Patchwork [V15,5/7] Add a TPM Passthrough backend driver implementation

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Submitter Stefan Berger
Date March 27, 2012, 8:24 p.m.
Message ID <1332879879-29460-6-git-send-email-stefanb@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/149042/
State New
Headers show

Comments

Stefan Berger - March 27, 2012, 8:24 p.m.
From Andreas Niederl's original posting with adaptations where necessary:

This patch is based of off version 9 of Stefan Berger's patch series
  "Qemu Trusted Platform Module (TPM) integration"
and adds a new backend driver for it.

This patch adds a passthrough backend driver for passing commands sent to the
emulated TPM device directly to a TPM device opened on the host machine.

Thus it is possible to use a hardware TPM device in a system running on QEMU,
providing the ability to access a TPM in a special state (e.g. after a Trusted
Boot).

This functionality is being used in the acTvSM Trusted Virtualization Platform
which is available on [1].

Usage example:
  qemu-system-x86_64 -tpmdev passthrough,id=tpm0,path=/dev/tpm0 \
                     -device tpm-tis,tpmdev=tpm0 \
                     -cdrom test.iso -boot d

Some notes about the host TPM:
The TPM needs to be enabled and activated. If that's not the case one
has to go through the BIOS/UEFI and enable and activate that TPM for TPM
commands to work as expected.
It may be necessary to boot the kernel using tpm_tis.force=1 in the boot
command line or 'modprobe tpm_tis force=1' in case of using it as a module.

Regards,
Andreas Niederl, Stefan Berger

[1] http://trustedjava.sourceforge.net/

Signed-off-by: Andreas Niederl <andreas.niederl@iaik.tugraz.at>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
---
 Makefile.target      |    3 +-
 configure            |    3 +
 hw/tpm_backend.c     |   58 +++++++
 hw/tpm_backend.h     |   43 +++++
 hw/tpm_passthrough.c |  419 ++++++++++++++++++++++++++++++++++++++++++++++++++
 qemu-options.hx      |   37 +++++-
 tpm.c                |   17 ++
 tpm.h                |   33 ++++
 8 files changed, 611 insertions(+), 2 deletions(-)
 create mode 100644 hw/tpm_backend.c
 create mode 100644 hw/tpm_backend.h
 create mode 100644 hw/tpm_passthrough.c
Stefan Berger - March 28, 2012, 3:38 p.m.
On 03/27/2012 05:41 PM, Anthony Liguori wrote:
> On 03/27/2012 03:24 PM, Stefan Berger wrote:
>>  From Andreas Niederl's original posting with adaptations where 
>> necessary:
>>
>> This patch is based of off version 9 of Stefan Berger's patch series
>>    "Qemu Trusted Platform Module (TPM) integration"
>> and adds a new backend driver for it.
>>
[...]

>> +/* borrowed from qemu-char.c */
>
> Sharing code is better than borrowing code :-)
>

True. Fixed.


    Stefan

Patch

diff --git a/Makefile.target b/Makefile.target
index 7cb4588..7af3593 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -221,7 +221,8 @@  obj-$(CONFIG_NO_KVM) += kvm-stub.o
 obj-$(CONFIG_VGA) += vga.o
 obj-y += memory.o savevm.o
 obj-y += tpm.o
-obj-$(CONFIG_TPM) += tpm_tis.o
+obj-$(CONFIG_TPM) += tpm_tis.o tpm_backend.o
+obj-$(CONFIG_TPM_PASSTHROUGH) += tpm_passthrough.o
 LIBS+=-lz
 
 obj-i386-$(CONFIG_KVM) += hyperv.o
diff --git a/configure b/configure
index c728b89..6395f06 100755
--- a/configure
+++ b/configure
@@ -3847,6 +3847,9 @@  fi
 
