Re: [PATCH Part1 v5 38/38] virt: sevguest: Add support to get extended report

Dr. David Alan Gilbert
2021-09-15T10:02:14Z

* Brijesh Singh (brijesh.singh@amd.com) wrote:
> Version 2 of GHCB specification defines NAE to get the extended guest
> request. It is similar to the SNP_GET_REPORT ioctl. The main difference

^^^^^^^^^ is that 'report' not request?

> is related to the additional data that be returned. The additional
> data returned is a certificate blob that can be used by the SNP guest
> user. The certificate blob layout is defined in the GHCB specification.
> The driver simply treats the blob as a opaque data and copies it to
> userspace.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>

I'm confused by snp_dev->certs_data - who writes to that, and when?
I see it's allocated as shared by the probe function but then passed in
input data in get_ext_report - but get_ext_report memset's it.
What happens if two threads were to try and get an extended report at
the same time?

Dave

> ---
> Documentation/virt/coco/sevguest.rst | 22 +++++
> drivers/virt/coco/sevguest/sevguest.c | 126 ++++++++++++++++++++++++++
> include/uapi/linux/sev-guest.h | 13 +++
> 3 files changed, 161 insertions(+)
>
> diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
> index 25446670d816..7acb8696fca4 100644
> --- a/Documentation/virt/coco/sevguest.rst
> +++ b/Documentation/virt/coco/sevguest.rst
> @@ -85,3 +85,25 @@ on the various fileds passed in the key derivation request.
>
> On success, the snp_derived_key_resp.data will contains the derived key
> value.
> +
> +2.2 SNP_GET_EXT_REPORT
> +----------------------
> +:Technology: sev-snp
> +:Type: guest ioctl
> +:Parameters (in/out): struct snp_ext_report_req
> +:Returns (out): struct snp_report_resp on success, -negative on error
> +
> +The SNP_GET_EXT_REPORT ioctl is similar to the SNP_GET_REPORT. The difference is
> +related to the additional certificate data that is returned with the report.
> +The certificate data returned is being provided by the hypervisor through the
> +SNP_SET_EXT_CONFIG.
> +
> +The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command provided by the SEV-SNP
> +firmware to get the attestation report.
> +
> +On success, the snp_ext_report_resp.data will contains the attestation report
> +and snp_ext_report_req.certs_address will contains the certificate blob. If the
> +length of the blob is lesser than expected then snp_ext_report_req.certs_len will
> +be updated with the expected value.
> +
> +See GHCB specification for further detail on how to parse the certificate blob.
> diff --git a/drivers/virt/coco/sevguest/sevguest.c b/drivers/virt/coco/sevguest/sevguest.c
> index 621b1c5a9cfc..d978eb432c4c 100644
> --- a/drivers/virt/coco/sevguest/sevguest.c
> +++ b/drivers/virt/coco/sevguest/sevguest.c
> @@ -39,6 +39,7 @@ struct snp_guest_dev {
> struct device *dev;
> struct miscdevice misc;
>
> + void *certs_data;
> struct snp_guest_crypto *crypto;
> struct snp_guest_msg *request, *response;
> };
> @@ -347,6 +348,117 @@ static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_user_guest_
> return rc;
> }
>
> +static int get_ext_report(struct snp_guest_dev *snp_dev, struct snp_user_guest_request *arg)
> +{
> + struct snp_guest_crypto *crypto = snp_dev->crypto;
> + struct snp_guest_request_data input = {};
> + struct snp_ext_report_req req;
> + int ret, npages = 0, resp_len;
> + struct snp_report_resp *resp;
> + struct snp_report_req *rreq;
> + unsigned long fw_err = 0;
> +
> + 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;
> +
> + rreq = &req.data;
> +
> + /* Message version must be non-zero */
> + if (!rreq->msg_version)
> + return -EINVAL;
> +
> + if (req.certs_len) {
> + if (req.certs_len > SEV_FW_BLOB_MAX_SIZE ||
> + !IS_ALIGNED(req.certs_len, PAGE_SIZE))
> + return -EINVAL;
> + }
> +
> + if (req.certs_address && req.certs_len) {
> + if (!access_ok(req.certs_address, req.certs_len))
> + return -EFAULT;
> +
> + /*
> + * Initialize the intermediate buffer with all zero's. This buffer
> + * is used in the guest request message to get the certs blob from
> + * the host. If host does not supply any certs in it, then we copy
> + * zeros to indicate that certificate data was not provided.
> + */
