Re: [PATCH v2 2/5] tsm: Introduce a shared ABI for attestation reports

[Jeremi Piotrowski]

On 8/14/2023 9:43 AM, Dan Williams wrote:
> One of the common operations of a TSM (Trusted Security Module) is to
> provide a way for a TVM (confidential computing guest execution
> environment) to take a measurement of its launch state, sign it and
> submit it to a verifying party. Upon successful attestation that
> verifies the integrity of the TVM additional secrets may be deployed.
> The concept is common across TSMs, but the implementations are
> unfortunately vendor specific. While the industry grapples with a common
> definition of this attestation format [1], Linux need not make this
> problem worse by defining a new ABI per TSM that wants to perform a
> similar operation. The current momentum has been to invent new ioctl-ABI
> per TSM per function which at best is an abdication of the kernel's
> responsibility to make common infrastructure concepts share common ABI.
> 
> The proposal, targeted to conceptually work with TDX, SEV, COVE if not
> more, is to define a sysfs interface to retrieve the TSM-specific blob.
> 
>     echo $hex_encoded_userdata_plus_nonce > /sys/class/tsm/tsm0/inhex
>     hexdump /sys/class/tsm/tsm0/outblob
> 
> This approach later allows for the standardization of the attestation
> blob format without needing to change the Linux ABI. Until then, the
> format of 'outblob' is determined by the parent device for 'tsm0'.
> 
> The expectation is that this is a boot time exchange that need not be
> regenerated, making it amenable to a sysfs interface. In case userspace
> does try to generate multiple attestation reports it includes conflict
> detection so userspace can be sure no other thread changed the
> parameters from its last configuration step to the blob retrieval.
> 
> TSM specific options are encoded as 'extra' attributes on the TSM device
> with the expectation that vendors reuse the same options for similar
> concepts. The current options are defined by SEV-SNP's need for a
> 'privilege level' concept (VMPL), and the option to retrieve a
> certificate chain in addition to the attestation report ("extended"
> format).
> 
> Link: http://lore.kernel.org/r/64961c3baf8ce_142af829436@xxxxxxxxxxxxxxxxxxxxxxxxx.notmuch [1]
> Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Cc: Dionna Amalie Glaze <dionnaglaze@xxxxxxxxxx>
> Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Gonda <pgonda@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Samuel Ortiz <sameo@xxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  Documentation/ABI/testing/sysfs-class-tsm |   47 +++++
>  MAINTAINERS                               |    8 +
>  drivers/virt/coco/Kconfig                 |    4 
>  drivers/virt/coco/Makefile                |    1 
>  drivers/virt/coco/tdx-guest/Kconfig       |    1 
>  drivers/virt/coco/tsm.c                   |  290 +++++++++++++++++++++++++++++
>  include/linux/tsm.h                       |   45 +++++
>  7 files changed, 396 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-class-tsm
>  create mode 100644 drivers/virt/coco/tsm.c
>  create mode 100644 include/linux/tsm.h
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-tsm b/Documentation/ABI/testing/sysfs-class-tsm
> new file mode 100644
> index 000000000000..37017bde626d
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-class-tsm
> @@ -0,0 +1,47 @@
> +What:		/sys/class/tsm/tsm0/inhex
> +Date:		August, 2023
> +KernelVersion:	v6.6
> +Contact:	linux-cxl@xxxxxxxxxxxxxxx
> +Description:
> +		(RW) Hex encoded userdata to be included in the attestation
> +		report. For replay protection this should include a nonce, but
> +		the kernel does not place any restrictions on the content.
> +
> +What:		/sys/class/tsm/tsm0/outblob
> +Date:		August, 2023
> +KernelVersion:	v6.6
> +Contact:	linux-cxl@xxxxxxxxxxxxxxx
> +Description:
> +		(RO) Binary attestation report generated from @inhex translated
> +		to binary and any options. The format of the report is vendor
> +		specific and determined by the parent device of 'tsm0'.
