From: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
To: Florian Weimer <fweimer@redhat.com>, Rich Felker <dalias@libc.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
libc-alpha@sourceware.org, Yann Droneaud <ydroneaud@opteya.com>,
jann@thejh.net, Michael@phoronix.com,
Paul Eggert <eggert@cs.ucla.edu>
Subject: Re: arc4random - are you sure we want these?
Date: Mon, 25 Jul 2022 13:49:47 -0300 [thread overview]
Message-ID: <cad79806-4757-2e05-2f69-6a6430b37e47@linaro.org> (raw)
In-Reply-To: <878rohp2ll.fsf@oldenburg.str.redhat.com>
On 25/07/22 13:40, Florian Weimer wrote:
> * Rich Felker:
>
>> On Sat, Jul 23, 2022 at 02:39:29PM -0300, Adhemerval Zanella Netto via Libc-alpha wrote:
>>> On 23/07/22 13:25, Jason A. Donenfeld wrote:
>>>> Firstly, for what use cases does this actually help? As of recent
>>>> changes to the Linux kernels -- now backported all the way to 4.9! --
>>>> getrandom() and /dev/urandom are extremely fast and operate over per-cpu
>>>> states locklessly. Sure you avoid a syscall by doing that in userspace,
>>>> but does it really matter? Who exactly benefits from this?
>>>
>>> Mainly performance, since glibc both export getrandom and getentropy.
>>> There were some discussion on maillist and we also decided to explicit
>>> state this is not a CSRNG on our documentation.
>>
>> This is an extreme documentation/specification bug that *hurts*
>> portability and security. The core contract of the historical
>> arc4random function is that it *is* a CSPRNG. Having a function by
>> that name that's allowed not to be one means now all software using it
>> has to add detection for the broken glibc variant.
>>
>> If the glibc implementation has flaws that actually make it not a
>> CSPRNG, this absolutely needs to be fixed. Not doing so is
>> irresponsible and will set everyone back a long ways.
>
> The core issue is that on some kernels/architectures, reading from
> /dev/urandom can degrade to GRND_INSECURE (approximately), and while the
> result is likely still unpredictable, not everyone would label that as a
> CSPRNG.
>
> If we document arc4random as a CSPRNG, this means that we would have to
> ditch the fallback code and abort the process if the getrandom system
> call is not available: when reading from /dev/urandom as a fallback, we
> have no way of knowing if we are in any of the impacted execution
> environments. Based on your other comments, it seems that you are
> interested in such fallbacks, too, but I don't think you can actually
> have both (CSPRNG + fallback).
It seems the best course of actions, specially form the fact that document
arc4random as a CSPRNG seems to a deal-breaker.
>
> And then there is the certification issue. We really want applications
> that already use OpenSSL for other cryptography to use RAND_bytes
> instead of arc4random. Likewise for GNUTLS and gnutls_rnd. What should
> authors of those cryptographic libraries? That's less clear, and really
> depends on the constraints they operate in (e.g., they may target only a
> subset of architectures and kernel versions).
