Re: [PATCH v7 44/45] virt: sevguest: Add support to derive key

[Peter Gonda <pgonda@google.com>]

On Wed, Nov 10, 2021 at 3:09 PM Brijesh Singh <brijesh.singh@amd.com> 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@amd.com>
> ---
>  Documentation/virt/coco/sevguest.rst  | 19 ++++++++++-
>  drivers/virt/coco/sevguest/sevguest.c | 49 +++++++++++++++++++++++++++
>  include/uapi/linux/sev-guest.h        | 24 +++++++++++++
>  3 files changed, 91 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
> index 002c90946b8a..0bd9a65e0370 100644
> --- a/Documentation/virt/coco/sevguest.rst
> +++ b/Documentation/virt/coco/sevguest.rst
> @@ -64,10 +64,27 @@ The SNP_GET_REPORT ioctl can be used to query the attestation report from the
>  SEV-SNP firmware. The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command
>  provided by the SEV-SNP firmware to query the attestation report.
>
> -On success, the snp_report_resp.data will contains the report. The report
> +On success, the snp_report_resp.data will contain the report. The report
>  will contain the format 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.

derived

> +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 fields passed in the key derivation request.
> +
> +On success, the snp_derived_key_resp.data will contains the derived key value. See
".data will contain the..." or ".data contains the derived key..."


> +the SEV-SNP specification for further details.
>
>  Reference
>  ---------
> diff --git a/drivers/virt/coco/sevguest/sevguest.c b/drivers/virt/coco/sevguest/sevguest.c
> index 982714c1b4ca..bece6856573e 100644
> --- a/drivers/virt/coco/sevguest/sevguest.c
> +++ b/drivers/virt/coco/sevguest/sevguest.c
> @@ -392,6 +392,52 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
>         return rc;
>  }
>
> +static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
> +{
> +       struct snp_guest_crypto *crypto = snp_dev->crypto;
> +       struct snp_derived_key_resp resp = {0};
> +       struct snp_derived_key_req req;
> +       int rc, resp_len;
> +       u8 buf[89];

Could we document this magic number?

> +
> +       if (!arg->req_data || !arg->resp_data)
> +               return -EINVAL;
> +
> +       /* Copy the request payload from 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;
> +       if (sizeof(buf) < resp_len)
> +               return -ENOMEM;
> +
> +       /* Issue the command to get the attestation report */
> +       rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, req.msg_version,
> +                                 SNP_MSG_KEY_REQ, &req.data, sizeof(req.data), buf, resp_len,
> +                                 &arg->fw_err);
> +       if (rc)
> +               goto e_free;

Should we check the first 32 bits of |data| here since that is a
status field? If we see 16h here we could return -EINVAL, or better to
let userspace deal with that error handling?

> +
> +       /* Copy the response payload to userspace */
> +       memcpy(resp.data, buf, sizeof(resp.data));
> +       if (copy_to_user((void __user *)arg->resp_data, &resp, sizeof(resp)))
> +               rc = -EFAULT;
> +
> +e_free:
> +       memzero_explicit(buf, sizeof(buf));
> +       memzero_explicit(&resp, sizeof(resp));
> +       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);
> @@ -417,6 +463,9 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>         case SNP_GET_REPORT:
>                 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 eda7edcffda8..f6d9c136ff4d 100644
> --- a/include/uapi/linux/sev-guest.h
> +++ b/include/uapi/linux/sev-guest.h
> @@ -36,9 +36,33 @@ struct snp_guest_request_ioctl {
>         __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_guest_request_ioctl)
>
> +/* Get a derived key from the root */
> +#define SNP_GET_DERIVED_KEY _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x1, struct snp_guest_request_ioctl)
> +
>  #endif /* __UAPI_LINUX_SEV_GUEST_H_ */
> --
> 2.25.1
>