> +
> +What:		/sys/class/tsm/tsm0/generation
> +Date:		August, 2023
> +KernelVersion:	v6.6
> +Contact:	linux-cxl@xxxxxxxxxxxxxxx
> +Description:
> +		(RO) The value in this attribute increments each time @inhex or
> +		any option is written. Userspace can detect conflicts by
> +		checking generation before writing to any attribute and making
> +		sure the number of writes matches expectations after reading
> +		@outblob.
> +
> +What:		/sys/class/tsm/tsm0/privlevel
> +Date:		August, 2023
> +KernelVersion:	v6.6
> +Contact:	linux-cxl@xxxxxxxxxxxxxxx
> +Description:
> +		(RW) If a TSM implementation supports the concept of attestation
> +		reports for TVMs running at different privilege levels, like
> +		SEV-SNP "VMPL", specify the privilege level via this attribute.
> +
> +What:		/sys/class/tsm/tsm0/format
> +Date:		August, 2023
> +KernelVersion:	v6.6
> +Contact:	linux-cxl@xxxxxxxxxxxxxxx
> +Description:
> +		(RW) If a TSM implementation supports the concept of attestation
> +		reports with "extended" contents, like SEV-SNP extended reports
> +		with certificate chains, specify "extended" vs "default" via
> +		this attribute.
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 3be1bdfe8ecc..97f74d344c8a 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -21625,6 +21625,14 @@ W:	https://github.com/srcres258/linux-doc
>  T:	git git://github.com/srcres258/linux-doc.git doc-zh-tw
>  F:	Documentation/translations/zh_TW/
>  
> +TRUSTED SECURITY MODULE (TSM) ATTESTATION REPORTS
> +M:	Dan Williams <dan.j.williams@xxxxxxxxx>
> +L:	linux-coco@xxxxxxxxxxxxxxx
> +S:	Maintained
> +F:	Documentation/ABI/testing/sysfs-class-tsm
> +F:	drivers/virt/coco/tsm.c
> +F:	include/linux/tsm.h
> +
>  TTY LAYER
>  M:	Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>  M:	Jiri Slaby <jirislaby@xxxxxxxxxx>
> diff --git a/drivers/virt/coco/Kconfig b/drivers/virt/coco/Kconfig
> index fc5c64f04c4a..d92f07019f38 100644
> --- a/drivers/virt/coco/Kconfig
> +++ b/drivers/virt/coco/Kconfig
> @@ -2,6 +2,10 @@
>  #
>  # Confidential computing related collateral
>  #
> +
> +config TSM_REPORTS
> +	tristate
> +
>  source "drivers/virt/coco/efi_secret/Kconfig"
>  
>  source "drivers/virt/coco/sev-guest/Kconfig"
> diff --git a/drivers/virt/coco/Makefile b/drivers/virt/coco/Makefile
> index 55302ef719ad..18c1aba5edb7 100644
> --- a/drivers/virt/coco/Makefile
> +++ b/drivers/virt/coco/Makefile
> @@ -2,6 +2,7 @@
>  #
>  # Confidential computing related collateral
>  #
> +obj-$(CONFIG_TSM_REPORTS)	+= tsm.o
>  obj-$(CONFIG_EFI_SECRET)	+= efi_secret/
>  obj-$(CONFIG_SEV_GUEST)		+= sev-guest/
>  obj-$(CONFIG_INTEL_TDX_GUEST)	+= tdx-guest/
> diff --git a/drivers/virt/coco/tdx-guest/Kconfig b/drivers/virt/coco/tdx-guest/Kconfig
> index 14246fc2fb02..22dd59e19431 100644
> --- a/drivers/virt/coco/tdx-guest/Kconfig
> +++ b/drivers/virt/coco/tdx-guest/Kconfig
> @@ -1,6 +1,7 @@
>  config TDX_GUEST_DRIVER
>  	tristate "TDX Guest driver"
>  	depends on INTEL_TDX_GUEST
> +	select TSM_REPORTS
>  	help
>  	  The driver provides userspace interface to communicate with
>  	  the TDX module to request the TDX guest details like attestation
> diff --git a/drivers/virt/coco/tsm.c b/drivers/virt/coco/tsm.c
> new file mode 100644
> index 000000000000..1bf2ee82eb94
> --- /dev/null
> +++ b/drivers/virt/coco/tsm.c
> @@ -0,0 +1,290 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright(c) 2023 Intel Corporation. All rights reserved. */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/tsm.h>
> +#include <linux/err.h>
> +#include <linux/slab.h>
> +#include <linux/rwsem.h>
