Re: [PATCH v2 1/1] x86/boot/compressed: Fix reloading of GDTR post-relocation

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 27 Feb 2020 at 16:54, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Thu, Feb 27, 2020 at 04:21:32PM +0100, Ard Biesheuvel wrote:
> > On Thu, 27 Feb 2020 at 16:16, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > >
> > > On Thu, Feb 27, 2020 at 09:12:29AM +0100, Ingo Molnar wrote:
> > > >
> > > > * Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > > >
> > > > > Commit ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
> > > > > introduced GDT setup into the 32-bit kernel's startup_32, and reloads
> > > > > the GDTR after relocating the kernel for paranoia's sake.
> > > > >
> > > > > Commit 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of
> > > > > the buffer") introduced a similar GDTR reload in the 64-bit kernel.
> > > > >
> > > > > The GDTR is adjusted by init_size - _end, however this may not be the
> > > > > correct offset to apply if the kernel was loaded at a misaligned address
> > > > > or below LOAD_PHYSICAL_ADDR, as in that case the decompression buffer
> > > > > has an additional offset from the original load address.
> > > > >
> > > > > This should never happen for a conformant bootloader, but we're being
> > > > > paranoid anyway, so just store the new GDT address in there instead of
> > > > > adding any offsets, which is simpler as well.
> > > > >
> > > > > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> > > > > Fixes: ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
> > > > > Fixes: 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of the buffer")
> > > >
> > > > Have you or anyone else observed this condition practice, or have a
> > > > suspicion that this could happen - or is this a mostly theoretical
> > > > concern?
> > > >
> > > > Thanks,
> > > >
> > > >       Ingo
> > >
> > > Right now it's a theoretical concern.
> > >
> > > I'm working on another patch, to tell the EFI firmware PE loader what
> > > the kernel's preferred address is, so that we can avoid having to
> > > relocate the kernel in the EFI stub in most cases (ie if the PE loader
> > > manages to load us at that address). With those changes, the required
> > > adjustment won't be init_size - _end any more, and while fixing it up
> > > there, I noticed that it could already be the case that the required
> > > adjustment is different.
> > >
> >
> > Do you mean setting the image address in the PE/COFF header to the
> > preferred address?
>
> Yep. I'm doing that and then making a few adjustments to the PE entry
> code and head_* so that it can decompress starting at image_base instead
> of startup_32.

Interesting. I am going to rip most of the EFI handover protocol stuff
out of OVMF, since it is mostly unnecessary, and having the PE/COFF
loader put the image in the correct place right away is a nice
complimentary improvement to that. (Note that the OVMF implementation
of the EFI handover protocol does not currently honor the preferred
address from the setup header anyway)