hw/tpm/tpm_passthrough.c - qemu - Git at Google

 /*
  *  passthrough TPM driver
  *
  *  Copyright (c) 2010 - 2013 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/osdep.h"
 #include "qemu-common.h"
 #include "qemu/error-report.h"
 #include "qemu/sockets.h"
 #include "sysemu/tpm_backend.h"
 #include "tpm_int.h"
 #include "hw/hw.h"
 #include "hw/i386/pc.h"
 #include "tpm_tis.h"
 #include "tpm_util.h"

 #define DEBUG_TPM 0

 #define DPRINTF(fmt, ...) do { \
     if (DEBUG_TPM) { \
         fprintf(stderr, fmt, ## __VA_ARGS__); \
     } \
 } while (0);

 #define TYPE_TPM_PASSTHROUGH "tpm-passthrough"
 #define TPM_PASSTHROUGH(obj) \
     OBJECT_CHECK(TPMPassthruState, (obj), TYPE_TPM_PASSTHROUGH)

 /* data structures */
 struct TPMPassthruState {
     TPMBackend parent;

     char *tpm_dev;
     int tpm_fd;
     bool tpm_executing;
     bool tpm_op_canceled;
     int cancel_fd;

     TPMVersion tpm_version;
 };

 typedef struct TPMPassthruState TPMPassthruState;

 #define TPM_PASSTHROUGH_DEFAULT_DEVICE "/dev/tpm0"

 /* functions */

 static void tpm_passthrough_cancel_cmd(TPMBackend *tb);

 static int tpm_passthrough_unix_write(int fd, const uint8_t *buf, uint32_t len)
 {
     int ret, remain;

     remain = len;
     while (remain > 0) {
         ret = write(fd, buf, remain);
         if (ret < 0) {
             if (errno != EINTR && errno != EAGAIN) {
                 return -1;
             }
         } else if (ret == 0) {
             break;
         } else {
             buf += ret;
             remain -= ret;
         }
     }
     return len - remain;
 }

 static int tpm_passthrough_unix_read(int fd, uint8_t *buf, uint32_t len)
 {
     int ret;
  reread:
     ret = read(fd, buf, len);
     if (ret < 0) {
         if (errno != EINTR && errno != EAGAIN) {
             return -1;
         }
         goto reread;
     }
     return ret;
 }

 static uint32_t tpm_passthrough_get_size_from_buffer(const uint8_t *buf)
 {
     struct tpm_resp_hdr *resp = (struct tpm_resp_hdr *)buf;

     return be32_to_cpu(resp->len);
 }

 /*
  * Write an error message in the given output buffer.
  */
 static 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);
     }
 }

 static bool tpm_passthrough_is_selftest(const uint8_t *in, uint32_t in_len)
 {
     struct tpm_req_hdr *hdr = (struct tpm_req_hdr *)in;

     if (in_len >= sizeof(*hdr)) {
         return (be32_to_cpu(hdr->ordinal) == TPM_ORD_ContinueSelfTest);
     }

     return false;
 }

 static int tpm_passthrough_unix_tx_bufs(TPMPassthruState *tpm_pt,
                                         const uint8_t *in, uint32_t in_len,
                                         uint8_t *out, uint32_t out_len,
                                         bool *selftest_done)
 {
     int ret;
     bool is_selftest;
     const struct tpm_resp_hdr *hdr;

     tpm_pt->tpm_op_canceled = false;
     tpm_pt->tpm_executing = true;
     *selftest_done = false;

     is_selftest = tpm_passthrough_is_selftest(in, in_len);

     ret = tpm_passthrough_unix_write(tpm_pt->tpm_fd, in, in_len);
     if (ret != in_len) {
         if (!tpm_pt->tpm_op_canceled || errno != ECANCELED) {
             error_report("tpm_passthrough: error while transmitting data "
                          "to TPM: %s (%i)",
                          strerror(errno), errno);
         }
         goto err_exit;
     }

     tpm_pt->tpm_executing = false;

     ret = tpm_passthrough_unix_read(tpm_pt->tpm_fd, out, out_len);
     if (ret < 0) {
         if (!tpm_pt->tpm_op_canceled || errno != ECANCELED) {
             error_report("tpm_passthrough: error while reading data from "
                          "TPM: %s (%i)",
                          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");
     }

     if (is_selftest && (ret >= sizeof(struct tpm_resp_hdr))) {
         hdr = (struct tpm_resp_hdr *)out;
         *selftest_done = (be32_to_cpu(hdr->errcode) == 0);
     }

