diff --git a/sys/dev/tpm/tpm20.h b/sys/dev/tpm/tpm20.h
--- a/sys/dev/tpm/tpm20.h
+++ b/sys/dev/tpm/tpm20.h
@@ -124,6 +124,7 @@
 
 	uint8_t 	*buf;
 	size_t		pending_data_length;
+	size_t		total_length;
 	lwpid_t		owner_tid;
 
 	struct callout 	discard_buffer_callout;
diff --git a/sys/dev/tpm/tpm20.c b/sys/dev/tpm/tpm20.c
--- a/sys/dev/tpm/tpm20.c
+++ b/sys/dev/tpm/tpm20.c
@@ -68,6 +68,7 @@
 {
 	struct tpm_sc *sc;
 	size_t bytes_to_transfer;
+	size_t offset;
 	int result = 0;
 
 	sc = (struct tpm_sc *)dev->si_drv1;
@@ -80,10 +81,10 @@
 	}
 
 	bytes_to_transfer = MIN(sc->pending_data_length, uio->uio_resid);
+	offset = sc->total_length - sc->pending_data_length;
 	if (bytes_to_transfer > 0) {
-		result = uiomove((caddr_t) sc->buf, bytes_to_transfer, uio);
-		memset(sc->buf, 0, TPM_BUFSIZE);
-		sc->pending_data_length = 0;
+		result = uiomove((caddr_t) sc->buf + offset, bytes_to_transfer, uio);
+		sc->pending_data_length -= bytes_to_transfer;
 		cv_signal(&sc->buf_cv);
 	} else {
 		result = ETIMEDOUT;
@@ -152,6 +153,7 @@
 
 	memset(sc->buf, 0, TPM_BUFSIZE);
 	sc->pending_data_length = 0;
+	sc->total_length = 0;
 
 	cv_signal(&sc->buf_cv);
 	sx_xunlock(&sc->dev_lock);
@@ -191,6 +193,7 @@
 	cv_init(&sc->buf_cv, "TPM buffer cv");
 	callout_init(&sc->discard_buffer_callout, 1);
 	sc->pending_data_length = 0;
+	sc->total_length = 0;
 
 	make_dev_args_init(&args);
 	args.mda_devsw = &tpm20_cdevsw;
@@ -275,6 +278,7 @@
 
 	/* Ignore response size */
 	sc->pending_data_length = 0;
+	sc->total_length = 0;
 
 	/* The number of random bytes we got is placed right after the header */
 	entropy_size = (uint16_t) sc->buf[TPM_HEADER_SIZE + 1];
diff --git a/sys/dev/tpm/tpm_crb.c b/sys/dev/tpm/tpm_crb.c
--- a/sys/dev/tpm/tpm_crb.c
+++ b/sys/dev/tpm/tpm_crb.c
@@ -398,6 +398,7 @@
 
 	tpmcrb_relinquish_locality(sc);
 	sc->pending_data_length = bytes_available;
+	sc->total_length = bytes_available;
 
 	return (0);
 }
diff --git a/sys/dev/tpm/tpm_tis_core.c b/sys/dev/tpm/tpm_tis_core.c
--- a/sys/dev/tpm/tpm_tis_core.c
+++ b/sys/dev/tpm/tpm_tis_core.c
@@ -469,6 +469,7 @@
 	}
 	tpmtis_relinquish_locality(sc);
 	sc->pending_data_length = bytes_available;
+	sc->total_length = bytes_available;
 
 	return (0);
 }