linux-kernel - Re: [PATCH] ELF: implement AT

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Date:	Mon, 6 Oct 2008 17:31:19 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>, libc-alpha@...rceware.org
Subject: Re: [PATCH] ELF: implement AT_RANDOM for future glibc use

On Mon, Oct 06, 2008 at 04:58:27PM -0700, Roland McGrath wrote:
> What mmap randomization and stack randomization actually use is
> get_random_int(), not get_random_bytes().  This is one of those weaker
> flavors seeded occasionally from the real entropy pool.  (As is, it's not a
> good choice for getting 16 bytes of random at once, since it usually
> returns the same 4 bytes each time when called 4 times in quick succession.)
> 
> What glibc wants is some bits with a strength of randomness chosen by the
> kernel, and not to worry about the details.  I think the strength applied
> to mmap and stack randomization is good enough for AT_RANDOM.

Is this email a vote for or against doing:

+       k_rand_bytes[0] = get_random_int();
+       k_rand_bytes[1] = get_random_int();
+       k_rand_bytes[2] = get_random_int();
+       k_rand_bytes[3] = get_random_int();

It sounds like it's not very safe, but on the other hand, glibc doesn't
really care?

-- 
Kees Cook
Ubuntu Security Team
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