Re: [PATCH Part1 v5 35/38] x86/sev: Register SNP guest request platform device

[Dov Murik <dovmurik@linux.ibm.com>]

On 02/09/2021 22:58, Brijesh Singh wrote:
> 
> 
> On 9/2/21 11:40 AM, Borislav Petkov wrote:

[...]

>>
>>> +static u64 find_secrets_paddr(void)
>>> +{
>>> +    u64 pa_data = boot_params.cc_blob_address;
>>> +    struct cc_blob_sev_info info;
>>> +    void *map;
>>> +
>>> +    /*
>>> +     * The CC blob contains the address of the secrets page, check
>>> if the
>>> +     * blob is present.
>>> +     */
>>> +    if (!pa_data)
>>> +        return 0;
>>> +
>>> +    map = early_memremap(pa_data, sizeof(info));
>>> +    memcpy(&info, map, sizeof(info));
>>> +    early_memunmap(map, sizeof(info));
>>> +
>>> +    /* Verify that secrets page address is passed */
>>
>> That's hardly verifying something - if anything, it should say
>>
>>     /* smoke-test the secrets page passed */
>>
> Noted.
> 
>>> +    if (info.secrets_phys && info.secrets_len == PAGE_SIZE)
>>> +        return info.secrets_phys;
>>
>> ... which begs the question: how do we verify the HV is not passing some
>> garbage instead of an actual secrets page?
>>
> 
> Unfortunately, the secrets page does not contain a magic header or uuid
> which a guest can read to verify that the page is actually populated by
> the PSP. 

In the SNP FW ABI document section 8.14.2.5 there's a Table 61 titled
Secrets Page Format, which states that the first field in that page is a
u32 VERSION field which should equal 2h.

While not as strict as GUID header, this can help detect early that the
content of the SNP secrets page is invalid.

-Dov

> But since the page is encrypted before the launch so this page
> is always accessed encrypted. If hypervisor is tricking us then all that
> means is guest OS will get a wrong key and will not be able to
> communicate with the PSP to get the attestation reports etc.
> 
> 
>> I guess it is that:
>>
>> "SNP_LAUNCH_UPDATE can insert two special pages into the guest’s
>> memory: the secrets page and the CPUID page. The secrets page contains
>> encryption keys used by the guest to interact with the firmware. Because
>> the secrets page is encrypted with the guest’s memory encryption
>> key, the hypervisor cannot read the keys. The CPUID page contains
>> hypervisor provided CPUID function values that it passes to the guest.
>> The firmware validates these values to ensure the hypervisor is not
>> providing out-of-range values."
>>
>>  From "4.5 Launching a Guest" in the SNP FW ABI spec.
>>
>> I think that explanation above is very important wrt to explaining the
>> big picture how this all works with those pages injected into the guest
>> so I guess somewhere around here a comment should say
>>
> 
> I will add more explanation.
> 
>> "See section 4.5 Launching a Guest in the SNP FW ABI spec for details
>> about those special pages."
>>
>> or so.
>>