linux-kernel - Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg

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Date:   Thu, 9 Sep 2021 11:17:10 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        Marc Orr <marcorr@...gle.com>,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper



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