> +#include <linux/device.h>
> +#include <linux/string.h>
> +#include <linux/module.h>
> +#include <linux/cleanup.h>
> +
> +struct class *tsm_class;
> +static struct tsm_provider {
> +	const struct tsm_ops *ops;
> +	struct device *dev;
> +} provider;
> +static DECLARE_RWSEM(tsm_rwsem);
> +
> +/**
> + * DOC: Trusted Security Module (TSM) Attestation Report Interface
> + *
> + * The TSM report interface is a common provider of blobs that facilitate
> + * attestation of a TVM (confidential computing guest) by an attestation
> + * service. A TSM report combines a user-defined blob (likely a public-key with
> + * a nonce for a key-exchange protocol) with a signed attestation report. That
> + * combined blob is then used to obtain secrets provided by an agent that can
> + * validate the attestation report. The expectation is that this interface is
> + * invoked infrequently, likely only once at TVM boot time.
> + *
> + * The attestation report format is TSM provider specific, when / if a standard
> + * materializes that can be published instead of the vendor layout.
> + */
> +
> +/**
> + * struct tsm_report - track state of report generation relative to options
> + * @desc: report generation options / cached report state
> + * @outblob: generated evidence to provider to the attestation agent
> + * @outblob_len: sizeof(outblob)
> + * @write_generation: conflict detection, and report regeneration tracking
> + * @read_generation: cached report invalidation tracking
> + */
> +struct tsm_report {
> +	struct tsm_desc desc;
> +	size_t outblob_len;
> +	u8 *outblob;
> +	unsigned long write_generation;
> +	unsigned long read_generation;
> +} tsm_report;
> +
> +static ssize_t privlevel_store(struct device *dev,
> +			       struct device_attribute *attr, const char *buf,
> +			       size_t len)
> +{
> +	unsigned int val;
> +	int rc;
> +
> +	rc = kstrtouint(buf, 0, &val);
> +	if (rc)
> +		return rc;
> +
> +	guard(rwsem_write)(&tsm_rwsem);
> +	if (tsm_report.desc.privlevel == val)
> +		return len;
> +	tsm_report.desc.privlevel = val;
> +	tsm_report.write_generation++;
> +
> +	return len;
> +}
> +
> +static ssize_t privlevel_show(struct device *dev, struct device_attribute *attr,
> +			      char *buf)
> +{
> +	return sysfs_emit(buf, "%u\n", tsm_report.desc.privlevel);
> +}
> +
> +static DEVICE_ATTR_RW(privlevel);
> +
> +static ssize_t format_store(struct device *dev, struct device_attribute *attr,
> +			    const char *buf, size_t len)
> +{
> +	enum tsm_format format;
> +
> +	if (sysfs_streq(buf, "default"))
> +		format = TSM_FORMAT_DEFAULT;
> +	else if (sysfs_streq(buf, "extended"))
> +		format = TSM_FORMAT_EXTENDED;
> +	else
> +		return -EINVAL;
> +
> +	guard(rwsem_write)(&tsm_rwsem);
> +	if (tsm_report.desc.outblob_format == format)
> +		return len;
> +	tsm_report.desc.outblob_format = format;
> +	tsm_report.write_generation++;
> +
> +	return len;
> +}
> +
> +static ssize_t format_show(struct device *dev, struct device_attribute *attr,
> +			   char *buf)
> +{
> +	if (tsm_report.desc.outblob_format == TSM_FORMAT_DEFAULT)
> +		return sysfs_emit(buf, "default\n");
> +	return sysfs_emit(buf, "extended\n");
> +}
> +
> +static DEVICE_ATTR_RW(format);
> +
> +static struct attribute *tsm_extra_attributes[] = {
> +	&dev_attr_format.attr,
> +	&dev_attr_privlevel.attr,
> +	NULL,
> +};
> +
> +struct attribute_group tsm_extra_attribute_group = {
> +	.attrs = tsm_extra_attributes,
> +};
> +EXPORT_SYMBOL_GPL(tsm_extra_attribute_group);
> +
> +/*
> + * Input is a small hex blob, rather than a writable binary attribute, so that
> + * it is conveyed atomically.
