linux-kernel - Re: [PATCH v4] binfmt_elf.c: use get_random

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Date:	Wed, 14 Nov 2012 23:32:25 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Jeff Liu <jeff.liu@...cle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andreas Dilger <aedilger@...il.com>,
	John Sobecki <john.sobecki@...cle.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	Alan Cox <alan@...ux.intel.com>,
	"arnd@...db.de" <arnd@...db.de>,
	James Morris <james.l.morris@...cle.com>,
	"Ted Ts'o" <tytso@....edu>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	jakub@...hat.com, drepper@...hat.com,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v4] binfmt_elf.c: use get_random_int() to fix entropy depleting

On Wed, Nov 14, 2012 at 8:12 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
> Hello,
>
> The problems have been fixed in this version as per Kees's comments for v3.
>
> Hi Kees,
> Would you please ACK this patch if you think it is ok except the strength
> of these various RNGs you are concerned?
>
>
> Changes:
> --------
> v4->v3:
> - s/random_stack_user()/get_atrandom_bytes()/
> - Move this function to ahead of its use to avoid the predeclaration.
>
> v3->v2:
> - Tweak code comments of random_stack_user().
> - Remove redundant bits mask and shift upon the random variable.
>
> v2->v1:
> - Fix random copy to check up buffer length that are not 4-byte multiples.
>
> v3 can be found at:
> http://www.spinics.net/lists/linux-fsdevel/msg59597.html
> v2 can be found at:
> http://www.spinics.net/lists/linux-fsdevel/msg59418.html
> v1 can be found at:
> http://www.spinics.net/lists/linux-fsdevel/msg59128.html
>
>
> Thanks,
> -Jeff
>
>
> Entropy is quickly depleted under normal operations like ls(1), cat(1),
> etc...  between 2.6.30 to current mainline, for instance:
>
> $ cat /proc/sys/kernel/random/entropy_avail
> 3428
> $ cat /proc/sys/kernel/random/entropy_avail
> 2911
> $cat /proc/sys/kernel/random/entropy_avail
> 2620
>
> We observed this problem has been occurring since 2.6.30 with
> fs/binfmt_elf.c: create_elf_tables()->get_random_bytes(), introduced by
> f06295b44c296c8f ("ELF: implement AT_RANDOM for glibc PRNG seeding").
>
> /*
>  * Generate 16 random bytes for userspace PRNG seeding.
>  */
> get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes));
>
> The patch introduces a wrapper around get_random_int() which has lower
> overhead than calling get_random_bytes() directly.
>
> With this patch applied:
> $ cat /proc/sys/kernel/random/entropy_avail
> 2731
> $ cat /proc/sys/kernel/random/entropy_avail
> 2802
> $ cat /proc/sys/kernel/random/entropy_avail
> 2878
>
> Analyzed by John Sobecki.
>
> Signed-off-by: Jie Liu <jeff.liu@...cle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Andreas Dilger <aedilger@...il.com>
> Cc: Alan Cox <alan@...ux.intel.com>
> Cc: Arnd Bergmann <arnn@...db.de>
> Cc: John Sobecki <john.sobecki@...cle.com>
> Cc: James Morris <james.l.morris@...cle.com>
> Cc: Jakub Jelinek <jakub@...hat.com>
> Cc: Ted Ts'o <tytso@....edu>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Ulrich Drepper <drepper@...hat.com>

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook
Chrome OS Security
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