 err_exit:
     if (ret < 0) {
         tpm_write_fatal_error_response(out, out_len);
     }

     tpm_pt->tpm_executing = false;

     return ret;
 }

 static int tpm_passthrough_unix_transfer(TPMPassthruState *tpm_pt,
                                          const TPMLocality *locty_data,
                                          bool *selftest_done)
 {
     return tpm_passthrough_unix_tx_bufs(tpm_pt,
                                         locty_data->w_buffer.buffer,
                                         locty_data->w_offset,
                                         locty_data->r_buffer.buffer,
                                         locty_data->r_buffer.size,
                                         selftest_done);
 }

 static void tpm_passthrough_handle_request(TPMBackend *tb, TPMBackendCmd cmd)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
     bool selftest_done = false;

     DPRINTF("tpm_passthrough: processing command type %d\n", cmd);

     switch (cmd) {
     case TPM_BACKEND_CMD_PROCESS_CMD:
         tpm_passthrough_unix_transfer(tpm_pt,
                                       tb->tpm_state->locty_data,
                                       &selftest_done);

         tb->recv_data_callback(tb->tpm_state,
                                tb->tpm_state->locty_number,
                                selftest_done);
         break;
     case TPM_BACKEND_CMD_INIT:
     case TPM_BACKEND_CMD_END:
     case TPM_BACKEND_CMD_TPM_RESET:
         /* nothing to do */
         break;
     }
 }

 static void tpm_passthrough_reset(TPMBackend *tb)
 {
     DPRINTF("tpm_passthrough: CALL TO TPM_RESET!\n");

     tpm_passthrough_cancel_cmd(tb);
 }

 static bool tpm_passthrough_get_tpm_established_flag(TPMBackend *tb)
 {
     return false;
 }

 static int tpm_passthrough_reset_tpm_established_flag(TPMBackend *tb,
                                                       uint8_t locty)
 {
     /* only a TPM 2.0 will support this */
     return 0;
 }

 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_cancel_cmd(TPMBackend *tb)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
     int n;

     /*
      * As of Linux 3.7 the tpm_tis driver does not properly cancel
      * commands on all TPM manufacturers' TPMs.
      * Only cancel if we're busy so we don't cancel someone else's
      * command, e.g., a command executed on the host.
      */
     if (tpm_pt->tpm_executing) {
         if (tpm_pt->cancel_fd >= 0) {
             n = write(tpm_pt->cancel_fd, "-", 1);
             if (n != 1) {
                 error_report("Canceling TPM command failed: %s",
                              strerror(errno));
             } else {
                 tpm_pt->tpm_op_canceled = true;
             }
         } else {
             error_report("Cannot cancel TPM command due to missing "
                          "TPM sysfs cancel entry");
         }
     }
 }

 static TPMVersion tpm_passthrough_get_tpm_version(TPMBackend *tb)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);

     return tpm_pt->tpm_version;
 }

 /*
  * Unless path or file descriptor set has been provided by user,
  * determine the sysfs cancel file following kernel documentation
  * in Documentation/ABI/stable/sysfs-class-tpm.
  * From /dev/tpm0 create /sys/class/misc/tpm0/device/cancel
  */
 static int tpm_passthrough_open_sysfs_cancel(TPMBackend *tb)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
     int fd = -1;
     char *dev;
     char path[PATH_MAX];

     if (tb->cancel_path) {
         fd = qemu_open(tb->cancel_path, O_WRONLY);
         if (fd < 0) {
             error_report("Could not open TPM cancel path : %s",
                          strerror(errno));
         }
         return fd;
     }

     dev = strrchr(tpm_pt->tpm_dev, '/');
     if (dev) {
         dev++;
         if (snprintf(path, sizeof(path), "/sys/class/misc/%s/device/cancel",
                      dev) < sizeof(path)) {
             fd = qemu_open(path, O_WRONLY);
             if (fd >= 0) {
                 tb->cancel_path = g_strdup(path);
             } else {
                 error_report("tpm_passthrough: Could not open TPM cancel "
                              "path %s : %s", path, strerror(errno));
             }
         }
     } else {
        error_report("tpm_passthrough: Bad TPM device path %s",
                     tpm_pt->tpm_dev);
     }

     return fd;
 }

 static int tpm_passthrough_handle_device_opts(QemuOpts *opts, TPMBackend *tb)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
     const char *value;

