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

[Lucas De Marchi]

On Sun, Mar 18, 2012 at 4:27 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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... :-/


Lucas De Marchi
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