[RFC PATCH 4/9] x86/purgatory: Avoid absolute reference to GDT

[Brian Gerst]

On Wed, Apr 24, 2024 at 1:53 PM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> Hi Brian,
>
> Thanks for taking a look.
>
> On Wed, 24 Apr 2024 at 19:39, Brian Gerst <brgerst at gmail.com> wrote:
> >
> > On Wed, Apr 24, 2024 at 12:06 PM Ard Biesheuvel <ardb+git at google.com> wrote:
> > >
> > > From: Ard Biesheuvel <ardb at kernel.org>
> > >
> > > The purgatory is almost entirely position independent, without any need
> > > for any relocation processing at load time except for the reference to
> > > the GDT in the entry code. Generate this reference at runtime instead,
> > > to remove the last R_X86_64_64 relocation from this code.
> > >
> > > While the GDT itself needs to be preserved in memory as long as it is
> > > live, the GDT descriptor that is used to program the GDT can be
> > > discarded so it can be allocated on the stack.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > > ---
> > >  arch/x86/purgatory/entry64.S | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/purgatory/entry64.S b/arch/x86/purgatory/entry64.S
> > > index 9913877b0dbe..888661d9db9c 100644
> > > --- a/arch/x86/purgatory/entry64.S
> > > +++ b/arch/x86/purgatory/entry64.S
> > > @@ -16,7 +16,11 @@
> > >
> > >  SYM_CODE_START(entry64)
> > >         /* Setup a gdt that should be preserved */
> > > -       lgdt gdt(%rip)
> > > +       leaq    gdt(%rip), %rax
> > > +       pushq   %rax
> > > +       pushw   $gdt_end - gdt - 1
> > > +       lgdt    (%rsp)
> > > +       addq    $10, %rsp
> >
> > This misaligns the stack, pushing 16 bytes on the stack but only
> > removing 10 (decimal).
> >
>
> pushw subtracts 2 from RSP and stores a word. So the total size stored
> is 10 decimal not 16.

I didn't notice the pushw, since I didn't think a 16-bit push was even
possible in 64-bit mode. Unexpected, but clever.


Brian Gerst