> + */
> +static ssize_t inhex_store(struct device *dev, struct device_attribute *attr,
> +			   const char *buf, size_t len)
> +{
> +	u8 inblob[TSM_INBLOB_MAX];
> +	size_t inblob_len;
> +	int rc;
> +
> +	inblob_len = len;
> +	if (buf[len - 1] == '\n')
> +		inblob_len--;
> +	if (inblob_len & 1)
> +		return -EINVAL;
> +	inblob_len /= 2;
> +	if (inblob_len > TSM_INBLOB_MAX)
> +		return -EINVAL;
> +
> +	rc = hex2bin(inblob, buf, inblob_len);
> +	if (rc < 0)
> +		return rc;
> +
> +	guard(rwsem_write)(&tsm_rwsem);
> +	if (memcmp(tsm_report.desc.inblob, inblob, inblob_len) == 0)
> +		return len;
> +	memcpy(tsm_report.desc.inblob, inblob, inblob_len);
> +	tsm_report.desc.inblob_len = inblob_len;
> +	tsm_report.write_generation++;
> +
> +	return len;
> +}
> +
> +static ssize_t inhex_show(struct device *dev, struct device_attribute *attr,
> +			  char *buf)
> +{
> +	char *end;
> +
> +	guard(rwsem_read)(&tsm_rwsem);
> +	if (!tsm_report.desc.inblob_len)
> +		return 0;
> +	end = bin2hex(buf, tsm_report.desc.inblob, tsm_report.desc.inblob_len);
> +	*end++ = '\n';
> +	return end - buf;
> +}
> +static DEVICE_ATTR_RW(inhex);
> +
> +static ssize_t generation_show(struct device *dev,
> +			       struct device_attribute *attr, char *buf)
> +{
> +	guard(rwsem_read)(&tsm_rwsem);
> +	return sysfs_emit(buf, "%lu\n", tsm_report.write_generation);
> +}
> +static DEVICE_ATTR_RO(generation);
> +
> +static struct attribute *tsm_attributes[] = {
> +	&dev_attr_inhex.attr,
> +	&dev_attr_generation.attr,
> +	NULL,
> +};
> +
> +static ssize_t outblob_read(struct file *f, struct kobject *kobj,
> +			    struct bin_attribute *bin_attr, char *buf,
> +			    loff_t offset, size_t count)
> +{
> +	guard(rwsem_read)(&tsm_rwsem);

This is unfortunate but it would need to be a rwsem_write otherwise two
processes can race to reach the kvfree and both call report_new at the
same time (unlikely as it may be).

