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

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Date:   Wed, 16 Nov 2022 10:58:33 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Peter Gonda <pgonda@...gle.com>, Borislav Petkov <bp@...e.de>
Cc:     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 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.

> 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)