Re: [PATCH v6 40/42] virt: Add SEV-SNP guest driver

Brijesh Singh
2021-10-27T21:13:08Z

On 10/27/21 4:05 PM, Peter Gonda wrote:
....

>>>>>
>>>>> Thanks for updating this sequence number logic. But I still have some
>>>>> concerns. In verify_and_dec_payload() we check the encryption header
>>>>> but all these fields are accessible to the hypervisor, meaning it can
>>>>> change the header and cause this sequence number to not get
>>>>> incremented. We then will reuse the sequence number for the next
>>>>> command, which isn't great for AES GCM. It seems very hard to tell if
>>>>> the FW actually got our request and created a response there by
>>>>> incrementing the sequence number by 2, or if the hypervisor is acting
>>>>> in bad faith. It seems like to be safe we need to completely stop
>>>>> using this vmpck if we cannot confirm the PSP has gotten our request
>>>>> and created a response. Thoughts?
>>>>>
>>>>
>>>> Very good point, I think we can detect this condition by rearranging the
>>>> checks. The verify_and_dec_payload() is called only after the command is
>>>> succesful and does the following checks
>>>>
>>>> 1) Verifies the header
>>>> 2) Decrypts the payload
>>>> 3) Later we increment the sequence
>>>>
>>>> If we arrange to the below order then we can avoid this condition.
>>>> 1) Decrypt the payload
>>>> 2) Increment the sequence number
>>>> 3) Verify the header
>>>>
>>>> The descryption will succeed only if PSP constructed the payload.
>>>>
>>>> Does this make sense ?
>>>
>>> Either ordering seems fine to me. I don't think it changes much though
>>> since the header (bytes 30-50 according to the spec) are included in
>>> the authenticated data of the encryption. So any hypervisor modictions
>>> will lead to a decryption failure right?
>>>
>>> Either case if we do fail the decryption, what are your thoughts on
>>> not allowing further use of that VMPCK?
>>>
>>
>> We have limited number of VMPCK (total 3). I am not sure switching to
>> different will change much. HV can quickly exaust it. Once we have SVSM
>> in-place then its possible that SVSM may use of the VMPCK. If the
>> decryption failed, then maybe its safe to erase the key from the secrets
>> page (in other words guest OS cannot use that key for any further
>> communication). A guest can reload the driver will different VMPCK id
>> and try again.
>
> SNP cannot really cover DOS at all since the VMM could just never
> schedule the VM. In this case we know that the hypervisor is trying to
> mess with the guest, so my preference would be to stop sending guest
> messages to prevent that duplicated IV usage. If one caller gets an
> EBADMSG it knows its in this case but the rest of userspace has no
> idea. Maybe log an error?
>
>>

Yap, we cannot protect against the DOS. This is why I was saying that we
zero the key from secrets page so that guest cannot use that key for any
future communication (whether its from rest of userspace or kexec
kernel). I can update the driver to log the message and ensure that
future messages will *not* use that key. The VMPCK ID is a module
params, so a guest can reload the driver to use different VMPCK.

>> thanks