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    Subject[PATCH Part1 RFC v4 35/36] virt: sevguest: Add support to derive key
    Date
    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 | 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 f3f86f9b5b22..2f20db80490a 100644
    --- a/drivers/virt/coco/sevguest/sevguest.c
    +++ b/drivers/virt/coco/sevguest/sevguest.c
    @@ -304,6 +304,50 @@ 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);
    + 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);
    + 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);
    @@ -321,6 +365,10 @@ 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_ */
    --
    2.17.1
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