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Date: Mon, 5 Aug 2019 10:50:30 -0700
From: Thomas Garnier <thgarnie@...omium.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Kristen Carlson Accardi <kristen@...ux.intel.com>, Kees Cook <keescook@...omium.org>, 
	Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	"H. Peter Anvin" <hpa@...or.com>, "the arch/x86 maintainers" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 04/11] x86/entry/64: Adapt assembly for PIE support

On Mon, Aug 5, 2019 at 10:28 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Jul 30, 2019 at 12:12:48PM -0700, Thomas Garnier wrote:
> > Change the assembly code to use only relative references of symbols for the
> > kernel to be PIE compatible.
> >
> > Position Independent Executable (PIE) support will allow to extend the
> > KASLR randomization range below 0xffffffff80000000.
> >
> > Signed-off-by: Thomas Garnier <thgarnie@...omium.org>
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > ---
> >  arch/x86/entry/entry_64.S | 16 +++++++++++-----
> >  1 file changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index 3f5a978a02a7..4b588a902009 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -1317,7 +1317,8 @@ ENTRY(error_entry)
> >       movl    %ecx, %eax                      /* zero extend */
> >       cmpq    %rax, RIP+8(%rsp)
> >       je      .Lbstep_iret
> > -     cmpq    $.Lgs_change, RIP+8(%rsp)
> > +     leaq    .Lgs_change(%rip), %rcx
> > +     cmpq    %rcx, RIP+8(%rsp)
> >       jne     .Lerror_entry_done
> >
> >       /*
> > @@ -1514,10 +1515,10 @@ ENTRY(nmi)
> >        * resume the outer NMI.
> >        */
> >
> > -     movq    $repeat_nmi, %rdx
> > +     leaq    repeat_nmi(%rip), %rdx
> >       cmpq    8(%rsp), %rdx
> >       ja      1f
> > -     movq    $end_repeat_nmi, %rdx
> > +     leaq    end_repeat_nmi(%rip), %rdx
> >       cmpq    8(%rsp), %rdx
> >       ja      nested_nmi_out
> >  1:
> > @@ -1571,7 +1572,8 @@ nested_nmi:
> >       pushq   %rdx
> >       pushfq
> >       pushq   $__KERNEL_CS
> > -     pushq   $repeat_nmi
> > +     leaq    repeat_nmi(%rip), %rdx
> > +     pushq   %rdx
> >
> >       /* Put stack back */
> >       addq    $(6*8), %rsp
> > @@ -1610,7 +1612,11 @@ first_nmi:
> >       addq    $8, (%rsp)      /* Fix up RSP */
> >       pushfq                  /* RFLAGS */
> >       pushq   $__KERNEL_CS    /* CS */
> > -     pushq   $1f             /* RIP */
> > +     pushq   $0              /* Future return address */
> > +     pushq   %rax            /* Save RAX */
> > +     leaq    1f(%rip), %rax  /* RIP */
> > +     movq    %rax, 8(%rsp)   /* Put 1f on return address */
> > +     popq    %rax            /* Restore RAX */
>
> Can't you just use a callee-clobbered reg here instead of preserving
> %rax?

I saw that %rdx was used for temporary usage and restored before the
end so I assumed that it was not an option.

>
> --
> Regards/Gruss,
>     Boris.
>
> Good mailing practices for 400: avoid top-posting and trim the reply.

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