Re: [PATCH v7 09/12] x86/boot/64: Adapt assembly for PIE support

[Kees Cook <keescook@chromium.org>]

On Mon, May 20, 2019 at 04:19:34PM -0700, Thomas Garnier wrote:
> From: Thomas Garnier <thgarnie@google.com>
> 
> Change the assembly code to use only relative references of symbols for the
> kernel to be PIE compatible.
> 
> Early at boot, the kernel is mapped at a temporary address while preparing
> the page table. To know the changes needed for the page table with KASLR,
> the boot code calculate the difference between the expected address of the
> kernel and the one chosen by KASLR. It does not work with PIE because all
> symbols in code are relatives. Instead of getting the future relocated
> virtual address, you will get the current temporary mapping.
> Instructions were changed to have absolute 64-bit references.
> 
> Position Independent Executable (PIE) support will allow to extend the
> KASLR randomization range below 0xffffffff80000000.
> 
> Signed-off-by: Thomas Garnier <thgarnie@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  arch/x86/kernel/head_64.S | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index bcd206c8ac90..64a4f0a22b20 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -90,8 +90,10 @@ startup_64:
>  	popq	%rsi
>  
>  	/* Form the CR3 value being sure to include the CR3 modifier */
> -	addq	$(early_top_pgt - __START_KERNEL_map), %rax
> +	movabs  $(early_top_pgt - __START_KERNEL_map), %rcx
> +	addq    %rcx, %rax
>  	jmp 1f
> +
>  ENTRY(secondary_startup_64)
>  	UNWIND_HINT_EMPTY
>  	/*
> @@ -120,7 +122,8 @@ ENTRY(secondary_startup_64)
>  	popq	%rsi
>  
>  	/* Form the CR3 value being sure to include the CR3 modifier */
> -	addq	$(init_top_pgt - __START_KERNEL_map), %rax
> +	movabs	$(init_top_pgt - __START_KERNEL_map), %rcx
> +	addq    %rcx, %rax
>  1:
>  
>  	/* Enable PAE mode, PGE and LA57 */
> @@ -138,7 +141,7 @@ ENTRY(secondary_startup_64)
>  	movq	%rax, %cr3
>  
>  	/* Ensure I am executing from virtual addresses */
> -	movq	$1f, %rax
> +	movabs  $1f, %rax
>  	ANNOTATE_RETPOLINE_SAFE
>  	jmp	*%rax
>  1:
> @@ -235,11 +238,12 @@ ENTRY(secondary_startup_64)
>  	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
>  	 *		address given in m16:64.
>  	 */
> -	pushq	$.Lafter_lret	# put return address on stack for unwinder
> +	movabs  $.Lafter_lret, %rax
> +	pushq	%rax		# put return address on stack for unwinder
>  	xorl	%ebp, %ebp	# clear frame pointer
> -	movq	initial_code(%rip), %rax
> +	leaq	initial_code(%rip), %rax
>  	pushq	$__KERNEL_CS	# set correct cs
> -	pushq	%rax		# target address in negative space
> +	pushq	(%rax)		# target address in negative space
>  	lretq
>  .Lafter_lret:
>  END(secondary_startup_64)
> -- 
> 2.21.0.1020.gf2820cf01a-goog
> 

-- 
Kees Cook