Re: [PATCH 1/2] efi/x86: Move efi stub globals from .bss to .data

Arvind Sankar
2020-04-09T14:39:10Z

On Wed, Apr 08, 2020 at 09:49:15AM +0200, Ard Biesheuvel wrote:
> (add Peter, Leif and Daniel)
>
> On Wed, 8 Apr 2020 at 09:43, Dave Young <dyoung@redhat.com> wrote:
> >
> > On 04/06/20 at 02:06pm, Arvind Sankar wrote:
> > > Commit
> > >
> > > 3ee372ccce4d ("x86/boot/compressed/64: Remove .bss/.pgtable from
> > > bzImage")
> > >
> > > removed the .bss section from the bzImage.
> > >
> > > However, while a PE loader is required to zero-initialize the .bss
> > > section before calling the PE entry point, the EFI handover protocol
> > > does not currently document any requirement that .bss be initialized by
> > > the bootloader prior to calling the handover entry.
> > >
> > > When systemd-boot is used to boot a unified kernel image [1], the image
> > > is constructed by embedding the bzImage as a .linux section in a PE
> > > executable that contains a small stub loader from systemd together with
> > > additional sections and potentially an initrd. As the .bss section
> > > within the bzImage is no longer explicitly present as part of the file,
> > > it is not initialized before calling the EFI handover entry.
> > > Furthermore, as the size of the embedded .linux section is only the size
> > > of the bzImage file itself, the .bss section's memory may not even have
> > > been allocated.
> >
> > I did not follow up the old report, maybe I missed something. But not
> > sure why only systemd-boot is mentioned here. I also have similar issue
> > with early efi failure. With these two patches applied, it works well
> > then.
> >
> > BTW, I use Fedora 31 + Grub2
> >
>
> OK, so I take it this means that GRUB's PE/COFF loader does not
> zero-initialize BSS either? Does it honor the image size in memory if
> it exceeds the file size?

Dave, that comment was because the previous report was for systemd-boot
stub.

Ard, should I revise the commit message to make it clear it's not
restricted to systemd-boot but anything using handover entry may be
affected? Maybe just a "for example, when systemd-boot..." and then a
line to say grub2 with the EFI stub patches is also impacted?

https://src.fedoraproject.org/rpms/grub2/blob/f31/f/0001-Add-support-for-Linux-EFI-stub-loading.patch#_743

+ kernel_mem = grub_efi_allocate_pages_max(lh.pref_address,
+ BYTES_TO_PAGES(lh.init_size));

Looking at this, grub does allocate init_size for the image, but it
doesn't zero it out.

This call also looks wrong to me though. It allocates at max address of
pref_address, which, if it succeeds, will guarantee that the kernel gets
loaded entirely below pref_address == LOAD_PHYSICAL_ADDR. In native
mode, if it weren't for the EFI stub copying the kernel again, this
would cause the startup code to relocate the kernel into unallocated
memory. On a mixed-mode boot, this would cause the early page tables
setup prior to transitioning to 64-bit mode to be in unallocated memory
and potentially get clobbered by the EFI stub.

The first try to allocate pref_address should be calling
grub_efi_allocate_fixed instead.