F-4673 - Bind context sequence counter to blob and reject replay

src/fwtpm/fwtpm_command.c

@@ -3077,7 +3077,7 @@ static TPM_RC FwCmd_ContextLoad(FWTPM_CTX* ctx, TPM2_Packet* cmd,
     UINT32 magic = 0, version = 0;
 
     (void)cmdTag;
-    (void)seqHi; (void)seqLo; (void)hierarchy;
+    (void)hierarchy;
 
     if (cmdSize < TPM2_HEADER_SIZE + 18) {
         rc = TPM_RC_COMMAND_SIZE;
@@ -3091,6 +3091,13 @@ static TPM_RC FwCmd_ContextLoad(FWTPM_CTX* ctx, TPM2_Packet* cmd,
         TPM2_Packet_ParseU16(cmd, &blobSz);
     }
 
+    /* Replay protection: a saved context is bound to the sequence counter
+     * value it was created with, and each successful load consumes it. */
+    if (rc == 0 &&
+            (((UINT64)seqHi << 32) | (UINT64)seqLo) != ctx->contextSeqCounter) {
+        rc = TPM_RC_INTEGRITY;
+    }
+
     /* Validate minimum blob size (magic + version = 8 bytes) */
     if (rc == 0 && blobSz < 8) {
         rc = TPM_RC_SIZE;
@@ -3189,6 +3196,8 @@ static TPM_RC FwCmd_ContextLoad(FWTPM_CTX* ctx, TPM2_Packet* cmd,
     #ifdef DEBUG_WOLFTPM
         printf("fwTPM: ContextLoad(handle=0x%x)\n", savedHandle);
     #endif
+        /* Consume this sequence value so the same blob cannot be replayed */
+        ctx->contextSeqCounter++;
         TPM2_Packet_AppendU32(rsp, savedHandle);
         FwRspFinalize(rsp, TPM_ST_NO_SESSIONS, TPM_RC_SUCCESS);
     }

src/fwtpm/fwtpm_crypto.c

@@ -2219,6 +2219,18 @@ int FwUnwrapPrivate(FWTPM_Object* parent,
 /* Context blob wrap/unwrap (ContextSave/ContextLoad)                  */
 /* ================================================================== */
 
+/* Fold a 64-bit value into an HMAC as big-endian, used to bind the context
+ * sequence counter into the blob MAC for replay protection. */
+static int FwHmacUpdateU64(Hmac* hmac, UINT64 v)
+{
+    byte b[8];
+    int i;
+    for (i = 0; i < 8; i++) {
+        b[i] = (byte)(v >> (56 - 8 * i));
+    }
+    return wc_HmacUpdate(hmac, b, (word32)sizeof(b));
+}
+
 /* Encrypt-then-MAC context blob protection using the per-boot key.
  * Layout: iv(16) | ciphertext(plainSz) | hmac(32)
  * Returns 0 on success, sets *outSz. */
@@ -2271,6 +2283,9 @@ int FwWrapContextBlob(FWTPM_CTX* ctx,
     if (rc == 0) {
         rc = wc_HmacUpdate(hmac, out, AES_BLOCK_SIZE + plainSz);
     }
+    if (rc == 0) {
+        rc = FwHmacUpdateU64(hmac, ctx->contextSeqCounter);
+    }
     if (rc == 0) {
         rc = wc_HmacFinal(hmac, out + AES_BLOCK_SIZE + plainSz);
     }
@@ -2328,6 +2343,9 @@ int FwUnwrapContextBlob(FWTPM_CTX* ctx,
     if (rc == 0) {
         rc = wc_HmacUpdate(hmac, in, AES_BLOCK_SIZE + cipherSz);
     }
+    if (rc == 0) {
+        rc = FwHmacUpdateU64(hmac, ctx->contextSeqCounter);
+    }
     if (rc == 0) {
         rc = wc_HmacFinal(hmac, computedHmac);
     }

tests/fwtpm_unit_tests.c

@@ -8482,6 +8482,53 @@ static void test_fwtpm_context_save(void)
     fwtpm_pass("ContextSave:", 0);
 }
 
+/* A saved context must load at most once; replaying the same blob is
+ * rejected to prevent resurrecting a satisfied policy session. */
+static void test_fwtpm_context_load_replay_rejected(void)
+{
+    FWTPM_CTX ctx;
+    int rspSize = 0, ctxSz, pos;
+    UINT32 keyH;
+    byte savedCtx[512];
+
+    memset(&ctx, 0, sizeof(ctx));
+    AssertIntEQ(fwtpm_test_startup(&ctx), 0);
+#ifdef HAVE_ECC
+    keyH = CreatePrimaryHelper(&ctx, TPM_ALG_ECC);
+#else
+    keyH = CreatePrimaryHelper(&ctx, TPM_ALG_RSA);
+#endif
+    AssertIntNE(keyH, 0);
+
+    BuildCmdHeader(gCmd, TPM_ST_NO_SESSIONS, 14, TPM_CC_ContextSave);
+    PutU32BE(gCmd + 10, keyH);
+    FWTPM_ProcessCommand(&ctx, gCmd, 14, gRsp, &rspSize, 0);
+    AssertIntEQ(GetRspRC(gRsp), TPM_RC_SUCCESS);
+    ctxSz = rspSize - TPM2_HEADER_SIZE; /* TPMS_CONTEXT follows the header */
+    AssertIntGT(ctxSz, 0);
+    memcpy(savedCtx, gRsp + TPM2_HEADER_SIZE, ctxSz);
+
+    /* First load succeeds */
+    pos = BuildCmdHeader(gCmd, TPM_ST_NO_SESSIONS, 0, TPM_CC_ContextLoad);
+    memcpy(gCmd + pos, savedCtx, ctxSz); pos += ctxSz;
+    PutU32BE(gCmd + 2, (UINT32)pos);
+    rspSize = 0;
+    FWTPM_ProcessCommand(&ctx, gCmd, pos, gRsp, &rspSize, 0);
+    AssertIntEQ(GetRspRC(gRsp), TPM_RC_SUCCESS);
+
+    /* Replaying the identical blob is rejected */
+    pos = BuildCmdHeader(gCmd, TPM_ST_NO_SESSIONS, 0, TPM_CC_ContextLoad);
+    memcpy(gCmd + pos, savedCtx, ctxSz); pos += ctxSz;
+    PutU32BE(gCmd + 2, (UINT32)pos);
+    rspSize = 0;
+    FWTPM_ProcessCommand(&ctx, gCmd, pos, gRsp, &rspSize, 0);
+    AssertIntNE(GetRspRC(gRsp), TPM_RC_SUCCESS);
+
+    FlushHandle(&ctx, keyH);
+    FWTPM_Cleanup(&ctx);
+    printf("Test fwTPM:\tContextLoad replay rejected:\tPassed\n");
+}
+
 static void test_fwtpm_evict_control(void)
 {
     FWTPM_CTX ctx;
@@ -9017,6 +9064,7 @@ int fwtpm_unit_tests(int argc, char *argv[])
     test_fwtpm_evict_control_bad_persistent_handle_rejected();
     test_fwtpm_evict_control_persistent_object_rejected();
     test_fwtpm_context_save();
+    test_fwtpm_context_load_replay_rejected();
 
     /* Crypto */
     test_fwtpm_hash();