Re: [PATCH Part1 v5 37/38] virt: sevguest: Add support to derive key

[Dov Murik]

Hi Brijesh,

On 20/08/2021 18:19, Brijesh Singh wrote:
> The SNP_GET_DERIVED_KEY ioctl interface can be used by the SNP guest to
> ask the firmware to provide a key derived from a root key. The derived
> key may be used by the guest for any purposes it choose, such as a
> sealing key or communicating with the external entities.
> 
> See SEV-SNP firmware spec for more information.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  Documentation/virt/coco/sevguest.rst  | 18 ++++++++++
>  drivers/virt/coco/sevguest/sevguest.c | 48 +++++++++++++++++++++++++++
>  include/uapi/linux/sev-guest.h        | 24 ++++++++++++++
>  3 files changed, 90 insertions(+)
> 
> diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
> index 52d5915037ef..25446670d816 100644
> --- a/Documentation/virt/coco/sevguest.rst
> +++ b/Documentation/virt/coco/sevguest.rst
> @@ -67,3 +67,21 @@ provided by the SEV-SNP firmware to query the attestation report.
>  On success, the snp_report_resp.data will contains the report. The report
>  format is described in the SEV-SNP specification. See the SEV-SNP specification
>  for further details.
> +
> +2.2 SNP_GET_DERIVED_KEY
> +-----------------------
> +:Technology: sev-snp
> +:Type: guest ioctl
> +:Parameters (in): struct snp_derived_key_req
> +:Returns (out): struct snp_derived_key_req on success, -negative on error
> +
> +The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key.
> +The derived key can be used by the guest for any purpose, such as sealing keys
> +or communicating with external entities.
> +
> +The ioctl uses the SNP_GUEST_REQUEST (MSG_KEY_REQ) command provided by the
> +SEV-SNP firmware to derive the key. See SEV-SNP specification for further details
> +on the various fileds passed in the key derivation request.
> +
> +On success, the snp_derived_key_resp.data will contains the derived key
> +value.
> diff --git a/drivers/virt/coco/sevguest/sevguest.c b/drivers/virt/coco/sevguest/sevguest.c
> index d029a98ad088..621b1c5a9cfc 100644
> --- a/drivers/virt/coco/sevguest/sevguest.c
> +++ b/drivers/virt/coco/sevguest/sevguest.c
> @@ -303,6 +303,50 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_user_guest_reque
>  	return rc;
>  }
>  
> +static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_user_guest_request *arg)
> +{
> +	struct snp_guest_crypto *crypto = snp_dev->crypto;
> +	struct snp_derived_key_resp *resp;
> +	struct snp_derived_key_req req;
> +	int rc, resp_len;
> +
> +	if (!arg->req_data || !arg->resp_data)
> +		return -EINVAL;
> +
> +	/* Copy the request payload from the userspace */
> +	if (copy_from_user(&req, (void __user *)arg->req_data, sizeof(req)))
> +		return -EFAULT;
> +
> +	/* Message version must be non-zero */
> +	if (!req.msg_version)
> +		return -EINVAL;
> +
> +	/*
> +	 * The intermediate response buffer is used while decrypting the
> +	 * response payload. Make sure that it has enough space to cover the
> +	 * authtag.
> +	 */
> +	resp_len = sizeof(resp->data) + crypto->a_len;
> +	resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);

The length of resp->data is 64 bytes; I assume crypto->a_len is not a
lot more (and probably known in advance for AES GCM).  Maybe use a
buffer on the stack instead of allocating and freeing?


> +	if (!resp)
> +		return -ENOMEM;
> +
> +	/* Issue the command to get the attestation report */
> +	rc = handle_guest_request(snp_dev, req.msg_version, SNP_MSG_KEY_REQ,
> +				  &req.data, sizeof(req.data), resp->data, resp_len,
> +				  &arg->fw_err);
> +	if (rc)
> +		goto e_free;
> +
> +	/* Copy the response payload to userspace */
> +	if (copy_to_user((void __user *)arg->resp_data, resp, sizeof(*resp)))
> +		rc = -EFAULT;
> +
> +e_free:
> +	kfree(resp);

Since resp contains key material, I think you should explicit_memzero()
it before freeing, so the key bytes don't linger around in unused
memory.  I'm not sure if any copies are made inside the
handle_guest_request call above; maybe zero these as well.

-Dov


> +	return rc;
> +}
> +
>  static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
>  {
>  	struct snp_guest_dev *snp_dev = to_snp_dev(file);
> @@ -320,6 +364,10 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>  		ret = get_report(snp_dev, &input);
>  		break;
>  	}
> +	case SNP_GET_DERIVED_KEY: {
> +		ret = get_derived_key(snp_dev, &input);
> +		break;
> +	}
>  	default:
>  		break;
>  	}
> diff --git a/include/uapi/linux/sev-guest.h b/include/uapi/linux/sev-guest.h
> index e8cfd15133f3..621a9167df7a 100644
> --- a/include/uapi/linux/sev-guest.h
> +++ b/include/uapi/linux/sev-guest.h
> @@ -36,9 +36,33 @@ struct snp_user_guest_request {
>  	__u64 fw_err;
>  };
>  
> +struct __snp_derived_key_req {
> +	__u32 root_key_select;
> +	__u32 rsvd;
> +	__u64 guest_field_select;
> +	__u32 vmpl;
> +	__u32 guest_svn;
> +	__u64 tcb_version;
> +};
> +
> +struct snp_derived_key_req {
> +	/* message version number (must be non-zero) */
> +	__u8 msg_version;
> +
> +	struct __snp_derived_key_req data;
> +};
> +
> +struct snp_derived_key_resp {
> +	/* response data, see SEV-SNP spec for the format */
> +	__u8 data[64];
> +};
> +
>  #define SNP_GUEST_REQ_IOC_TYPE	'S'
>  
>  /* Get SNP attestation report */
>  #define SNP_GET_REPORT _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x0, struct snp_user_guest_request)
>  
> +/* Get a derived key from the root */
> +#define SNP_GET_DERIVED_KEY _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x1, struct snp_user_guest_request)
> +
>  #endif /* __UAPI_LINUX_SEV_GUEST_H_ */
>