linux-kernel - Re: [PATCH V4] virt: sev: Prevent IV reuse in SNP guest driver

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Message-ID: <CAMkAt6rHTgJX1KTjYmbii6dyG7QMxXJxNy1E_eZ8vRWLK9Vc1g@mail.gmail.com>
Date:   Wed, 16 Nov 2022 10:10:58 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Borislav Petkov <bp@...e.de>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Haowen Bai <baihaowen@...zu.com>,
        Yang Yingliang <yangyingliang@...wei.com>,
        Marc Orr <marcorr@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Ashish Kalra <Ashish.Kalra@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V4] virt: sev: Prevent IV reuse in SNP guest driver

On Wed, Nov 16, 2022 at 9:58 AM Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 11/16/22 10:23, Peter Gonda wrote:
> > On Wed, Nov 16, 2022 at 5:20 AM Borislav Petkov <bp@...e.de> wrote:
> >>
> >> On Tue, Nov 15, 2022 at 02:47:31PM -0700, Peter Gonda wrote:
> >>>>> +      * certificate data buffer retry the same guest request without the
> >>>>> +      * extended data request.
> >>>>> +      */
> >>>>> +     if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
> >>>>> +         err == SNP_GUEST_REQ_INVALID_LEN) {
> >>>>> +             const unsigned int certs_npages = snp_dev->input.data_npages;
> >>>>> +
> >>>>> +             exit_code = SVM_VMGEXIT_GUEST_REQUEST;
> >>>>> +             rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
> >>>>> +
> >>>>> +             err = SNP_GUEST_REQ_INVALID_LEN;
> >>>>
> >>>> Huh, why are we overwriting err here?
> >>>
> >>> I have added a comment for the next revision.
> >>>
> >>> We are overwriting err here so that userspace is alerted that they
> >>> supplied a buffer too small.
> >>
> >> Sure but you're not checking rc either. What if that reissue fails for
> >> whatever other reason? -EIO for example...
> >
> > If we get any error here we have to wipe the VMPCK here so I thought
>
> More accurate to say that you will wipe the VMPCK, since the value of rc
> is checked a bit further down in the code and the -EIO (or other non-zero)
> will be result in a call to snp_disable_vmpck() and rc being propagated
> back to the user as an ioctl() return code.
>
> Might be worth a comment above that second snp_issue_guest_request()
> explaining that.

I'll add a comment above the second snp_issue_guest_request(), good idea thanks.

I think another comment above the first snp_issue_guest_request()
could help too. Saying once we call this function we either need to
increment the sequence number or wipe the VMPCK to ensure the
encryption scheme is safe.

>
> > this always override @err was OK.
> >
> > I can update this to only override @err if after the secondary
> > SVM_VMGEXIT_GUEST_REQUEST rc and err are OK. Thoughts?
>
> I think it's ok to set it no matter what, but I don't have a strong
> opinion either way.
>
> Thanks,
> Tom
>
> >
> >>
> >> --
> >> Regards/Gruss,
> >>      Boris.
> >>
> >> SUSE Software Solutions Germany GmbH
> >> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Martje Boudien Moerman
> >> (HRB 36809, AG Nürnberg)