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Date: Mon, 11 Apr 2016 09:00:37 +0100
From: James Morse <james.morse@....com>
To: Kees Cook <keescook@...omium.org>
CC: Linus Torvalds <torvalds@...ux-foundation.org>, 
 Ard Biesheuvel <ard.biesheuvel@...aro.org>,
 Matt Redfearn <matt.redfearn@...tec.com>, 
 Yves-Alexis Perez <corsac@...ian.org>,
 Emrah Demir <ed@...sec.com>, Jonathan Corbet <corbet@....net>, 
 x86@...nel.org, "Rafael J. Wysocki" <rjw@...ysocki.net>, 
 Len Brown <len.brown@...el.com>,
 Pavel Machek <pavel@....cz>, Borislav Petkov <bp@...e.de>, 
 Andy Lutomirski <luto@...nel.org>,
 linux-doc@...r.kernel.org, linux-pm@...r.kernel.org, 
 linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] Prefer kASLR over Hibernation

Hi Kees,

On 06/04/16 20:44, Kees Cook wrote:
> When building with both CONFIG_HIBERNATION and CONFIG_RANDOMIZE_BASE,
> one or the other must be chosen at boot-time. Until now, hibernation
> was selected when no choice was made on the command line.
> 
> To make the security benefits of kASLR more widely available to end
> users (since the use of hibernation is becoming more rare and kASLR,
> already available on x86, will be available on arm64 and MIPS soon),
> this changes the default to preferring kASLR over hibernation. Users
> wanting hibernation can turn off kASLR by adding "nokaslr" to the kernel
> command line.

While hibernate isn't yet merged for arm64, it does work with kASLR in v4.6-rc*,
it would be a shame to have to choose at boot time, (but that's my problem to
fix if/when its merged).


> diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
> index fca9254280ee..be5041354b1e 100644
> --- a/kernel/power/hibernate.c
> +++ b/kernel/power/hibernate.c
> @@ -1159,6 +1164,13 @@ static int __init kaslr_nohibernate_setup(char *str)
>  	return nohibernate_setup(str);
>  }
>  
> +static int __init nokaslr_hibernate_setup(char *str)
> +{
> +	noresume = 0;
> +	nohibernate = 0;
> +	return 1;
> +}
> +
>  static int __init page_poison_nohibernate_setup(char *str)
>  {
>  #ifdef CONFIG_PAGE_POISONING_ZERO
> @@ -1183,4 +1195,5 @@ __setup("resumewait", resumewait_setup);
>  __setup("resumedelay=", resumedelay_setup);
>  __setup("nohibernate", nohibernate_setup);
>  __setup("kaslr", kaslr_nohibernate_setup);
> +__setup("nokaslr", nokaslr_hibernate_setup);

So one or the other option has to be specified at boot?

The kASLR patches for arm64 in v4.6-rc1 enable kASLR at boot if you chose to
select it at compile time. I guess this patch is preparation for doing the same
on x86?

I didn't hit this kaslr_nohibernate_setup() call during testing because of the
on by default behaviour. Is it worth exposing this via Kconfig? Something like:
ARCH_RANDOMIZE_BASE_DEFAULT_ON ?


Thanks,

James

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