linux-kernel - Re: use-after-free in sctp_do

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Message-ID: <f7ttwnyus8l.fsf@aconole.bos.csb>
Date:	Fri, 04 Dec 2015 10:51:38 -0500
From:	Aaron Conole <aconole@...hat.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Joe Perches <joe@...ches.com>, Jason Baron <jbaron@...mai.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	linux-sctp@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in sctp_do_sm

Vlad Yasevich <vyasevich@...il.com> writes:
> On 12/04/2015 07:55 AM, Marcelo Ricardo Leitner wrote:
>> On Fri, Dec 04, 2015 at 11:40:02AM +0100, Dmitry Vyukov wrote:
>>> On Thu, Dec 3, 2015 at 9:51 PM, Joe Perches <joe@...ches.com> wrote:
>>>> (adding lkml as this is likely better discussed there)
>>>>
>>>> On Thu, 2015-12-03 at 15:42 -0500, Jason Baron wrote:
>>>>> On 12/03/2015 03:24 PM, Joe Perches wrote:
>>>>>> On Thu, 2015-12-03 at 15:10 -0500, Jason Baron wrote:
>>>>>>> On 12/03/2015 03:03 PM, Joe Perches wrote:
>>>>>>>> On Thu, 2015-12-03 at 14:32 -0500, Jason Baron wrote:
>>>>>>>>> On 12/03/2015 01:52 PM, Aaron Conole wrote:
>>>>>>>>>> I think that as a minimum, the following patch should be evaluted,
>>>>>>>>>> but am unsure to whom I should submit it (after I test):
>>>>>>>> []
>>>>>>>>> Agreed - the intention here is certainly to have no side effects. It
>>>>>>>>> looks like 'no_printk()' is used in quite a few other places that would
>>>>>>>>> benefit from this change. So we probably want a generic
>>>>>>>>> 'really_no_printk()' macro.
>>>>>>>>
>>>>>>>> https://lkml.org/lkml/2012/6/17/231
>>>>>>>
>>>>>>> I don't see this in the tree.
>>>>>>
>>>>>> It never got applied.
>>>>>>
>>>>>>> Also maybe we should just convert
>>>>>>> no_printk() to do what your 'eliminated_printk()'.
>>>>>>
>>>>>> Some of them at least.
>>>>>>
>>>>>>> So we can convert all users with this change?
>>>>>>
>>>>>> I don't think so, I think there are some
>>>>>> function evaluation/side effects that are
>>>>>> required.  I believe some do hardware I/O.
>>>>>>
>>>>>> It'd be good to at least isolate them.
>>>>>>
>>>>>> I'm not sure how to find them via some
>>>>>> automated tool/mechanism though.
>>>>>>
>>>>>> I asked Julia Lawall about it once in this
>>>>>> thread:  https://lkml.org/lkml/2014/12/3/696
>>>>>>
>>>>>
>>>>> Seems rather fragile to have side effects that we rely
>>>>> upon hidden in a printk().
>>>>
>>>> Yup.
>>>>
>>>>> Just convert them and see what breaks :)
>>>>
>>>> I appreciate your optimism.  It's very 1995.
>>>> Try it and see what happens.
>>>
>>>
>>> Whatever is the resolution for pr_debug, we still need to fix this
>>> particular use-after-free. It affects stability of debug builds, gives
>>> invalid debug output, prevents us from finding more bugs in SCTP. And
>>> maybe somebody uses CONFIG_DYNAMIC_DEBUG in production.
>> 
>> Agreed. I'm already working on a fix for this particular use-after-free.
>> 
>> Another interesting thing about this is that sctp_do_sm() is called for
>> nearly every movement that happens on a sctp socket. Said that, that
>> always-running IDR search hidden on that debug statement do have some
>> nasty performance impact, specially because it's serialized on a
>> spinlock.
>
> YUCK!  I didn't really pay much attention to those debug macros before, but
> debug_post_sfx() is truly awful.
>
> This wasn't such a bad thing where these macros depended on CONFIG_SCTP_DEBUG,
> but now that they are always built, we need fix them.

I've proposed a patch to linux-kernel to fix them, but I don't think
it's really as bad as folks imagine. Ubuntu, RHEL, and Fedora all use
DYNAMIC_DEBUG configuration option, which means that the code is getting
emitted anyway (correctly, I'll add) and is shunted out by a dynamic
debug flag. So for the average user, it's not even really a blip.

That does mean there's a cool side-effect of the entire print-macro setup
which implies we execute less code when running with DYNAMIC_DEBUG=y in
the "normal" case. "Turn on the dynamic debugging config and watch
everything get better" isn't the worst mantra, is it? :)

> -vlad
>
>
>
>> This wouldn't be happening if it was fully ellided and would
>> be ok if that pr_debug() was really being printed, but not as it is.
>> Kudos to this report that I could notice this. I'm trying to fix this on
>> SCTP-side as well.
>> 
>>   Marcelo
>> 
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