linux-kernel - Re: [PATCH] avoid entropy starvation due to stack protection

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Message-ID: <50CD00E2.5050104@chronox.de>
Date:	Sat, 15 Dec 2012 23:59:46 +0100
From:	Stephan Müller <smueller@...onox.de>
To:	Ondřej Bílka <neleai@...nam.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Ted Ts'o <tytso@....edu>, lkml <linux-kernel@...r.kernel.org>,
	Jeff Liu <jeff.liu@...cle.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] avoid entropy starvation due to stack protection

Am 15.12.2012 20:15, schrieb Ondřej Bílka:
> Why not use nonblocking pool and seed nonblocking pool only with half of
> collected entropy to get /dev/random in almost all practical scenarios
> nonblocking?

I would not recommend changing /dev/urandom. First, we would change the 
characteristic of a kernel interface a lot of user space cryptographic 
components rely on. According to Linus that is typically a no-go. 
Moreover, the question can be raised, where do we pick the number of 
50%, why not 30% or 70%, why (re)seeding it at all?

Also, let us assume we pick 50% and we leave the create_elf_tables 
function as is (i.e. it pulls from get_random_bytes), I fear that we do 
not win at all. Our discussed problem is the depletion of the entropy 
via nonblocking_pool due to every execve() syscall requires 128 bits of 
data from nonblocking_pool. Even if we seed nonblocking_pool more 
rarely, we still deplete the entropy of the input_pool and thus deplete 
the entropy we want for cryptographic purposes a particular user has.

Thus, my recommendation is to disconnect the system entropy requirements 
from the user entropy requirements as much as possible. I am aware that 
there are in-kernel cryptographic requirements that must seed itself via 
the good entropy. And those users shall be rather left untouched -- i.e. 
they should still call get_random_bytes.

But for users that do not require cryptographic strength, but a strength 
against guessing of a random number on the local system for a decent 
time (like the stack protection or ASLR), we can use a slightly less 
perfect DRNG which is seeded with good entropy and never thereafter.

Ciao
Stephan
>
> On Thu, Dec 13, 2012 at 08:44:36AM +0100, Stephan Mueller wrote:
>> On 13.12.2012 01:43:21, +0100, Andrew Morton
>> <akpm@...ux-foundation.org>  wrote:
>>
>> Hi Andrew,
>>> On Tue, 11 Dec 2012 13:33:04 +0100
>>> Stephan Mueller<smueller@...onox.de>  wrote:
>>>
>>>> Some time ago, I noticed the fact that for every newly
>>>> executed process, the function create_elf_tables requests 16 bytes of
>>>> randomness from get_random_bytes. This is easily visible when calling
>>>>
>>>> while [ 1 ]
>>>> do
>>>> 	cat /proc/sys/kernel/random/entropy_avail
>>>> 	sleep 1
>>>> done
>>> Please see
>>> http://ozlabs.org/~akpm/mmotm/broken-out/binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch
>>>
>>> That patch is about one week from a mainline merge, btw.
>> Initially I was also thinking about get_random_int. But stack protection
>> depends on non-predictable numbers to ensure it cannot be defeated. As
>> get_random_int depends on MD5 which is assumed to be broken now, I
>> discarded the idea of using get_random_int.
>>
>> Moreover, please consider that get_cycles is an architecture-specific
>> function that on some architectures only returns 0 (For all
>> architectures where this is implemented, you have no guarantee that it
>> increments as a high-resolution timer). So, the quality of
>> get_random_int is questionable IMHO for the use as a stack protector.
>>
>> Also note, that other in-kernel users of get_random_bytes may be
>> converted to using the proposed kernel pool to avoid more entropy drainage.
>>
>> Please note that the suggested approach of fully seeding a deterministic
>> RNG never followed by a re-seeding is used elsewhere (e.g. the OpenSSL
>> RNG). Therefore, I think the suggested approach is viable.
>>
>> Ciao
>> Stephan
>>
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