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 16:45:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...e.de>,
        Brijesh Singh <brijesh.singh@....com>, thomas.lendacky@....com
Cc:     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,
        devel@...uxdriverproject.org, tglx@...utronix.de,
        mchehab@...nel.org, iamjoonsoo.kim@....com,
        labbott@...oraproject.org, tony.luck@...el.com,
        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 16:35, Borislav Petkov wrote:
>> > @@ -230,6 +230,10 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
>> >  	efi_scratch.efi_pgt = (pgd_t *)__sme_pa(efi_pgd);
>> >  	pgd = efi_pgd;
>> >  
>> > +	flags = _PAGE_NX | _PAGE_RW;
>> > +	if (sev_active)
>> > +		flags |= _PAGE_ENC;
> So this is confusing me. There's this patch which says EFI data is
> accessed in the clear:
> 
> https://lkml.kernel.org/r/20160822223738.29880.6909.stgit@tlendack-t1.amdoffice.net
> 
> but now here it is encrypted when SEV is enabled.
> 
> Do you mean, it is encrypted here because we're in the guest kernel?

I suspect this patch is untested, and also wrong. :)

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.

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.

	Important: I don't know what you guys are doing for SEV and
	Windows guests, but if you are doing something I would really
	appreciate doing things in the open.  If Linux and Windows end
	up doing different things with EFI boot data, ACPI tables, etc.
	it will be a huge pain.  On the other hand, if we can enjoy
	being first, that's great.

In fact, I have suggested in the QEMU list that SEV guests should always
use UEFI; because BIOS runs in real-mode or 32-bit non-paging protected
mode, BIOS must always write encrypted data, which becomes painful in
the kernel.

And regarding the above "important" point, all I know is that Microsoft
for sure will be happy to restrict SEV to UEFI guests. :)

There are still some differences, mostly around the real mode trampoline
executed by the kernel, but they should be much smaller.

Paolo