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

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Date:   Thu, 9 Sep 2021 14:26:49 -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 11:21 AM, Peter Gonda wrote:
> On Thu, Sep 9, 2021 at 10:17 AM Brijesh Singh <brijesh.singh@....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
>