 if test "$tpm" = "yes"; then
   if test "$target_softmmu" = "yes" ; then
+    if test "$linux" = "yes" ; then
+      echo "CONFIG_TPM_PASSTHROUGH=y" >> $config_target_mak
+    fi
     echo "CONFIG_TPM=y" >> $config_host_mak
   fi
 fi
diff --git a/hw/tpm_backend.c b/hw/tpm_backend.c
new file mode 100644
index 0000000..4cf0809
--- /dev/null
+++ b/hw/tpm_backend.c
@@ -0,0 +1,58 @@ 
+/*
+ *  common TPM backend driver functions
+ *
+ *  Copyright (c) 2012 IBM Corporation
+ *  Authors:
+ *    Stefan Berger <stefanb@us.ibm.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>
+ */
+
+#include "tpm.h"
+#include "qemu-thread.h"
+#include "hw/tpm_backend.h"
+
+void tpm_backend_thread_deliver_request(TPMBackendThread *tbt)
+{
+   g_thread_pool_push(tbt->pool, (gpointer)TPM_BACKEND_CMD_PROCESS_CMD, NULL);
+}
+
+void tpm_backend_thread_create(TPMBackendThread *tbt,
+                               GFunc func, gpointer user_data)
+{
+    if (!tbt->pool) {
+        tbt->pool = g_thread_pool_new(func, user_data, 1, TRUE, NULL);
+        g_thread_pool_push(tbt->pool, (gpointer)TPM_BACKEND_CMD_INIT, NULL);
+    }
+}
+
+void tpm_backend_thread_end(TPMBackendThread *tbt)
+{
+    if (tbt->pool) {
+        g_thread_pool_push(tbt->pool, (gpointer)TPM_BACKEND_CMD_END, NULL);
+        g_thread_pool_free(tbt->pool, FALSE, TRUE);
+        tbt->pool = NULL;
+    }
+}
+
+void tpm_backend_thread_tpm_reset(TPMBackendThread *tbt,
+                                  GFunc func, gpointer user_data)
+{
+    if (!tbt->pool) {
+        tpm_backend_thread_create(tbt, func, user_data);
+    } else {
+        g_thread_pool_push(tbt->pool, (gpointer)TPM_BACKEND_CMD_TPM_RESET,
+                           NULL);
+    }
+}
diff --git a/hw/tpm_backend.h b/hw/tpm_backend.h
new file mode 100644
index 0000000..f5fe198
--- /dev/null
+++ b/hw/tpm_backend.h
@@ -0,0 +1,43 @@ 
+/*
+ *  common TPM backend driver functions
+ *
+ *  Copyright (c) 2012 IBM Corporation
+ *  Authors:
+ *    Stefan Berger <stefanb@us.ibm.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>
+ */
+
+#ifndef HW_TPM_BACKEND_H
+#define HW_TPM_BACKEND_H
+
+typedef struct TPMBackendThread {
+    GThreadPool *pool;
+} TPMBackendThread;
+
+void tpm_backend_thread_deliver_request(TPMBackendThread *tbt);
+void tpm_backend_thread_create(TPMBackendThread *tbt,
+                               GFunc func, gpointer user_data);
+void tpm_backend_thread_end(TPMBackendThread *tbt);
+void tpm_backend_thread_tpm_reset(TPMBackendThread *tbt,
+                                  GFunc func, gpointer user_data);
+
+typedef enum TPMBackendCmd {
+    TPM_BACKEND_CMD_INIT = 1,
+    TPM_BACKEND_CMD_PROCESS_CMD,
+    TPM_BACKEND_CMD_END,
+    TPM_BACKEND_CMD_TPM_RESET,
+} TPMBackendCmd;
+
+#endif /* HW_TPM_BACKEND_H */
diff --git a/hw/tpm_passthrough.c b/hw/tpm_passthrough.c
new file mode 100644
index 0000000..dee7418
--- /dev/null
+++ b/hw/tpm_passthrough.c
@@ -0,0 +1,419 @@ 
+/*
+ *  passthrough TPM driver
+ *
+ *  Copyright (c) 2010, 2011 IBM Corporation
+ *  Authors:
+ *    Stefan Berger <stefanb@us.ibm.com>
+ *
+ *  Copyright (C) 2011 IAIK, Graz University of Technology
+ *    Author: Andreas Niederl
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>
+ */
+
+#include "qemu-common.h"
+#include "qemu-error.h"
+#include "tpm.h"
+#include "hw/hw.h"
+#include "hw/tpm_tis.h"
+#include "hw/tpm_backend.h"
+#include "hw/pc.h"
+
+/* #define DEBUG_TPM */
+
+#ifdef DEBUG_TPM
+#define dprintf(fmt, ...) \