Jeremi

> +	if (!tsm_report.desc.inblob_len)
> +		return -EINVAL;
> +
> +	if (!tsm_report.outblob ||
> +	    tsm_report.read_generation != tsm_report.write_generation) {
> +		const struct tsm_ops *ops = provider.ops;
> +		size_t outblob_len;
> +		u8 *outblob;
> +
> +		kvfree(tsm_report.outblob);
> +		outblob = ops->report_new(provider.dev->parent,
> +					  &tsm_report.desc, &outblob_len);
> +		if (IS_ERR(outblob))
> +			return PTR_ERR(outblob);
> +		tsm_report.outblob_len = outblob_len;
> +		tsm_report.outblob = outblob;
> +		tsm_report.read_generation = tsm_report.write_generation;
> +	}
> +
> +	return memory_read_from_buffer(buf, count, &offset,
> +				       tsm_report.outblob,
> +				       tsm_report.outblob_len);
> +}
> +static BIN_ATTR_RO(outblob, 0);
> +
> +static struct bin_attribute *tsm_bin_attributes[] = {
> +	&bin_attr_outblob,
> +	NULL,
> +};
> +
> +struct attribute_group tsm_default_attribute_group = {
> +	.bin_attrs = tsm_bin_attributes,
> +	.attrs = tsm_attributes,
> +};
> +EXPORT_SYMBOL_GPL(tsm_default_attribute_group);
> +
> +static const struct attribute_group *tsm_default_attribute_groups[] = {
> +	&tsm_default_attribute_group,
> +	NULL,
> +};
> +
> +int register_tsm(const struct tsm_ops *ops, struct device *parent,
> +		 const struct attribute_group **groups)
> +{
> +	const struct tsm_ops *conflict;
> +	struct device *dev;
> +	int rc;
> +
> +	if (!parent)
> +		return -EINVAL;
> +
> +	if (!groups)
> +		groups = tsm_default_attribute_groups;
> +
> +	guard(rwsem_write)(&tsm_rwsem);
> +	conflict = provider.ops;
> +	if (conflict) {
> +		pr_err("\"%s\" ops already registered\n", conflict->name);
> +		return rc;
> +	}
> +
> +	dev = device_create_with_groups(tsm_class, parent, 0, NULL, groups,
> +					"tsm0");
> +	if (IS_ERR(dev))
> +		return PTR_ERR(dev);
> +
> +	provider.ops = ops;
> +	provider.dev = dev;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(register_tsm);
> +
> +int unregister_tsm(const struct tsm_ops *ops)
> +{
> +	guard(rwsem_write)(&tsm_rwsem);
> +	if (ops != provider.ops)
> +		return -EBUSY;
> +	provider.ops = NULL;
> +	device_unregister(provider.dev);
> +	provider.dev = NULL;
> +	kvfree(tsm_report.outblob);
> +	tsm_report.outblob = NULL;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(unregister_tsm);
> +
> +static int __init tsm_init(void)
> +{
> +	tsm_class = class_create("tsm");
> +	return PTR_ERR_OR_ZERO(tsm_class);
> +}
> +module_init(tsm_init);
> +
> +static void __exit tsm_exit(void)
> +{
> +	class_destroy(tsm_class);
> +}
> +module_exit(tsm_exit);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_DESCRIPTION("Provide Trusted Security Module attestation reports via sysfs");
> diff --git a/include/linux/tsm.h b/include/linux/tsm.h
> new file mode 100644
> index 000000000000..6dc2f07543b8
> --- /dev/null
> +++ b/include/linux/tsm.h
> @@ -0,0 +1,45 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __TSM_H
> +#define __TSM_H
> +
> +#include <linux/types.h>
> +#include <linux/device.h>
> +
> +#define TSM_INBLOB_MAX 64
> +
> +enum tsm_format {
> +	TSM_FORMAT_DEFAULT,
> +	TSM_FORMAT_EXTENDED,
> +};
> +
> +/**
> + * struct tsm_desc - option descriptor for generating tsm report blobs
> + * @privlevel: optional privilege level to associate with @outblob
> + * @inblob_len: sizeof @inblob
> + * @inblob: arbitrary input data
> + * @outblob_format: for TSMs with an "extended" format
> + */
> +struct tsm_desc {
> +	unsigned int privlevel;
> +	size_t inblob_len;
> +	u8 inblob[TSM_INBLOB_MAX];
> +	enum tsm_format outblob_format;
> +};
> +
> +/*
> + * arch specific ops, only one is expected to be registered at a time
> + * i.e. only one of SEV, TDX, COVE, etc.
> + */
> +struct tsm_ops {
> +	const char *name;
> +	u8 *(*report_new)(struct device *dev, const struct tsm_desc *desc,
> +			  size_t *outblob_len);
> +};
> +
> +extern struct attribute_group tsm_default_attribute_group;
> +extern struct attribute_group tsm_extra_attribute_group;
> +
> +int register_tsm(const struct tsm_ops *ops, struct device *parent,
> +		 const struct attribute_group **groups);
> +int unregister_tsm(const struct tsm_ops *ops);
> +#endif /* __TSM_H */
>