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

Brijesh Singh
2021-06-14T20:51:07Z

On 6/14/21 12:23 PM, Dr. David Alan Gilbert wrote:
> * 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?

That's bug. I noticed it after you pointed the INT_MAX check and asked
question on why 2. I will fix in next iteration.

>>> + /*
>>> + * 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;
>>> +}
>>> +