+    do { fprintf(stderr, fmt, ## __VA_ARGS__); } while (0)
+#else
+#define dprintf(fmt, ...) \
+    do { } while (0)
+#endif
+
+/* data structures */
+
+typedef struct TPMPassthruThreadParams {
+    TPMState *tpm_state;
+
+    TPMRecvDataCB *recv_data_callback;
+    TPMBackend *tb;
+} TPMPassthruThreadParams;
+
+struct TPMPassthruState {
+    QemuThread thread;
+    TPMBackendThread tbt;
+
+    TPMPassthruThreadParams tpm_thread_params;
+
+    char *tpm_dev;
+    int tpm_fd;
+    bool had_startup_error;
+};
+
+#define TPM_PASSTHROUGH_DEFAULT_DEVICE "/dev/tpm0"
+
+/* borrowed from qemu-char.c */
+static int tpm_passthrough_unix_write(int fd, const uint8_t *buf, uint32_t len)
+{
+    int ret, len1;
+
+    len1 = len;
+    while (len1 > 0) {
+        ret = write(fd, buf, len1);
+        if (ret < 0) {
+            if (errno != EINTR && errno != EAGAIN) {
+                return -1;
+            }
+        } else if (ret == 0) {
+            break;
+        } else {
+            buf  += ret;
+            len1 -= ret;
+        }
+    }
+    return len - len1;
+}
+
+static int tpm_passthrough_unix_read(int fd, uint8_t *buf, uint32_t len)
+{
+    int ret, len1;
+    uint8_t *buf1;
+
+    len1 = len;
+    buf1 = buf;
+    while ((len1 > 0) && (ret = read(fd, buf1, len1)) != 0) {
+        if (ret < 0) {
+            if (errno != EINTR && errno != EAGAIN) {
+                return -1;
+            }
+        } else {
+            buf1 += ret;
+            len1 -= ret;
+        }
+    }
+    return len - len1;
+}
+
+static uint32_t tpm_passthrough_get_size_from_buffer(const uint8_t *buf)
+{
+    return be32_to_cpu(*(uint32_t *)&buf[2]);
+}
+
+static int tpm_passthrough_unix_tx_bufs(int tpm_fd,
+                                        const uint8_t *in, uint32_t in_len,
+                                        uint8_t *out, uint32_t out_len)
+{
+    int ret;
+
+    ret = tpm_passthrough_unix_write(tpm_fd, in, in_len);
+    if (ret < 0) {
+        error_report("tpm_passthrough: error while transmitting data "
+                     "to TPM: %s (%i)\n",
+                     strerror(errno), errno);
+        goto err_exit;
+    }
+
+    ret = tpm_passthrough_unix_read(tpm_fd, out, out_len);
+    if (ret < 0) {
+        error_report("tpm_passthrough: error while reading data from "
+                     "TPM: %s (%i)\n",
+                     strerror(errno), errno);
+    } else if (ret < sizeof(struct tpm_resp_hdr) ||
+               tpm_passthrough_get_size_from_buffer(out) != ret) {
+        ret = -1;
+        error_report("tpm_passthrough: received invalid response "
+                     "packet from TPM\n");
+    }
+
+err_exit:
+    if (ret < 0) {
+        tpm_write_fatal_error_response(out, out_len);
+    }
+
+    return ret;
+}
+
+static int tpm_passthrough_unix_transfer(int tpm_fd,
+                                         const TPMLocality *cmd_locty)
+{
+    return tpm_passthrough_unix_tx_bufs(tpm_fd,
+                                        cmd_locty->w_buffer.buffer,
+                                        cmd_locty->w_offset,
+                                        cmd_locty->r_buffer.buffer,
+                                        cmd_locty->r_buffer.size);
+}
+
+static void tpm_passthrough_worker_thread(gpointer data,
+                                          gpointer user_data)
+{
+    TPMPassthruThreadParams *thr_parms = user_data;
+    TPMPassthruState *tpm_pt = thr_parms->tb->s.tpm_pt;
