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

[Arvind Sankar]

On Thu, Apr 09, 2020 at 04:47:55PM +0200, Ard Biesheuvel wrote:
> On Thu, 9 Apr 2020 at 16:39, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxx> 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?
> >
> 
> Well, the fact the /some/ piece of software is used in production that
> relies on the ill-defined EFI handover protocol is sufficient
> justification, so I don't think it is hugely important to update it.
> 
> > 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.
> 
> Thanks Arvind. I'm sure the Fedora/RedHat folks on cc should be able
> to get these logged somewhere.

Ok. For dracut, the process for building the unified kernel image needs
a check to make sure the kernel can fit in the space provided for it --
there is 16MiB of space and the distro bzImage's are up to 10-11MiB in
size, so there's some slack left at present.

Additionally, in mixed-mode, the unified kernel images are quite likely
to end up with early pgtables from startup_32 clobbering the initrd,
independently of the recent kernel changes. Hopefully no-one actually
uses these in mixed-mode.