linux-kernel - Re: [PATCH 2/4] kaslr: check if the random addr is available

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Message-ID: <CAGXu5jJXvR9c3OvkX-gBMjhJEwXh5-wBdT+jjMwBFXDs8_-40Q@mail.gmail.com>
Date:	Fri, 5 Sep 2014 10:16:15 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Baoquan He <bhe@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Deutschmann <whissi@...ssi.de>,
	Dave Young <dyoung@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vivek Goyal <vgoyal@...hat.com>,
	WANG Chao <chaowang@...hat.com>
Subject: Re: [PATCH 2/4] kaslr: check if the random addr is available

On Fri, Sep 5, 2014 at 7:08 AM, Baoquan He <bhe@...hat.com> wrote:
> Currently kaslr enabling can extend the kernel virtual address space
> to 1G, next is for modules. So if kernel is loaded to above 1G, system
> running will be exceptional, This happened when kexec/kdump load kernel.
>
> So add a check to see if the decompression output region is contained
> in 1G.
>
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  arch/x86/boot/compressed/misc.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 7780a5b..d2a0eaa 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -250,6 +250,11 @@ static void handle_relocations(void *output, unsigned long output_len)
>         }
>  #endif
>
> +       if (max_addr > CONFIG_RANDOMIZE_BASE_MAX_OFFSET) {
> +                debug_putstr("Random addr is not allowed. No relocation needed... \n");
> +                return;
> +        }
> +

It's not clear to me what this is fixing. In aslr.c,
process_e820_entry() should already make it impossible to select
max_addr > CONFIG_RANDOMIZE_BASE_MAX_OFFSET. If you're trying to
detect a non-kaslr boot, I think this is better handled in 1/4 where I
suggest examining the "output" location before/after
choose_kernel_location.

-Kees

>         /*
>          * Calculate the delta between where vmlinux was linked to load
>          * and where it was actually loaded.
> --
> 1.8.5.3
>



-- 
Kees Cook
Chrome OS Security
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