Re: [PATCH v9 42/43] virt: sevguest: Add support to derive key

Borislav Petkov
2022-02-07T08:55:35Z

On Fri, Jan 28, 2022 at 11:18:03AM -0600, Brijesh Singh wrote:
> +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 */

verify_comment_style: Warning: No tail comments please:
drivers/virt/coco/sevguest/sevguest.c:401 [+ 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 */

That comment looks useless.

> + 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;

That test can happen before the copy_from_user() above.

> +
> + /* Issue the command to get the attestation report */

Also useless.

> + 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 */

Ditto.

> + 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));

Those are allocated on stack, why are you clearing them?

> + return rc;
> +}

--
Regards/Gruss,
Boris.