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Subject: Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper
To:     Peter Gonda <pgonda@google.com>
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 <20210820151933.22401-35-brijesh.singh@amd.com>
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From:   Brijesh Singh <brijesh.singh@amd.com>
Message-ID: <f54f3fc3-14c3-5c36-2712-62eb625a958b@amd.com>
Date:   Thu, 9 Sep 2021 11:17:10 -0500
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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.

thanks