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

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Message-ID: <CAMkAt6pAcM-+odnagFTiaY7PPGE1CfAt27x=tG=-4UU9c+dQXA@mail.gmail.com>
Date:   Fri, 19 Nov 2021 09:16:20 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v7 43/45] virt: Add SEV-SNP guest driver

On Thu, Nov 18, 2021 at 10:32 AM Brijesh Singh <brijesh.singh@....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