Re: [PATCH Part1 RFC v3 21/22] x86/sev: Register SNP guest request platform device

Dr. David Alan Gilbert
2021-06-14T17:23:40Z

* Brijesh Singh (brijesh.singh@amd.com) wrote:
> I see that Tom answered few comments. I will cover others.
>
>
> On 6/9/21 2:24 PM, Dr. David Alan Gilbert wrote:
> + /*
> >> + * The message sequence counter for the SNP guest request is a 64-bit value
> >> + * but the version 2 of GHCB specification defines the 32-bit storage for the
> >> + * it.
> >> + */
> >> + if ((count + 1) >= INT_MAX)
> >> + return 0;
> > Is that UINT_MAX?
>
> Good catch. It should be UINT_MAX.

OK, but I'm also confused by two things:
a) Why +1 given that Tom's reply says this gets incremented by 2 each
time (once for the message, once for the reply)
b) Why >= ? I think here is count was INT_MAX-1 you'd skip to 0,
skipping INT_MAX - is that what you want?

>
> > + /*
> > + * The secret page contains the VM encryption key used for encrypting the
> > + * messages between the guest and the PSP. The secrets page location is
> > + * available either through the setup_data or EFI configuration table.
> > + */
> > + if (hdr->cc_blob_address) {
> > + paddr = hdr->cc_blob_address;
> > Can you trust the paddr the host has given you or do you need to do some
> > form of validation?
> The paddr is mapped encrypted. That means that data  in the paddr must
> be encrypted either through the guest or PSP. After locating the paddr,
> we perform a simply sanity check (32-bit magic string "AMDE"). See the
> verify header check below. Unfortunately the secrets page itself does
> not contain any magic key which we can use to ensure that
> hdr->secret_paddr is actually pointing to the secrets pages but all of
> these memory is accessed encrypted so its safe to access it. If VMM
> lying to us that basically means guest will not be able to communicate
> with the PSP and can't do the attestation etc.

OK; that nails pretty much anything bad that can happen - I was just
thinking if the host did something odd like give you an address in the
middle of some other useful structure.

Dave

> >
> > Dave
> > + } else if (efi_enabled(EFI_CONFIG_TABLES)) {
> > +#ifdef CONFIG_EFI
> > + paddr = cc_blob_phys;
> > +#else
> > + return -ENODEV;
> > +#endif
> > + } else {
> > + return -ENODEV;
> > + }
> > +
> > + info = memremap(paddr, sizeof(*info), MEMREMAP_WB);
> > + if (!info)
> > + return -ENOMEM;
> > +
> > + /* Verify the header that its a valid SEV_SNP CC header */
> > + if ((info->magic == CC_BLOB_SEV_HDR_MAGIC) &&
> > + info->secrets_phys &&
> > + (info->secrets_len == PAGE_SIZE)) {
> > + res->start = info->secrets_phys;
> > + res->end = info->secrets_phys + info->secrets_len;
> > + res->flags = IORESOURCE_MEM;
> > + snp_secrets_phys = info->secrets_phys;
> > + ret = 0;
> > + }
> > +
> > + memunmap(info);
> > + return ret;
> > +}
> > +
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK