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

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Date:   Thu, 9 Sep 2021 10:21:20 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     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 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?

https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
(Section 8 page 18)

>
> thanks