linux-kernel - Re: [PATCH 1/2] x86: introduce kaslr

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Date:	Tue, 28 Apr 2015 07:08:38 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	x86@...nel.org, Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org
Subject: Re: [PATCH 1/2] x86: introduce kaslr_offset()

On Mon, Apr 27, 2015 at 04:28:07PM +0200, Jiri Kosina wrote:
> Offset that has been chosen for kaslr during kernel decompression can be 
> easily computed as a difference between _text and __START_KERNEL. We are 
> already making use of this in dump_kernel_offset() notifier.
> 
> Introduce kaslr_offset() that makes this computation instead of 
> hard-coding it, so that other kernel code (such as live patching) can make 
> use of it.
> 
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> ---
>  arch/x86/include/asm/setup.h | 6 ++++++
>  arch/x86/kernel/setup.c      | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
> index f69e06b..785ac2f 100644
> --- a/arch/x86/include/asm/setup.h
> +++ b/arch/x86/include/asm/setup.h
> @@ -65,12 +65,18 @@ static inline void x86_ce4100_early_setup(void) { }
>   * This is set up by the setup-routine at boot-time
>   */
>  extern struct boot_params boot_params;
> +extern char _text[];
>  
>  static inline bool kaslr_enabled(void)
>  {
>  	return !!(boot_params.hdr.loadflags & KASLR_FLAG);
>  }
>  
> +static inline unsigned long kaslr_offset(void)
> +{
> +	return (unsigned long)&_text - __START_KERNEL;
> +}
> +
>  /*
>   * Do NOT EVER look at the BIOS memory size location.
>   * It does not work on many machines.
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index d74ac33..5056d3c 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -834,7 +834,7 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>  {
>  	if (kaslr_enabled()) {
>  		pr_emerg("Kernel Offset: 0x%lx from 0x%lx (relocation range: 0x%lx-0x%lx)\n",
> -			 (unsigned long)&_text - __START_KERNEL,
> +			 kaslr_offset(),
>  			 __START_KERNEL,
>  			 __START_KERNEL_map,
>  			 MODULES_VADDR-1);

It looks like kaslr_offset() can also be used by
arch_crash_save_vmcoreinfo() in machine_kexec_64.c.

-- 
Josh
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