linux-kernel - Re: [PATCH v4] x86, kaslr: Access the correct kaslr

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Date:	Sun, 15 Mar 2015 20:18:45 +0800
From:	Minfei Huang <mhuang@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Borislav Petkov <bp@...e.de>, Baoquan He <bhe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v4] x86, kaslr: Access the correct kaslr_enabled variable

On 03/15/15 at 12:49am, Yinghai Lu wrote:
> While trying to optimize setup_data handling code, found commit f47233c2d34f
> ("x86/mm/ASLR: Propagate base load address calculation"), uses a physical
> address as the value.
> 
> It leads to wrong kaslr status. That kaslr status is passed for controlling
> module load offset. So when kernel has kasl support compiled in, and user
> use nokaslr to disable kaslr, wrong value cause unwanted offset for module
> loading.
> 

Hi, Yinghai!

It confuses me with the virtual address in function parse_kaslr_setup.
When we are in here(parse_kaslr_setup), we already use the virtual
address, instead of physical address. Is it all right?

In the other words, using physical address in parse_kaslr_setup is
always a mistake, whatever the kaslr is on or off.

Thanks
Minfei

> We should keep kaslr status consistent between aslr randrom kernel base,
> and late module random offset. That is old behavior before that commit.
> 
> The setup_data linked list and thus the element which contains
> kaslr_enabled is chained together using physical addresses. At the time
> when we access it in the kernel, we're already running with paging enabled
> and therefore must access it through its virtual address.
> 
> This patch changes the code to use early_memmap() and access the value,
> as it is still in kernel early stage, we don't have kernel mapping setup
> yet.
> 
> Fixes: f47233c2d34f ("x86/mm/ASLR: Propagate base load address calculation")
> Cc: Matt Fleming <matt.fleming@...el.com>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Jiri Kosina <jkosina@...e.cz>
> Acked-by: Jiri Kosina <jkosina@...e.cz>
> Signed-off-by: Yinghai Lu <yinghai@...nel.org>
> 
> ---
> -v3: add checking return from early_memmap according to Boris.
> -v4: add description about setup_data accessing from Boris to change log.
> 
> ---
>  arch/x86/kernel/setup.c |   13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/arch/x86/kernel/setup.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/setup.c
> +++ linux-2.6/arch/x86/kernel/setup.c
> @@ -429,7 +429,18 @@ static void __init reserve_initrd(void)
>  
>  static void __init parse_kaslr_setup(u64 pa_data, u32 data_len)
>  {
> -	kaslr_enabled = (bool)(pa_data + sizeof(struct setup_data));
> +	/* kaslr_setup_data is defined in aslr.c */
> +	unsigned char *data;
> +	unsigned long offset = sizeof(struct setup_data);
> +
> +	data = early_memremap(pa_data, offset + 1);
> +	if (!data) {
> +		kaslr_enabled = true;
> +		return;
> +	}
> +
> +	kaslr_enabled = *(data + offset);
> +	early_memunmap(data, offset + 1);
>  }
>  
>  static void __init parse_setup_data(void)
> --
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