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

[Borislav Petkov]

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.

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