linux-kernel - Re: [PATCH] x86/compressed: remove unnecessary check

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Date:	Fri, 20 Feb 2015 10:38:28 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Alexander Kuleshov <kuleshovmail@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/compressed: remove unnecessary check

On Fri, Feb 20, 2015 at 9:55 AM, Alexander Kuleshov
<kuleshovmail@...il.com> wrote:
> region.start is entry->addr, but we already did check for case when
> entry->addr + entry->size < minimum, so no need to check that
> region.start is less than minimum.

Nope, that's not correct. Those are separate checks. For example, a
region 20..130 would be seen as:

start = 20
size = 110

With minimum = 50, the first check eliminates anything with size <= 30
(since the region would be entirely (20...50) below the minimum).

The second check (the one you're suggesting to remove) raises the
bottom of the region up to the minimum:

start = 50

And later on ("Reduce size by any delta from the original address."),
the size is also adjusted to notice any changes:

size = 80

Now the region is 50...130: the bottom is cut off to adapt to the
minimum location.

Is there some situation you found where this logic isn't working?

-Kees

>
> Signed-off-by: Alexander Kuleshov <kuleshovmail@...il.com>
> ---
>  arch/x86/boot/compressed/aslr.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
> index bb13763..13424a9 100644
> --- a/arch/x86/boot/compressed/aslr.c
> +++ b/arch/x86/boot/compressed/aslr.c
> @@ -249,11 +249,7 @@ static void process_e820_entry(struct e820entry *entry,
>
>         region.start = entry->addr;
>         region.size = entry->size;
> -
> -       /* Potentially raise address to minimum location. */
> -       if (region.start < minimum)
> -               region.start = minimum;
> -
> +
>         /* Potentially raise address to meet alignment requirements. */
>         region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
>
> --
> 2.3.0.80.g18d0fec
>



-- 
Kees Cook
Chrome OS Security
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