Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for kexec kernel

[Borislav Petkov]

On Thu, Aug 03, 2023 at 01:11:54PM +0200, Ard Biesheuvel wrote:
> Sadly, not only 'old' grubs - GRUB mainline only recently added
> support for booting Linux/x86 via the EFI stub (because I wrote the
> code for them),

haha.

> but it will still fall back to the previous mode for kernels that are
> built without EFI stub support, or which are older than ~v5.8 (because
> their EFI stub does not implement the generic EFI initrd loading
> mechanism)

The thing is, those SNP kernels pretty much use the EFI boot mechanism.
I mean, don't take my word for it as I run SNP guests only from time to
time but that's what everyone uses AFAIK.

> Yeah. what seems to be saving our ass here is that startup_32 maps the
> first 1G of physical address space 4 times, and x86_64 EFI usually
> puts firmware tables below 4G. This means the cc blob check doesn't
> fault, but it may dereference bogus memory traversing the config table
> array looking for the cc blob GUID. However, the system table field
> holding the size of the array may also appear as bogus so this may
> still break in weird ways.

Oh fun.

> I don't think we can kill it yet, but it would be nice if we could
> avoid the need to support SNP boot when entering that way.

That's what I meant - not boot SNP guests through the 32-bit entry path.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette