linux-kernel - Re: [RESEND PATCH V3] binfmt_elf.c: use get_random

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Message-ID: <CAGXu5jKYD35B=TBXjACQiFS5gjNNjWYiY0MTq_ZH5qugw9ML4g@mail.gmail.com>
Date:	Wed, 14 Nov 2012 13:14:07 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jeff Liu <jeff.liu@...cle.com>,
	LKML <linux-kernel@...r.kernel.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: [RESEND PATCH V3] binfmt_elf.c: use get_random_int() to fix
 entropy depleting

On Wed, Nov 14, 2012 at 1:09 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Tue, 6 Nov 2012 23:13:54 -0800
> Kees Cook <keescook@...omium.org> wrote:
>
>> On Tue, Nov 6, 2012 at 11:02 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
>> > On 11/07/2012 02:21 PM, Kees Cook wrote:
>> >> I still want to hear at least from Ted about this changes -- we would
>> >> be potentially increasing the predictability of these bytes...
>> >
>> > We would not increasing that if this routine would be used for AT_RANDOM
>> > only(and if the array keeping aligned to 4 bytes).
>> > Otherwise, it would be, so let's waiting for further feedbacks.
>>
>> get_random_int() comes from a different pool than get_random_bytes(),
>> IIUC. I'd like to hear some convincing reasoning as to why this change
>> doesn't compromise predictability. :)
>
> But the original "ELF: implement AT_RANDOM for glibc PRNG seeding"
> compromised predictability.  That's the whole point of this patch.

It doesn't compromise predictability. It just used entropy. The idea
was that userspace had an actual need for it.

> What was so important about that patch that justified gobbling down so
> much of the system's entropy accumulation?

That does seem to be the core question here. If Ted doesn't think this
patch is a problem, then I don't object. Mostly I just had questions
about the strength of these various RNGs.

-Kees

-- 
Kees Cook
Chrome OS Security
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