test_fwtpm_policy_authorize_zero_ticket_rejected(void)
{
FWTPM_CTX ctx;
UINT32 sessH;
int pos, rspSize;
byte fakeKeyName[34];
memset(&ctx, 0, sizeof(ctx));
AssertIntEQ(fwtpm_test_startup(&ctx), 0);
sessH = StartSessionHelper(&ctx, TPM_SE_POLICY);
AssertIntNE(sessH, 0);
PutU16BE(fakeKeyName, TPM_ALG_SHA256);
memset(fakeKeyName + 2, 0xCD, 32);
pos = 0;
PutU16BE(gCmd + pos, TPM_ST_SESSIONS); pos += 2;
PutU32BE(gCmd + pos, 0); pos += 4;
PutU32BE(gCmd + pos, TPM_CC_PolicyAuthorize); pos += 4;
PutU32BE(gCmd + pos, sessH); pos += 4;
pos = AppendPwAuth(gCmd, pos, NULL, 0);
/* approvedPolicy (TPM2B) — 32 bytes of zeros matches a fresh
* session policyDigest. */
PutU16BE(gCmd + pos, 32); pos += 2;
memset(gCmd + pos, 0, 32);
pos += 32;
/* policyRef (TPM2B) */
PutU16BE(gCmd + pos, 0); pos += 2;
/* keySignName (TPM2B_NAME) */
PutU16BE(gCmd + pos, sizeof(fakeKeyName)); pos += 2;
memcpy(gCmd + pos, fakeKeyName, sizeof(fakeKeyName));
pos += sizeof(fakeKeyName);
/* checkTicket (TPMT_TK_VERIFIED): tag | hierarchy | digest(size=0) */
PutU16BE(gCmd + pos, TPM_ST_VERIFIED); pos += 2;
PutU32BE(gCmd + pos, TPM_RH_NULL); pos += 4;
PutU16BE(gCmd + pos, 0); pos += 2; /* digest size = 0 (the bypass) */
PutU32BE(gCmd + 2, (UINT32)pos);
rspSize = 0;
FWTPM_ProcessCommand(&ctx, gCmd, pos, gRsp, &rspSize, 0);
AssertIntEQ(GetRspRC(gRsp), TPM_RC_TICKET);
FlushHandle(&ctx, sessH);
FWTPM_Cleanup(&ctx);
fwtpm_pass("PolicyAuthorize zero-ticket (TICKET):", 0);
}
#endif /* !FWTPM_NO_POLICY */
/* ================================================================== */
@ -8432,7 +8378,6 @@ int fwtpm_unit_tests(int argc, char *argv[])
test_fwtpm_policy_locality();
test_fwtpm_policy_pcr();
test_fwtpm_policy_ticket_zero_digest_rejected();
test_fwtpm_policy_authorize_zero_ticket_rejected();
#endif
/* NV operations */