> + memset(snp_dev->certs_data, 0, req.certs_len);
> +
> + input.data_gpa = __pa(snp_dev->certs_data);
> + npages = req.certs_len >> PAGE_SHIFT;
> + }
> +
> + /*
> + * 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;
> +
> + if (copy_from_user(resp, (void __user *)arg->resp_data, sizeof(*resp))) {
> + ret = -EFAULT;
> + goto e_free;
> + }
> +
> + /* Encrypt the userspace provided payload */
> + ret = enc_payload(snp_dev, rreq->msg_version, SNP_MSG_REPORT_REQ,
> + &rreq->user_data, sizeof(rreq->user_data));
> + if (ret)
> + goto e_free;
> +
> + /* Call firmware to process the request */
> + input.req_gpa = __pa(snp_dev->request);
> + input.resp_gpa = __pa(snp_dev->response);
> + input.data_npages = npages;
> + memset(snp_dev->response, 0, sizeof(*snp_dev->response));
> + ret = snp_issue_guest_request(EXT_GUEST_REQUEST, &input, &fw_err);
> +
> + /* Popogate any firmware error to the userspace */
> + arg->fw_err = fw_err;
> +
> + /* If certs length is invalid then copy the returned length */
> + if (arg->fw_err == SNP_GUEST_REQ_INVALID_LEN) {
> + req.certs_len = input.data_npages << PAGE_SHIFT;
> +
> + if (copy_to_user((void __user *)arg->req_data, &req, sizeof(req)))
> + ret = -EFAULT;
> +
> + goto e_free;
> + }
> +
> + if (ret)
> + goto e_free;
> +
> + /* Decrypt the response payload */
> + ret = verify_and_dec_payload(snp_dev, resp->data, resp_len);
> + if (ret)
> + goto e_free;
> +
> + /* Copy the certificate data blob to userspace */
> + if (req.certs_address &&
> + copy_to_user((void __user *)req.certs_address, snp_dev->certs_data,
> + req.certs_len)) {
> + ret = -EFAULT;
> + goto e_free;
> + }
> +
> + /* Copy the response payload to userspace */
> + if (copy_to_user((void __user *)arg->resp_data, resp, sizeof(*resp)))
> + ret = -EFAULT;
> +
> +e_free:
> + kfree(resp);
> + return ret;
> +}
> +
> static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
> {
> struct snp_guest_dev *snp_dev = to_snp_dev(file);
> @@ -368,6 +480,10 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
> ret = get_derived_key(snp_dev, &input);
> break;
> }
> + case SNP_GET_EXT_REPORT: {
> + ret = get_ext_report(snp_dev, &input);
> + break;
> + }
> default:
> break;
> }
> @@ -453,6 +569,12 @@ static int __init snp_guest_probe(struct platform_device *pdev)
> goto e_free_req;
> }
>
> + snp_dev->certs_data = alloc_shared_pages(SEV_FW_BLOB_MAX_SIZE);
> + if (IS_ERR(snp_dev->certs_data)) {
> + ret = PTR_ERR(snp_dev->certs_data);
> + goto e_free_resp;
> + }
> +
> misc = &snp_dev->misc;
> misc->minor = MISC_DYNAMIC_MINOR;
> misc->name = DEVICE_NAME;
> @@ -460,6 +582,9 @@ static int __init snp_guest_probe(struct platform_device *pdev)
>
> return misc_register(misc);
>
> +e_free_resp:
> + free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
> +
> e_free_req:
> free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
>
> @@ -475,6 +600,7 @@ static int __exit snp_guest_remove(struct platform_device *pdev)
>
> free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
> free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
> + free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
> deinit_crypto(snp_dev->crypto);
> misc_deregister(&snp_dev->misc);
>
> diff --git a/include/uapi/linux/sev-guest.h b/include/uapi/linux/sev-guest.h
> index 621a9167df7a..23659215fcfb 100644
> --- a/include/uapi/linux/sev-guest.h
> +++ b/include/uapi/linux/sev-guest.h
> @@ -57,6 +57,16 @@ struct snp_derived_key_resp {
> __u8 data[64];
> };
>
> +struct snp_ext_report_req {
> + struct snp_report_req data;
> +
> + /* where to copy the certificate blob */
> + __u64 certs_address;
> +
> + /* length of the certificate blob */
> + __u32 certs_len;
> +};
> +
> #define SNP_GUEST_REQ_IOC_TYPE 'S'
>
> /* Get SNP attestation report */
> @@ -65,4 +75,7 @@ struct snp_derived_key_resp {
> /* Get a derived key from the root */
> #define SNP_GET_DERIVED_KEY _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x1, struct snp_user_guest_request)
>
> +/* Get SNP extended report as defined in the GHCB specification version 2. */
> +#define SNP_GET_EXT_REPORT _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x2, struct snp_user_guest_request)
> +
> #endif /* __UAPI_LINUX_SEV_GUEST_H_ */
> --
> 2.17.1
>
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK