Re: [PATCH v7 43/45] virt: Add SEV-SNP guest driver

Peter Gonda
2021-11-19T16:16:35Z

On Thu, Nov 18, 2021 at 10:32 AM Brijesh Singh <brijesh.singh@amd.com> wrote:
>
>
>
> On 11/17/21 5:34 PM, Peter Gonda wrote:
>
>
> >> +The guest ioctl should be issued on a file descriptor of the /dev/sev-guest device.
> >> +The ioctl accepts struct snp_user_guest_request. The input and output structure is
> >> +specified through the req_data and resp_data field respectively. If the ioctl fails
> >> +to execute due to a firmware error, then fw_err code will be set.
> >
> > Should way say what it will be set to? Also Sean pointed out on CCP
> > driver that 0 is strange to set the error to, its a uint so we cannot
> > do -1 like we did there. What about all FFs?
> >
>
> Sure, all FF's works, I can document and use it.
>
>
> >> +static inline u64 __snp_get_msg_seqno(struct snp_guest_dev *snp_dev)
> >> +{
> >> + u64 count;
> >
> > I may be overly paranoid here but how about
> > `lockdep_assert_held(&snp_cmd_mutex);` when writing or reading
> > directly from this data?
> >
>
> Sure, I can do it.
>
> ...
>
> >> +
> >> + if (rc)
> >> + return rc;
> >> +
> >> + rc = verify_and_dec_payload(snp_dev, resp_buf, resp_sz);
> >> + if (rc) {
> >> + /*
> >> + * The verify_and_dec_payload() will fail only if the hypervisor is
> >> + * actively modifiying the message header or corrupting the encrypted payload.
> > modifiying
> >> + * This hints that hypervisor is acting in a bad faith. Disable the VMPCK so that
> >> + * the key cannot be used for any communication.
> >> + */
> >
> > This looks great, thanks for changes Brijesh. Should we mention in
> > comment here or at snp_disable_vmpck() the AES-GCM issues with
> > continuing to use the key? Or will future updaters to this code
> > understand already?
> >
>
> Sure, I can add comment about the AES-GCM.
>
> ...
>
> >> +
> >> +/* See SNP spec SNP_GUEST_REQUEST section for the structure */
> >> +enum msg_type {
> >> + SNP_MSG_TYPE_INVALID = 0,
> >> + SNP_MSG_CPUID_REQ,
> >> + SNP_MSG_CPUID_RSP,
> >> + SNP_MSG_KEY_REQ,
> >> + SNP_MSG_KEY_RSP,
> >> + SNP_MSG_REPORT_REQ,
> >> + SNP_MSG_REPORT_RSP,
> >> + SNP_MSG_EXPORT_REQ,
> >> + SNP_MSG_EXPORT_RSP,
> >> + SNP_MSG_IMPORT_REQ,
> >> + SNP_MSG_IMPORT_RSP,
> >> + SNP_MSG_ABSORB_REQ,
> >> + SNP_MSG_ABSORB_RSP,
> >> + SNP_MSG_VMRK_REQ,
> >> + SNP_MSG_VMRK_RSP,
> >
> > Did you want to include MSG_ABSORB_NOMA_REQ and MSG_ABSORB_NOMA_RESP here?
> >
>
> Yes, I can includes those for the completeness.
>
> ...
>
> >> +struct snp_report_req {
> >> + /* message version number (must be non-zero) */
> >> + __u8 msg_version;
> >> +
> >> + /* user data that should be included in the report */
> >> + __u8 user_data[64];
> >
> > Are we missing the 'vmpl' field here? Does those default all requests
> > to be signed with VMPL0? Users might want to change that, they could
> > be using a paravisor.
> >
>
> Good question, so far I was thinking that guest kernel will provide its
> vmpl level instead of accepted the vmpl level from the userspace. Do you
> see a need for a userspace to provide this information ?

That seems fine. I am just confused because we are just encrypting
this struct as the payload for the PSP. Doesn't the message require a
struct that looks like 'snp_report_req_user_data' below?

snp_report_req{
/* message version number (must be non-zero) */
__u8 msg_version;

/* user data that should be included in the report */
struct snp_report_req_user_data;
};

struct snp_report_req_user_data {
u8 user_data[64];
u32 vmpl;
u32 reserved;
};

>
>
> thanks