Re: [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from bzImage

[Arvind Sankar <nivedita-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>]

On Mon, Apr 06, 2020 at 03:29:18PM +0200, Ard Biesheuvel wrote:
> > >
> > > > Actually, it may be sufficient to #define __efistub_global to
> > > > __section(.data) like we already do for ARM, to ensure that these
> > > > global flags are always initialized correctly. (I'll wait for Sergey
> > > > to confirm that the spurious enabling of the PCI DMA protection
> > > > resulting from this BSS issue is causing the boot regression)
> >
> > Yeah I thought of that as the easiest fix, but it might be safer to
> > explicitly zero-init in efi_main to avoid future problems in case
> > someone adds another variable in bss and isn't aware of this obscure
> > requirement. We actually already have sys_table in bss, but that one is
> > always initialized. There's also other globals that aren't annotated
> > (but not in bss by virtue of having initializers). What do you think?
> >
> 
> *If* we zero init BSS, I'd prefer for it to be done in the EFI
> handover protocol entrypoint only. But does that fix the issue that
> BSS lives outside of the memory footprint of the kernel image?
> 

Yes, I was thinking of only doing it if we didn't come through the
pe_entry. We could also avoid re-parsing the command line if we add a
global flag to indicate that.

Regarding the memory footprint, if there's no initrd that might be a
problem, since in that case ImageSize will only cover the actual data
from the bzImage, so it would be safer to move them to data (including
sys_table).

We could just pull the stub's bss section all into data with objcopy
similar to what ARM64 does [1]? i.e. rename .bss to .bss.efistub and
then pull that into .data in the linker script for the compressed
kernel?

There is also this scary looking comment in gnu-efi's linker script:
	/* the EFI loader doesn't seem to like a .bss section, so we stick
	   it all into .data: */
I don't know what the history of that is.

[1] As an aside, why doesn't ARM do this as well rather than using the
section(.data) annotation?

> 
> > What do you think of the other problem -- that's actually worse to fix,
> > as it won't just be when kaslr is disabled, the startup_64 code will do
> > relocation to the end of init_size and clobber the initrd before getting
> > to the kaslr code, so it will break as soon as the firmware loads the
> > "unified kernel image" at a 2Mb-aligned address. The only thing I can
> > think of is to just unconditionally call efi_relocate_kernel if we were
> > entered via handover_entry?
> >
> 
> Yes, that seems to be the most robust approach.