     value = qemu_opt_get(opts, "cancel-path");
     tb->cancel_path = g_strdup(value);

     value = qemu_opt_get(opts, "path");
     if (!value) {
         value = TPM_PASSTHROUGH_DEFAULT_DEVICE;
     }

     tpm_pt->tpm_dev = g_strdup(value);

     tb->path = g_strdup(tpm_pt->tpm_dev);

     tpm_pt->tpm_fd = qemu_open(tpm_pt->tpm_dev, O_RDWR);
     if (tpm_pt->tpm_fd < 0) {
         error_report("Cannot access TPM device using '%s': %s",
                      tpm_pt->tpm_dev, strerror(errno));
         goto err_free_parameters;
     }

     if (tpm_util_test_tpmdev(tpm_pt->tpm_fd, &tpm_pt->tpm_version)) {
         error_report("'%s' is not a TPM device.",
                      tpm_pt->tpm_dev);
         goto err_close_tpmdev;
     }

     return 0;

  err_close_tpmdev:
     qemu_close(tpm_pt->tpm_fd);
     tpm_pt->tpm_fd = -1;

  err_free_parameters:
     g_free(tb->path);
     tb->path = NULL;

     g_free(tpm_pt->tpm_dev);
     tpm_pt->tpm_dev = NULL;

     return 1;
 }

 static TPMBackend *tpm_passthrough_create(QemuOpts *opts, const char *id)
 {
     Object *obj = object_new(TYPE_TPM_PASSTHROUGH);
     TPMBackend *tb = TPM_BACKEND(obj);
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);

     tb->id = g_strdup(id);

     if (tpm_passthrough_handle_device_opts(opts, tb)) {
         goto err_exit;
     }

     tpm_pt->cancel_fd = tpm_passthrough_open_sysfs_cancel(tb);
     if (tpm_pt->cancel_fd < 0) {
         goto err_exit;
     }

     return tb;

 err_exit:
     object_unref(obj);

     return NULL;
 }

 static const QemuOptDesc tpm_passthrough_cmdline_opts[] = {
     TPM_STANDARD_CMDLINE_OPTS,
     {
         .name = "cancel-path",
         .type = QEMU_OPT_STRING,
         .help = "Sysfs file entry for canceling TPM commands",
     },
     {
         .name = "path",
         .type = QEMU_OPT_STRING,
         .help = "Path to TPM device on the host",
     },
     { /* end of list */ },
 };

 static const TPMDriverOps tpm_passthrough_driver = {
     .type                     = TPM_TYPE_PASSTHROUGH,
     .opts                     = tpm_passthrough_cmdline_opts,
     .desc                     = "Passthrough TPM backend driver",
     .create                   = tpm_passthrough_create,
     .realloc_buffer           = tpm_passthrough_realloc_buffer,
     .reset                    = tpm_passthrough_reset,
     .cancel_cmd               = tpm_passthrough_cancel_cmd,
     .get_tpm_established_flag = tpm_passthrough_get_tpm_established_flag,
     .reset_tpm_established_flag = tpm_passthrough_reset_tpm_established_flag,
     .get_tpm_version          = tpm_passthrough_get_tpm_version,
 };

 static void tpm_passthrough_inst_init(Object *obj)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(obj);

     tpm_pt->tpm_fd = -1;
     tpm_pt->cancel_fd = -1;
 }

 static void tpm_passthrough_inst_finalize(Object *obj)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(obj);

     tpm_passthrough_cancel_cmd(TPM_BACKEND(obj));

     qemu_close(tpm_pt->tpm_fd);
     qemu_close(tpm_pt->cancel_fd);
     g_free(tpm_pt->tpm_dev);
 }

 static void tpm_passthrough_class_init(ObjectClass *klass, void *data)
 {
     TPMBackendClass *tbc = TPM_BACKEND_CLASS(klass);

     tbc->ops = &tpm_passthrough_driver;
     tbc->handle_request = tpm_passthrough_handle_request;
 }

 static const TypeInfo tpm_passthrough_info = {
     .name = TYPE_TPM_PASSTHROUGH,
     .parent = TYPE_TPM_BACKEND,
     .instance_size = sizeof(TPMPassthruState),
     .class_init = tpm_passthrough_class_init,
     .instance_init = tpm_passthrough_inst_init,
     .instance_finalize = tpm_passthrough_inst_finalize,
 };

 static void tpm_passthrough_register(void)
 {
     type_register_static(&tpm_passthrough_info);
     tpm_register_driver(&tpm_passthrough_driver);
 }

 type_init(tpm_passthrough_register)
	/*
	* passthrough TPM driver
	*
	* Copyright (c) 2010 - 2013 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/osdep.h"
	#include "qemu-common.h"
	#include "qemu/error-report.h"
	#include "qemu/sockets.h"
	#include "sysemu/tpm_backend.h"
	#include "tpm_int.h"
	#include "hw/hw.h"
	#include "hw/i386/pc.h"
	#include "tpm_tis.h"
	#include "tpm_util.h"

