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Message-ID: <CAMj1kXE4f7UERE-x4CghW3=wQrvT1b2ODRSX3z6QAoy_wSJjiw@mail.gmail.com>
Date: Wed, 24 Apr 2024 19:53:06 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Brian Gerst <brgerst@...il.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, x86@...nel.org,
Arnd Bergmann <arnd@...db.de>, Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org,
Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>,
Kees Cook <keescook@...omium.org>, Bill Wendling <morbo@...gle.com>,
Justin Stitt <justinstitt@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [RFC PATCH 4/9] x86/purgatory: Avoid absolute reference to GDT
Hi Brian,
Thanks for taking a look.
On Wed, 24 Apr 2024 at 19:39, Brian Gerst <brgerst@...il.com> wrote:
>
> On Wed, Apr 24, 2024 at 12:06 PM Ard Biesheuvel <ardb+git@...gle.com> wrote:
> >
> > From: Ard Biesheuvel <ardb@...nel.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@...nel.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.
> >
> > /* load the data segments */
> > movl $0x18, %eax /* data segment */
> > @@ -83,8 +87,8 @@ SYM_DATA_START_LOCAL(gdt)
> > * 0x08 unused
> > * so use them as gdt ptr
>
> obsolete comment
>
> > */
> > - .word gdt_end - gdt - 1
> > - .quad gdt
> > + .word 0
> > + .quad 0
> > .word 0, 0, 0
>
> This can be condensed down to:
> .quad 0, 0
>
This code and the comment are removed in the next patch.
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