[PATCH v2 2/2] x86/efi: Apply EFI Memory Attributes after kexec

[Dave Young]

Hi Nicolas,

On Thu, 28 Nov 2024 at 23:58, Nicolas Saenz Julienne <nsaenz at amazon.com> wrote:
>
> Hi Dave,
>
> On Fri Nov 22, 2024 at 1:03 PM UTC, Dave Young wrote:
> > On Wed, 13 Nov 2024 at 02:53, Nicolas Saenz Julienne <nsaenz at amazon.com> wrote:
> >>
> >> Kexec bypasses EFI's switch to virtual mode. In exchange, it has its own
> >> routine, kexec_enter_virtual_mode(), which replays the mappings made by
> >> the original kernel. Unfortunately, that function fails to reinstate
> >> EFI's memory attributes, which would've otherwise been set after
> >> entering virtual mode. Remediate this by calling
> >> efi_runtime_update_mappings() within kexec's routine.
> >
> > In the function __map_region(), there are playing with the flags
> > similar to the efi_runtime_update_mappings though it looks a little
> > different.  Is this extra callback really necessary?
>
> EFI Memory attributes aren't tracked through
> `/sys/firmware/efi/runtime-map`, and as such, whatever happens in
> `__map_region()` after kexec will not honor them.

>From the comment below the reason to do the mappings update is that
firmware could perform some fixups.  But for kexec case I think doing
the mapping correctly in the mapping code would be good enough.

        /*
         * Apply more restrictive page table mapping attributes now that
         * SVAM() has been called and the firmware has performed all
         * necessary relocation fixups for the new virtual addresses.
         */
        efi_runtime_update_mappings();

Otherwise /sys/firmware/efi/runtime-map is a copy for kexec-tools to
create the virtual efi memmap,  but I think the __map_region is called
after kexecing into the 2nd kernel, so I feel that at that time the
mem attr table should be usable.

Anyway thanks for explaining about this.  It is indeed something to
improve.  I have no strong opinion as your code will also work.


>
> > Have you seen a real bug happened?
>
> If lowered security posture after kexec counts as a bug, yes. The system
> remains stable otherwise.
>
> Nicolas
>

Thanks
Dave