Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper

Brijesh Singh
2021-09-09T19:26:56Z

On 9/9/21 11:21 AM, Peter Gonda wrote:
> On Thu, Sep 9, 2021 at 10:17 AM Brijesh Singh <brijesh.singh@amd.com> wrote:
>>
>>
>>
>> On 9/9/21 10:43 AM, Peter Gonda wrote:
>> ...
>>
>>>>
>>>> Does this address your concern?
>>>
>>> So the 'snp_msg_seqno()' call in 'enc_payload' will not increment the
>>> counter, its only incremented on 'snp_gen_msg_seqno()'? If thats
>>> correct, that addresses my first concern.
>>>
>>
>> Yes, that is goal.
>>
>>>>>
>>>>
>>>> So far, the only user for the snp_msg_seqno() is the attestation driver.
>>>> And the driver is designed to serialize the vmgexit request and thus we
>>>> should not run into concurrence issue.
>>>
>>> That seems a little dangerous as any module new code or out-of-tree
>>> module could use this function thus revealing this race condition
>>> right? Could we at least have a comment on these functions
>>> (snp_msg_seqno and snp_gen_msg_seqno) noting this?
>>>
>>
>> Yes, if the driver is not performing the serialization then we will get
>> into race condition.
>>
>> One way to avoid this requirement is to do all the crypto inside the
>> snp_issue_guest_request() and eliminate the need to export the
>> snp_msg_seqno().
>>
>> I will add the comment about it in the function.
>
> Actually I forgot that the sequence number is the only component of
> the AES-GCM IV. Seen in 'enc_payload'. Given the AES-GCM spec requires
> uniqueness of the IV. I think we should try a little harder than a
> comment to guarantee we never expose 2 requests encrypted with the
> same sequence number / IV. It's more than just a DOS against the
> guest's PSP request ability but also could be a guest security issue,
> thoughts?
>

Ah good point, we should avoid a request with same IV. May be move the
sequence number increment and save in sevguest drv. Then driver can do
the sequence get, vmgexit and increment with a protected lock.

thanks

> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fnvlpubs.nist.gov%2Fnistpubs%2FLegacy%2FSP%2Fnistspecialpublication800-38d.pdf&amp;data=04%7C01%7Cbrijesh.singh%40amd.com%7C46a05f4713834307706608d973ade616%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637668013461202204%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=KCsi5rTQX6L%2BqY07VdBtF8IH0TLNyHn6wTyidgWvXf4%3D&amp;reserved=0
> (Section 8 page 18)
>
>>
>> thanks
>