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

[Tom Lendacky]

On 8/14/23 02:43, 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


> +/*
> + * 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;

Is an array_index_nospec() call needed after this check?

> +
> +	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);

Should the portion of tsm_report.desc.inblob that is not updated be 
cleared if inblob_len != TSM_INBLOB_MAX?

> +	tsm_report.desc.inblob_len = inblob_len;
> +	tsm_report.write_generation++;
> +
> +	return len;
> +}
> +


> +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;

Looks like rc is used uninitialized.

> +	}
> +
> +	dev = device_create_with_groups(tsm_class, parent, 0, NULL, groups,
> +					"tsm0");

You can go out to 100 characters now, so this could all be one line.

Thanks,
Tom

> +	if (IS_ERR(dev))
> +		return PTR_ERR(dev);
> +
> +	provider.ops = ops;
> +	provider.dev = dev;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(register_tsm);
> +