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Date: Mon, 5 Aug 2019 19:28:54 +0200
From: Borislav Petkov <bp@...en8.de>
To: Thomas Garnier <thgarnie@...omium.org>
Cc: kernel-hardening@...ts.openwall.com, kristen@...ux.intel.com,
	keescook@...omium.org, Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 04/11] x86/entry/64: Adapt assembly for PIE support

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?

-- 
Regards/Gruss,
    Boris.

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