+    TPMBackendCmd cmd = (TPMBackendCmd)data;
+
+    dprintf("tpm_passthrough: processing command type %ld\n", cmd);
+
+    switch (cmd) {
+    case TPM_BACKEND_CMD_PROCESS_CMD:
+        tpm_passthrough_unix_transfer(tpm_pt->tpm_fd,
+                                      thr_parms->tpm_state->cmd_locty);
+
+        thr_parms->recv_data_callback(thr_parms->tpm_state,
+                                      thr_parms->tpm_state->command_locty);
+        break;
+    case TPM_BACKEND_CMD_INIT:
+    case TPM_BACKEND_CMD_END:
+    case TPM_BACKEND_CMD_TPM_RESET:
+        /* nothing to do */
+        break;
+    }
+}
+
+/*
+ * Start the TPM (thread). If it had been started before, then terminate
+ * and start it again.
+ */
+static int tpm_passthrough_do_startup_tpm(TPMBackend *tb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+
+    /* terminate a running TPM */
+    tpm_backend_thread_end(&tpm_pt->tbt);
+
+    tpm_backend_thread_create(&tpm_pt->tbt,
+                              tpm_passthrough_worker_thread,
+                              &tb->s.tpm_pt->tpm_thread_params);
+
+    return 0;
+}
+
+static int tpm_passthrough_startup_tpm(TPMBackend *tb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+    int rc;
+
+    rc = tpm_passthrough_do_startup_tpm(tb);
+    if (rc) {
+        tpm_pt->had_startup_error = true;
+    }
+    return rc;
+}
+
+static void tpm_passthrough_reset(TPMBackend *tb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+
+    dprintf("tpm_passthrough: CALL TO TPM_RESET!\n");
+
+    tpm_backend_thread_end(&tpm_pt->tbt);
+
+    tpm_pt->had_startup_error = false;
+}
+
+static int tpm_passthrough_init(TPMBackend *tb, TPMState *s,
+                                TPMRecvDataCB *recv_data_cb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+
+    tpm_pt->tpm_thread_params.tpm_state = s;
+    tpm_pt->tpm_thread_params.recv_data_callback = recv_data_cb;
+    tpm_pt->tpm_thread_params.tb = tb;
+
+    return 0;
+}
+
+static bool tpm_passthrough_get_tpm_established_flag(TPMBackend *tb)
+{
+    return false;
+}
+
+static bool tpm_passthrough_get_startup_error(TPMBackend *tb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+
+    return tpm_pt->had_startup_error;
+}
+
+static size_t tpm_passthrough_realloc_buffer(TPMSizedBuffer *sb)
+{
+    size_t wanted_size = 4096; /* Linux tpm.c buffer size */
+
+    if (sb->size != wanted_size) {
+        sb->buffer = g_realloc(sb->buffer, wanted_size);
+        sb->size = wanted_size;
+    }
+    return sb->size;
+}
+
+static void tpm_passthrough_deliver_request(TPMBackend *tb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+
+    tpm_backend_thread_deliver_request(&tpm_pt->tbt);
+}
+
+static const char *tpm_passthrough_create_desc(void)
+{
+    return "Passthrough TPM backend driver";
+}
+
+/*
+ * A basic test of a TPM device. We expect a well formatted response header
+ * (error response is fine) within one second.
+ */
+static int tpm_passthrough_test_tpmdev(int fd)
+{
+    struct tpm_req_hdr req = {
+        .tag = cpu_to_be16(TPM_TAG_RQU_COMMAND),
+        .len = cpu_to_be32(sizeof(req)),
+        .ordinal = cpu_to_be32(TPM_ORD_GetTicks),
+    };
+    struct tpm_resp_hdr *resp;
+    fd_set readfds;
+    int n;
+    struct timeval tv = {
+        .tv_sec = 1,
+        .tv_usec = 0,
+    };
+    unsigned char buf[1024];
+
+    n = write(fd, &req, sizeof(req));
+    if (n < 0) {
+        return errno;
+    }
+    if (n != sizeof(req)) {