	#define DEBUG_TPM 0

	#define DPRINTF(fmt, ...) do { \
	if (DEBUG_TPM) { \
	fprintf(stderr, fmt, ## __VA_ARGS__); \
	} \
	} while (0);

	#define TYPE_TPM_PASSTHROUGH "tpm-passthrough"
	#define TPM_PASSTHROUGH(obj) \
	OBJECT_CHECK(TPMPassthruState, (obj), TYPE_TPM_PASSTHROUGH)

	/* data structures */
	struct TPMPassthruState {
	TPMBackend parent;

	char *tpm_dev;
	int tpm_fd;
	bool tpm_executing;
	bool tpm_op_canceled;
	int cancel_fd;

	TPMVersion tpm_version;
	};

	typedef struct TPMPassthruState TPMPassthruState;

	#define TPM_PASSTHROUGH_DEFAULT_DEVICE "/dev/tpm0"

	/* functions */

	static void tpm_passthrough_cancel_cmd(TPMBackend *tb);

	static int tpm_passthrough_unix_write(int fd, const uint8_t *buf, uint32_t len)
	{
	int ret, remain;

	remain = len;
	while (remain > 0) {
	ret = write(fd, buf, remain);
	if (ret < 0) {
	if (errno != EINTR && errno != EAGAIN) {
	return -1;
	}
	} else if (ret == 0) {
	break;
	} else {
	buf += ret;
	remain -= ret;
	}
	}
	return len - remain;
	}

	static int tpm_passthrough_unix_read(int fd, uint8_t *buf, uint32_t len)
	{
	int ret;
	reread:
	ret = read(fd, buf, len);
	if (ret < 0) {
	if (errno != EINTR && errno != EAGAIN) {
	return -1;
	}
	goto reread;
	}
	return ret;
	}

	static uint32_t tpm_passthrough_get_size_from_buffer(const uint8_t *buf)
	{
	struct tpm_resp_hdr resp = (struct tpm_resp_hdr )buf;

	return be32_to_cpu(resp->len);
	}

	/*
	* Write an error message in the given output buffer.
	*/
	static 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);
	}
	}

	static bool tpm_passthrough_is_selftest(const uint8_t *in, uint32_t in_len)
	{
	struct tpm_req_hdr hdr = (struct tpm_req_hdr )in;

	if (in_len >= sizeof(*hdr)) {
	return (be32_to_cpu(hdr->ordinal) == TPM_ORD_ContinueSelfTest);
	}

	return false;
	}

	static int tpm_passthrough_unix_tx_bufs(TPMPassthruState *tpm_pt,
	const uint8_t *in, uint32_t in_len,
	uint8_t *out, uint32_t out_len,
	bool *selftest_done)
	{
	int ret;
	bool is_selftest;
	const struct tpm_resp_hdr *hdr;

	tpm_pt->tpm_op_canceled = false;
	tpm_pt->tpm_executing = true;
	*selftest_done = false;

	is_selftest = tpm_passthrough_is_selftest(in, in_len);

	ret = tpm_passthrough_unix_write(tpm_pt->tpm_fd, in, in_len);
	if (ret != in_len) {
	if (!tpm_pt->tpm_op_canceled \|\| errno != ECANCELED) {
	error_report("tpm_passthrough: error while transmitting data "
	"to TPM: %s (%i)",
	strerror(errno), errno);
	}
	goto err_exit;
	}

	tpm_pt->tpm_executing = false;

	ret = tpm_passthrough_unix_read(tpm_pt->tpm_fd, out, out_len);
	if (ret < 0) {
	if (!tpm_pt->tpm_op_canceled \|\| errno != ECANCELED) {
	error_report("tpm_passthrough: error while reading data from "
	"TPM: %s (%i)",
	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");
	}

	if (is_selftest && (ret >= sizeof(struct tpm_resp_hdr))) {
	hdr = (struct tpm_resp_hdr *)out;
	*selftest_done = (be32_to_cpu(hdr->errcode) == 0);
	}

