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

From: Paolo Bonzini
Date: Thu Sep 22 2016 - 10:46:56 EST

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:
> 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

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.