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

Dov Murik
2021-08-31T19:02:15Z

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@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 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_ */
>