On 09/22/2016 09:59 AM, Borislav Petkov wrote:

On Thu, Sep 22, 2016 at 04:45:51PM +0200, Paolo Bonzini wrote:

quoted

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. I
think the aim is to have the transparent mode be the default one, which
makes most sense anyway.

There is a new Transparent SME mode that is now part of the overall
SME support, but I'm not alluding to that in the documentation at all.
In TSME mode, everything that goes through the memory controller would
be encrypted and that would include EFI data, etc.  TSME would be
enabled through a BIOS option, thus allowing legacy OSes to benefit.

The EFI regions are unencrypted for obvious reasons and you need to
access them as such.

quoted

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.

This patch assumes that the EFI regions of a guest would be encrypted.

Thanks,
Tom