linux-kernel - Re: [3.3-rc7] sys_poll use after free (hibernate)

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Message-ID: <CAMOw1v4gszzV7F3z1m+RWEe9UDgR2Jrp9wX_w9z_1UkXT=FL5Q@mail.gmail.com>
Date:	Thu, 22 Mar 2012 19:12:26 -0300
From:	Lucas De Marchi <lucas.demarchi@...fusion.mobi>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [3.3-rc7] sys_poll use after free (hibernate)

On Thu, Mar 22, 2012 at 6:31 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Lucas De Marchi <lucas.demarchi@...fusion.mobi> writes:
>
>> On Sun, Mar 18, 2012 at 4:27 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>>> On Sun, Mar 18, 2012 at 12:02:04PM -0700, Linus Torvalds wrote:
>>>> and that load is from
>>>>
>>>>     poll_wait(filp, &table->poll->wait, wait);
>>>>
>>>> where the testing of %rsi and %rcx are the "if (p && wait_address)"
>>>> check in poll_wait(), and %rsi is "table->poll" if I read it all
>>>> correctly.
>>>>
>>>> And the 6b6b6b6b6b6b6b6b pattern is obviously POISON_FREE, so
>>>> apparently 'table' has already been freed.
>>>>
>>>> I suspect the whole sysctl 'poll' code is seriously broken, since it
>>>> seems to depend on those ctl_table pointers being stable over the
>>>> whole open/close sequence, but if somebody unregisters the sysctl,
>>>> it's all gone. The ctl_table doesn't have any refcounting etc, and I
>>>> suspect that your hibernate sequence ends up unregistering some sysctl
>>>> (perhaps as part of a module unload?)
>>
>> How could that happen if the only files that support poll  right now
>> on sysctl are kernel/hostname and kernel/domainname?
>>
>>>
>>> Ewww...  The way it was supposed to work (prio to ->poll() madness) was
>>> that actual IO gets wrapped into grab_header()/sysctl_head_finish()
>>> pair.  proc_sys_poll() doesn't do it, so yes, that post-mortem is
>>> very likely to be correct.
>>
>> Yes, it  seems like I forgot to call grab_header() there, sorry for
>> that. I'll prepare a patch and send you later today. I just wonder
>> what is happening to reach that code... :-/
>
> It looks like it was a combination of the fuzzer doing silly things
> and a removed ctl_table entry being poisoned and having .poll set
> to 6b6b6b6b6b6b6b6b so the guard against calling poll when it is
> nonsense did not trigger.  So your patch should be sufficient
> for now.

What I understood afterwards was:

1. fuzzer calling poll() on files that did support poll
2. modules that created that sysctl entries were removed
3. 'table' was entirely removed (not ->poll).

>
> Long term we still need a version of poll that is safe to use
> with modules.

I think the way it's now (with my patch taken by Andrew) is safe for
having poll() with modules.


Lucas De Marchi
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