+        return EFAULT;
+    }
+
+    FD_ZERO(&readfds);
+    FD_SET(fd, &readfds);
+
+    /* wait for a second */
+    n = select(fd + 1, &readfds, NULL, NULL, &tv);
+    if (n != 1) {
+        return errno;
+    }
+
+    n = read(fd, &buf, sizeof(buf));
+    if (n < sizeof(struct tpm_resp_hdr)) {
+        return EFAULT;
+    }
+
+    resp = (struct tpm_resp_hdr *)buf;
+    /* check the header */
+    if (be16_to_cpu(resp->tag) != TPM_TAG_RSP_COMMAND ||
+        be32_to_cpu(resp->len) != n) {
+        return EBADMSG;
+    }
+
+    return 0;
+}
+
+static int tpm_passthrough_handle_device_opts(QemuOpts *opts, TPMBackend *tb)
+{
+    const char *value;
+    size_t bufsize;
+
+    value = qemu_opt_get(opts, "path");
+    if (!value) {
+        value = TPM_PASSTHROUGH_DEFAULT_DEVICE;
+    }
+
+    tb->s.tpm_pt->tpm_dev = g_strdup(value);
+
+    bufsize = sizeof("path=") + strlen(tb->s.tpm_pt->tpm_dev) + 1;
+
+    tb->parameters = g_malloc0(bufsize);
+
+    snprintf(tb->parameters, bufsize, "path=%s", tb->s.tpm_pt->tpm_dev);
+
+    tb->s.tpm_pt->tpm_fd = open(tb->s.tpm_pt->tpm_dev, O_RDWR);
+    if (tb->s.tpm_pt->tpm_fd < 0) {
+        error_report("Cannot access TPM device using '%s'.\n",
+                     tb->s.tpm_pt->tpm_dev);
+        goto err_free_parameters;
+    }
+
+    if (tpm_passthrough_test_tpmdev(tb->s.tpm_pt->tpm_fd)) {
+        error_report("'%s' is not a TPM device.\n",
+                     tb->s.tpm_pt->tpm_dev);
+        goto err_close_tpmdev;
+    }
+
+    return 0;
+
+ err_close_tpmdev:
+    close(tb->s.tpm_pt->tpm_fd);
+    tb->s.tpm_pt->tpm_fd = -1;
+
+ err_free_parameters:
+    g_free(tb->parameters);
+    tb->parameters = NULL;
+
+    g_free(tb->s.tpm_pt->tpm_dev);
+    tb->s.tpm_pt->tpm_dev = NULL;
+
+    return 1;
+}
+
+static TPMBackend *tpm_passthrough_create(QemuOpts *opts, const char *id)
+{
+    TPMBackend *tb;
+
+    tb = g_new0(TPMBackend, 1);
+    tb->s.tpm_pt = g_new0(TPMPassthruState, 1);
+    tb->id = g_strdup(id);
+
+    tb->ops = &tpm_passthrough_driver;
+
+    if (tpm_passthrough_handle_device_opts(opts, tb)) {
+        goto err_exit;
+    }
+
+    return tb;
+
+err_exit:
+    g_free(tb->id);
+    g_free(tb->s.tpm_pt);
+    g_free(tb);
+
+    return NULL;
+}
+
+static void tpm_passthrough_destroy(TPMBackend *tb)
+{
+    TPMPassthruState *tpm_pt = tb->s.tpm_pt;
+
+    tpm_backend_thread_end(&tpm_pt->tbt);
+
+    close(tpm_pt->tpm_fd);
+
+    g_free(tb->id);
+    g_free(tb->parameters);
+    g_free(tb->s.tpm_pt->tpm_dev);
+    g_free(tb->s.tpm_pt);
+    g_free(tb);
+}
+
+const TPMDriverOps tpm_passthrough_driver = {
+    .id                       = "passthrough",
+    .desc                     = tpm_passthrough_create_desc,
+    .create                   = tpm_passthrough_create,
+    .destroy                  = tpm_passthrough_destroy,
+    .init                     = tpm_passthrough_init,
+    .startup_tpm              = tpm_passthrough_startup_tpm,
+    .realloc_buffer           = tpm_passthrough_realloc_buffer,
+    .reset                    = tpm_passthrough_reset,
+    .had_startup_error        = tpm_passthrough_get_startup_error,
+    .deliver_request          = tpm_passthrough_deliver_request,
+    .get_tpm_established_flag = tpm_passthrough_get_tpm_established_flag,
+};
diff --git a/qemu-options.hx b/qemu-options.hx
index 355a9c1..c8fbe07 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2006,7 +2006,9 @@  DEFHEADING()
 DEFHEADING(TPM device options:)
 