	err_exit:
	if (ret < 0) {
	tpm_write_fatal_error_response(out, out_len);
	}

	tpm_pt->tpm_executing = false;

	return ret;
	}

	static int tpm_passthrough_unix_transfer(TPMPassthruState *tpm_pt,
	const TPMLocality *locty_data,
	bool *selftest_done)
	{
	return tpm_passthrough_unix_tx_bufs(tpm_pt,
	locty_data->w_buffer.buffer,
	locty_data->w_offset,
	locty_data->r_buffer.buffer,
	locty_data->r_buffer.size,
	selftest_done);
	}

	static void tpm_passthrough_handle_request(TPMBackend *tb, TPMBackendCmd cmd)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
	bool selftest_done = false;

	DPRINTF("tpm_passthrough: processing command type %d\n", cmd);

	switch (cmd) {
	case TPM_BACKEND_CMD_PROCESS_CMD:
	tpm_passthrough_unix_transfer(tpm_pt,
	tb->tpm_state->locty_data,
	&selftest_done);

	tb->recv_data_callback(tb->tpm_state,
	tb->tpm_state->locty_number,
	selftest_done);
	break;
	case TPM_BACKEND_CMD_INIT:
	case TPM_BACKEND_CMD_END:
	case TPM_BACKEND_CMD_TPM_RESET:
	/* nothing to do */
	break;
	}
	}

	static void tpm_passthrough_reset(TPMBackend *tb)
	{
	DPRINTF("tpm_passthrough: CALL TO TPM_RESET!\n");

	tpm_passthrough_cancel_cmd(tb);
	}

	static bool tpm_passthrough_get_tpm_established_flag(TPMBackend *tb)
	{
	return false;
	}

	static int tpm_passthrough_reset_tpm_established_flag(TPMBackend *tb,
	uint8_t locty)
	{
	/* only a TPM 2.0 will support this */
	return 0;
	}

	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_cancel_cmd(TPMBackend *tb)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
	int n;

	/*
	* As of Linux 3.7 the tpm_tis driver does not properly cancel
	* commands on all TPM manufacturers' TPMs.
	* Only cancel if we're busy so we don't cancel someone else's
	* command, e.g., a command executed on the host.
	*/
	if (tpm_pt->tpm_executing) {
	if (tpm_pt->cancel_fd >= 0) {
	n = write(tpm_pt->cancel_fd, "-", 1);
	if (n != 1) {
	error_report("Canceling TPM command failed: %s",
	strerror(errno));
	} else {
	tpm_pt->tpm_op_canceled = true;
	}
	} else {
	error_report("Cannot cancel TPM command due to missing "
	"TPM sysfs cancel entry");
	}
	}
	}

	static TPMVersion tpm_passthrough_get_tpm_version(TPMBackend *tb)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);

	return tpm_pt->tpm_version;
	}

	/*
	* Unless path or file descriptor set has been provided by user,
	* determine the sysfs cancel file following kernel documentation
	* in Documentation/ABI/stable/sysfs-class-tpm.
	* From /dev/tpm0 create /sys/class/misc/tpm0/device/cancel
	*/
	static int tpm_passthrough_open_sysfs_cancel(TPMBackend *tb)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
	int fd = -1;
	char *dev;
	char path[PATH_MAX];

	if (tb->cancel_path) {
	fd = qemu_open(tb->cancel_path, O_WRONLY);
	if (fd < 0) {
	error_report("Could not open TPM cancel path : %s",
	strerror(errno));
	}
	return fd;
	}

	dev = strrchr(tpm_pt->tpm_dev, '/');
	if (dev) {
	dev++;
	if (snprintf(path, sizeof(path), "/sys/class/misc/%s/device/cancel",
	dev) < sizeof(path)) {
	fd = qemu_open(path, O_WRONLY);
	if (fd >= 0) {
	tb->cancel_path = g_strdup(path);
	} else {
	error_report("tpm_passthrough: Could not open TPM cancel "
	"path %s : %s", path, strerror(errno));
	}
	}
	} else {
	error_report("tpm_passthrough: Bad TPM device path %s",
	tpm_pt->tpm_dev);
	}

	return fd;
	}

	static int tpm_passthrough_handle_device_opts(QemuOpts opts, TPMBackend tb)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
	const char *value;

	value = qemu_opt_get(opts, "cancel-path");
	tb->cancel_path = g_strdup(value);

