linux-kernel - Re: [RFC PATCH v1 09/28] x86/efi: Access EFI data as encrypted when SEV is active

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Date:   Thu, 22 Sep 2016 17:05:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Brijesh Singh <brijesh.singh@....com>, thomas.lendacky@....com,
        simon.guinot@...uanux.org, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, rkrcmar@...hat.com, matt@...eblueprint.co.uk,
        linus.walleij@...aro.org, linux-mm@...ck.org,
        paul.gortmaker@...driver.com, hpa@...or.com,
        dan.j.williams@...el.com, aarcange@...hat.com,
        sfr@...b.auug.org.au, andriy.shevchenko@...ux.intel.com,
        herbert@...dor.apana.org.au, bhe@...hat.com, xemul@...allels.com,
        joro@...tes.org, x86@...nel.org, mingo@...hat.com,
        msalter@...hat.com, ross.zwisler@...ux.intel.com,
        dyoung@...hat.com, jroedel@...e.de, keescook@...omium.org,
        toshi.kani@....com, mathieu.desnoyers@...icios.com,
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        alexandre.bounine@....com, kuleshovmail@...il.com,
        linux-kernel@...r.kernel.org, mcgrof@...nel.org,
        linux-crypto@...r.kernel.org, akpm@...ux-foundation.org,
        davem@...emloft.net
Subject: Re: [RFC PATCH v1 09/28] x86/efi: Access EFI data as encrypted when
 SEV is active



On 22/09/2016 16:59, Borislav Petkov wrote:
> On Thu, Sep 22, 2016 at 04:45:51PM +0200, Paolo Bonzini wrote:
>> The main difference between the SME and SEV encryption, from the point
>> of view of the kernel, is that real-mode always writes unencrypted in
>> SME and always writes encrypted in SEV.  But UEFI can run in 64-bit mode
>> and learn about the C bit, so EFI boot data should be unprotected in SEV
>> guests.
> 
> Actually, it is different: you can start fully encrypted in SME, see:
> 
> https://lkml.kernel.org/r/20160822223539.29880.96739.stgit@tlendack-t1.amdoffice.net
> 
> The last paragraph alludes to a certain transparent mode where you're
> already encrypted and only certain pieces like EFI is not encrypted.

Which paragraph?

>> Because the firmware volume is written to high memory in encrypted
>> form, and because the PEI phase runs in 32-bit mode, the firmware
>> code will be encrypted; on the other hand, data that is placed in low
>> memory for the kernel can be unencrypted, thus limiting differences
>> between SME and SEV.
> 
> When you run fully encrypted, you still need to access EFI tables in the
> clear. That's why I'm confused about this patch here.

I might be wrong, but I don't think this patch was tested with OVMF or Duet.

Paolo