 DEF("tpmdev", HAS_ARG, QEMU_OPTION_tpmdev, \
-    "-tpmdev [<type>],id=str[,option][,option][,...]\n",
+    "-tpmdev [passthrough],id=id[,option][,option][,...]\n"
+    "-tpmdev passthrough,id=id[,path=path]\n"
+    "                use path to provide path to a character device; default is /dev/tpm0\n",
     QEMU_ARCH_ALL)
 STEXI
 
@@ -2016,6 +2018,7 @@  The general form of a TPM device option is:
 @item -tpmdev @var{backend} ,id=@var{id} [,@var{options}]
 @findex -tpmdev
 Backend type must be:
+@option{passthrough}.
 
 The specific backend type will determine the applicable options.
 The @code{-tpmdev} options requires a @code{-device} option.
@@ -2027,6 +2030,38 @@  Use ? to print all available TPM backend types.
 qemu -tpmdev ?
 @end example
 
+@item -tpmdev passthrough, id=@var{id}, path=@var{path}
+
+(Linux-host only) Enable access to the host's TPM using the passthrough
+driver.
+
+@option{path} specifies the path to the host's TPM device, i.e., on
+a Linux host this would be @code{/dev/tpm0}.
+@option{path} is optional and by default @code{/dev/tpm0} is used.
+
+Some notes about using the host's TPM with the passthrough driver:
+
+The TPM device accessed by the passthrough driver must not be
+used by any other application on the host.
+
+Since the host's firmware (BIOS/UEFI) has already initialized the TPM,
+the VM's firmware (BIOS/UEFI) will not be able to initialize the
+TPM again and may therefore not show a TPM-specific menu that would
+otherwise allow the user to configure the TPM, e.g., allow the user to
+enable/disable or activate/deactivate the TPM.
+Further, if TPM ownership is released from within a VM then the host's TPM
+will get disabled and deactivated. To enable and activate the
+TPM again afterwards, the host has to be rebooted and the user is
+required to enter the firmware's menu to enable and activate the TPM.
+If the TPM is left disabled and/or deactivated most TPM commands will fail.
+
+To create a passthrough TPM use the following two options:
+@example
+-tpmdev passthrough,id=tpm0 -device tpm-tis,tpmdev=tpm0
+@end example
+Note that the @code{-tpmdev} id is @code{tpm0} and is referenced by
+@code{tpmdev=tpm0} in the device option.
+
 @end table
 
 ETEXI
diff --git a/tpm.c b/tpm.c
index 6324c1e..e285eaa 100644
--- a/tpm.c
+++ b/tpm.c
@@ -24,9 +24,26 @@  static QLIST_HEAD(, TPMBackend) tpm_backends =
 #ifdef CONFIG_TPM
 
 static const TPMDriverOps *bes[] = {
+#ifdef CONFIG_TPM_PASSTHROUGH
+    &tpm_passthrough_driver,
+#endif
     NULL,
 };
 
+/*
+ * Write an error message in the given output buffer.
+ */
+void tpm_write_fatal_error_response(uint8_t *out, uint32_t out_len)
+{
+    if (out_len >= sizeof(struct tpm_resp_hdr)) {
+        struct tpm_resp_hdr *resp = (struct tpm_resp_hdr *)out;
+
+        resp->tag = cpu_to_be16(TPM_TAG_RSP_COMMAND);
+        resp->len = cpu_to_be32(sizeof(struct tpm_resp_hdr));
+        resp->errcode = cpu_to_be32(TPM_FAIL);
+    }
+}
+
 const TPMDriverOps *tpm_get_backend_driver(const char *id)
 {
     int i;
diff --git a/tpm.h b/tpm.h
index 4e871d9..c7de642 100644
--- a/tpm.h
+++ b/tpm.h
@@ -18,12 +18,18 @@ 
 struct TPMDriverOps;
 typedef struct TPMDriverOps TPMDriverOps;
 
+typedef struct TPMPassthruState TPMPassthruState;
+
 typedef struct TPMBackend {
     char *id;
     const char *fe_model;
     char *parameters;
     const TPMDriverOps *ops;
 
+    union {
+        TPMPassthruState *tpm_pt;
+    } s;
+
     QLIST_ENTRY(TPMBackend) list;
 } TPMBackend;
 
@@ -71,11 +77,38 @@  struct TPMDriverOps {
 
 #define TPM_DEFAULT_DEVICE_MODEL "tpm-tis"
 
+struct tpm_req_hdr {
+    uint16_t tag;
+    uint32_t len;
+    uint32_t ordinal;
+} __attribute__((packed));
+
+struct tpm_resp_hdr {
+    uint16_t tag;
+    uint32_t len;
+    uint32_t errcode;
+} __attribute__((packed));
+
+#define TPM_TAG_RQU_COMMAND       0xc1
+#define TPM_TAG_RQU_AUTH1_COMMAND 0xc2
+#define TPM_TAG_RQU_AUTH2_COMMAND 0xc3
+
+#define TPM_TAG_RSP_COMMAND       0xc4
+#define TPM_TAG_RSP_AUTH1_COMMAND 0xc5
+#define TPM_TAG_RSP_AUTH2_COMMAND 0xc6
+
+#define TPM_FAIL                  9
+
+#define TPM_ORD_GetTicks          0xf1
+
 int tpm_config_parse(QemuOptsList *opts_list, const char *optarg);
 int tpm_init(void);
 void tpm_cleanup(void);
 TPMBackend *qemu_find_tpm(const char *id);
 void tpm_display_backend_drivers(void);
 const TPMDriverOps *tpm_get_backend_driver(const char *id);
+void tpm_write_fatal_error_response(uint8_t *out, uint32_t out_len);
+
+extern const TPMDriverOps tpm_passthrough_driver;
 
 #endif /* QEMU_TPM_H */