	value = qemu_opt_get(opts, "path");
	if (!value) {
	value = TPM_PASSTHROUGH_DEFAULT_DEVICE;
	}

	tpm_pt->tpm_dev = g_strdup(value);

	tb->path = g_strdup(tpm_pt->tpm_dev);

	tpm_pt->tpm_fd = qemu_open(tpm_pt->tpm_dev, O_RDWR);
	if (tpm_pt->tpm_fd < 0) {
	error_report("Cannot access TPM device using '%s': %s",
	tpm_pt->tpm_dev, strerror(errno));
	goto err_free_parameters;
	}

	if (tpm_util_test_tpmdev(tpm_pt->tpm_fd, &tpm_pt->tpm_version)) {
	error_report("'%s' is not a TPM device.",
	tpm_pt->tpm_dev);
	goto err_close_tpmdev;
	}

	return 0;

	err_close_tpmdev:
	qemu_close(tpm_pt->tpm_fd);
	tpm_pt->tpm_fd = -1;

	err_free_parameters:
	g_free(tb->path);
	tb->path = NULL;

	g_free(tpm_pt->tpm_dev);
	tpm_pt->tpm_dev = NULL;

	return 1;
	}

	static TPMBackend tpm_passthrough_create(QemuOpts opts, const char *id)
	{
	Object *obj = object_new(TYPE_TPM_PASSTHROUGH);
	TPMBackend *tb = TPM_BACKEND(obj);
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);

	tb->id = g_strdup(id);

	if (tpm_passthrough_handle_device_opts(opts, tb)) {
	goto err_exit;
	}

	tpm_pt->cancel_fd = tpm_passthrough_open_sysfs_cancel(tb);
	if (tpm_pt->cancel_fd < 0) {
	goto err_exit;
	}

	return tb;

	err_exit:
	object_unref(obj);

	return NULL;
	}

	static const QemuOptDesc tpm_passthrough_cmdline_opts[] = {
	TPM_STANDARD_CMDLINE_OPTS,
	{
	.name = "cancel-path",
	.type = QEMU_OPT_STRING,
	.help = "Sysfs file entry for canceling TPM commands",
	},
	{
	.name = "path",
	.type = QEMU_OPT_STRING,
	.help = "Path to TPM device on the host",
	},
	{ /* end of list */ },
	};

	static const TPMDriverOps tpm_passthrough_driver = {
	.type = TPM_TYPE_PASSTHROUGH,
	.opts = tpm_passthrough_cmdline_opts,
	.desc = "Passthrough TPM backend driver",
	.create = tpm_passthrough_create,
	.realloc_buffer = tpm_passthrough_realloc_buffer,
	.reset = tpm_passthrough_reset,
	.cancel_cmd = tpm_passthrough_cancel_cmd,
	.get_tpm_established_flag = tpm_passthrough_get_tpm_established_flag,
	.reset_tpm_established_flag = tpm_passthrough_reset_tpm_established_flag,
	.get_tpm_version = tpm_passthrough_get_tpm_version,
	};

	static void tpm_passthrough_inst_init(Object *obj)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(obj);

	tpm_pt->tpm_fd = -1;
	tpm_pt->cancel_fd = -1;
	}

	static void tpm_passthrough_inst_finalize(Object *obj)
	{
	TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(obj);

	tpm_passthrough_cancel_cmd(TPM_BACKEND(obj));

	qemu_close(tpm_pt->tpm_fd);
	qemu_close(tpm_pt->cancel_fd);
	g_free(tpm_pt->tpm_dev);
	}

	static void tpm_passthrough_class_init(ObjectClass klass, void data)
	{
	TPMBackendClass *tbc = TPM_BACKEND_CLASS(klass);

	tbc->ops = &tpm_passthrough_driver;
	tbc->handle_request = tpm_passthrough_handle_request;
	}

	static const TypeInfo tpm_passthrough_info = {
	.name = TYPE_TPM_PASSTHROUGH,
	.parent = TYPE_TPM_BACKEND,
	.instance_size = sizeof(TPMPassthruState),
	.class_init = tpm_passthrough_class_init,
	.instance_init = tpm_passthrough_inst_init,
	.instance_finalize = tpm_passthrough_inst_finalize,
	};

	static void tpm_passthrough_register(void)
	{
	type_register_static(&tpm_passthrough_info);
	tpm_register_driver(&tpm_passthrough_driver);
	}

	type_init(tpm_passthrough_register)