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

From: Ard Biesheuvel
Date: Thu Aug 03 2023 - 10:28:19 EST

On Thu, 3 Aug 2023 at 13:11, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> On Wed, 2 Aug 2023 at 17:52, Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > On Wed, Aug 02, 2023 at 04:55:27PM +0200, Ard Biesheuvel wrote:
> > > ... because now, entering via startup_32 is broken, given that it only
> > > maps the kernel image itself and relies on the #PF handling for
> > > everything else it accesses, including firmware tables.
> > >
> > > AFAICT this also means that entering via startup_32 is broken entirely
> > > for any configuration that enables the cc blob config table check,
> > > regardless of the platform.
> >
> > Lemme brain-dump what Tom and I just talked on IRC.
> >
> > That startup_32 entry path for SNP guests was used with old grubs which
> > used to enter through there and not anymore, reportedly. Which means,
> > that must've worked at some point but Joerg would know. CCed.
> >
> 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), 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)
> This fallback still appears to enter via startup_32, even when GRUB
> itself runs in long mode in the context of EFI.
> > Newer grubs enter through the 64-bit entry point and thus are fine
> > - otherwise we would be seeing explosions left and right.
> >
> 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.
> > So dependent on what we wanna do, if we kill the 32-bit path, we can
> > kill the 32-bit C-bit verif code. But that's for later and an item on my
> > TODO list.
> >
> 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.

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