Re: [PATCH] x86/efi: Access EFI MMIO data as unencrypted when SEV is active

From: Borislav Petkov
Date: Tue Jul 03 2018 - 11:44:30 EST

(dropping stable@ as this is not how you send patches to stable).

On Tue, Jul 03, 2018 at 05:37:18PM +0200, Ard Biesheuvel wrote:
> On 3 July 2018 at 15:32, Brijesh Singh <brijesh.singh@xxxxxxx> wrote:
> > SEV guest fails to update the UEFI runtime variables stored in the
> > flash. commit 1379edd59673 ("x86/efi: Access EFI data as encrypted
> > when SEV is active") unconditionally maps all the UEFI runtime data
> > as 'encrypted' (C=1). When SEV is active the UEFI runtime data marked
> > as EFI_MEMORY_MAPPED_IO should be mapped as 'unencrypted' so that both
> > guest and hypervisor can access the data.
> >
> I'm uncomfortable having to carry these heuristics in the kernel. The
> UEFI memory map should be the definitive source of information
> regarding how the OS should map the regions it describes, and if we
> need to guess the encryption status, we are likely to get it wrong at
> least some of the times.

I think the problem here is that IO memory can't be encrypted, at least
at the moment. Thus this patch. I believe future versions will be able
to handle encrypted IO but that's something Brijesh can correct me on.

So it is not really about the UEFI spec but about what the hardware
does/supports currently.

And I don't think that change matters on anything else besides AMD with
SEV enabled...



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