>
> Thanks,
> Florian
>
next prev parent reply other threads:[~2022-07-25 16:49 UTC|newest]
Thread overview: 81+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <YtwgTySJyky0OcgG@zx2c4.com>
2022-07-23 16:25 ` Jason A. Donenfeld
2022-07-23 17:18 ` Paul Eggert
2022-07-24 23:55 ` Jason A. Donenfeld
2022-07-25 20:31 ` Paul Eggert
2022-07-23 17:39 ` Adhemerval Zanella Netto
2022-07-23 22:54 ` Jason A. Donenfeld
2022-07-25 15:33 ` Rich Felker
2022-07-25 15:59 ` Adhemerval Zanella Netto
2022-07-25 17:41 ` Rich Felker
2022-07-25 16:18 ` Sandy Harris
2022-07-25 16:40 ` Florian Weimer
2022-07-25 16:49 ` Adhemerval Zanella Netto [this message]
2022-07-25 16:51 ` Jason A. Donenfeld
2022-07-25 17:44 ` Rich Felker
2022-07-25 18:33 ` Cristian Rodríguez
2022-07-25 18:49 ` Rich Felker
2022-07-27 1:54 ` Theodore Ts'o
2022-07-27 2:16 ` Rich Felker
2022-07-27 2:45 ` Theodore Ts'o
2022-07-27 11:34 ` Adhemerval Zanella Netto
2022-07-27 12:32 ` Theodore Ts'o
2022-07-27 12:49 ` Florian Weimer
2022-07-27 20:15 ` Theodore Ts'o
2022-07-27 21:59 ` Rich Felker
2022-07-28 0:30 ` Theodore Ts'o
2022-07-28 0:39 ` Cristian Rodríguez
2022-07-27 15:39 ` Rich Felker
2022-07-23 19:04 ` Cristian Rodríguez
2022-07-23 22:59 ` Jason A. Donenfeld
2022-07-24 16:23 ` Cristian Rodríguez
2022-07-24 21:57 ` Jason A. Donenfeld
2022-07-25 10:14 ` Florian Weimer
2022-07-25 10:11 ` Florian Weimer
2022-07-25 11:04 ` Jason A. Donenfeld
2022-07-25 12:39 ` Florian Weimer
2022-07-25 13:43 ` Jason A. Donenfeld
2022-07-25 13:58 ` Cristian Rodríguez
2022-07-25 16:06 ` Rich Felker
2022-07-25 16:43 ` Florian Weimer
2022-07-26 14:27 ` Overwrittting AT_RANDOM after use (was Re: arc4random - are you sure we want these?) Yann Droneaud
2022-07-26 14:35 ` arc4random - are you sure we want these? Yann Droneaud
2022-07-25 13:25 ` Jeffrey Walton
2022-07-25 13:48 ` Jason A. Donenfeld
2022-07-25 14:56 ` Rich Felker
2022-07-25 22:57 ` [PATCH] arc4random: simplify design for better safety Jason A. Donenfeld
2022-07-25 23:11 ` Jason A. Donenfeld
2022-07-25 23:28 ` [PATCH v2] " Jason A. Donenfeld
2022-07-25 23:59 ` Eric Biggers
2022-07-26 10:26 ` Jason A. Donenfeld
2022-07-26 1:10 ` Mark Harris
2022-07-26 10:41 ` Jason A. Donenfeld
2022-07-26 11:06 ` Florian Weimer
2022-07-26 16:51 ` Mark Harris
2022-07-26 18:42 ` Jason A. Donenfeld
2022-07-26 19:18 ` Adhemerval Zanella Netto
2022-07-26 19:24 ` Jason A. Donenfeld
2022-07-26 9:55 ` Florian Weimer
2022-07-26 11:04 ` Jason A. Donenfeld
2022-07-26 11:07 ` [PATCH v3] " Jason A. Donenfeld
2022-07-26 11:11 ` Jason A. Donenfeld
2022-07-26 11:12 ` [PATCH v2] " Florian Weimer
2022-07-26 11:20 ` Jason A. Donenfeld
2022-07-26 11:35 ` Adhemerval Zanella Netto
2022-07-26 11:33 ` Adhemerval Zanella Netto
2022-07-26 11:54 ` Jason A. Donenfeld
2022-07-26 12:08 ` Jason A. Donenfeld
2022-07-26 12:20 ` Jason A. Donenfeld
2022-07-26 12:34 ` Adhemerval Zanella Netto
2022-07-26 12:47 ` Jason A. Donenfeld
2022-07-26 13:11 ` Adhemerval Zanella Netto
2022-07-26 13:30 ` [PATCH v4] " Jason A. Donenfeld
2022-07-26 15:21 ` Yann Droneaud
2022-07-26 16:20 ` Adhemerval Zanella Netto
2022-07-26 18:36 ` Jason A. Donenfeld
2022-07-26 19:08 ` [PATCH v5] " Jason A. Donenfeld
2022-07-26 19:58 ` [PATCH v6] " Jason A. Donenfeld
2022-07-26 20:17 ` Adhemerval Zanella Netto
2022-07-26 20:56 ` Adhemerval Zanella Netto
2022-07-28 10:29 ` Szabolcs Nagy
2022-07-28 10:36 ` Szabolcs Nagy
2022-07-28 11:01 ` Adhemerval Zanella
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