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

From: Ard Biesheuvel <ardb@kernel.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Ingo Molnar <mingo@kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH v2 1/1] x86/boot/compressed: Fix reloading of GDTR post-relocation
Date: Thu, 27 Feb 2020 16:21:32 +0100	[thread overview]
Message-ID: <CAKv+Gu8BiW6P6Xv3EAPUEmbS3GQMJW=eRr-yygRbForaGDQyyw@mail.gmail.com> (raw)
In-Reply-To: <20200227151643.GA3498170@rani.riverdale.lan>

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?