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

[Arvind Sankar]

On Thu, Feb 27, 2020 at 09:12:29AM +0100, Ingo Molnar wrote:
> 
> * Arvind Sankar <nivedita@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > 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.

Thanks.