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    Subject[PATCH v9 42/43] virt: sevguest: Add support to derive key
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    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 chooses, such as a
    sealing key or communicating with the external entities.

    See SEV-SNP firmware spec for more information.

    Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
    Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    ---
    Documentation/virt/coco/sevguest.rst | 17 ++++++++++
    drivers/virt/coco/sevguest/sevguest.c | 45 +++++++++++++++++++++++++++
    include/uapi/linux/sev-guest.h | 17 ++++++++++
    3 files changed, 79 insertions(+)

    diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
    index 47ef3b0821d5..aafc9bce9aef 100644
    --- a/Documentation/virt/coco/sevguest.rst
    +++ b/Documentation/virt/coco/sevguest.rst
    @@ -72,6 +72,23 @@ On success, the snp_report_resp.data will contains the report. The report
    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_resp 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 fields passed in the key derivation request.
    +
    +On success, the snp_derived_key_resp.data contains the derived key value. See
    +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 6dc0785ddd4b..4369e55df9a6 100644
    --- a/drivers/virt/coco/sevguest/sevguest.c
    +++ b/drivers/virt/coco/sevguest/sevguest.c
    @@ -392,6 +392,48 @@ 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 = {0};
    + int rc, resp_len;
    + u8 buf[64+16]; /* Response data is 64 bytes and max authsize for GCM is 16 bytes */
    +
    + 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;
    +
    + /*
    + * 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, arg->msg_version,
    + SNP_MSG_KEY_REQ, &req, sizeof(req), buf, resp_len,
    + &arg->fw_err);
    + if (rc)
    + goto e_free;
    +
    + /* 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);
    @@ -421,6 +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 081d314a6279..bcd00a6d4501 100644
    --- a/include/uapi/linux/sev-guest.h
    +++ b/include/uapi/linux/sev-guest.h
    @@ -30,6 +30,20 @@ struct snp_report_resp {
    __u8 data[4000];
    };

    +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_resp {
    + /* response data, see SEV-SNP spec for the format */
    + __u8 data[64];
    +};
    +
    struct snp_guest_request_ioctl {
    /* message version number (must be non-zero) */
    __u8 msg_version;
    @@ -47,4 +61,7 @@ struct snp_guest_request_ioctl {
    /* 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
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