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.
>>
