Re: [PATCH] Kprobe: Fixed a leak when a retprobe entry function returnsnon-zero

[Masami Hiramatsu]

Hi Juhani,

I'd like to have your signed-off-by for this patch.
Could you update it against the latest kernel and
add your signed-off-by?

Thank you,

(2011/04/16 17:23), Masami Hiramatsu wrote:
> (2011/04/14 23:21), Hannu Heikkinen wrote:
>> Hi,
>>
>> any progress on this Juhani's fix?
> 
> I think this basically good for me. Could you revise it as
> usual style of commit patch on the latest -tip or linus tree?
> 
> Thank you,
> 
>>
>> br,
>> Hannu
>>
>> 2010/12/24 Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
>>
>>> (2010/12/23 17:12), Juhani Mäkelä wrote:
>>>> Dear Sirs,
>>>>
>>>> Here is a little fix I found necessary to implement in order to perform
>>>> some probing:
>>>>
>>>> ----
>>>> A kretprobe can install an optional entry probe that is called when
>>>> the probed function is entered. If the callback returns non zero,
>>>> the return probe will not be called and the instance is forgotten.
>>>> However, the allocated instance of struct kretprobe_instance was
>>>> not returned in the free_instances list. Fixed this by returning
>>>> the unused instance to the free list if it was not needed.
>>>
>>> Right. That must be a memory-leak path!
>>> Thank you very much for pointing it out :-)
>>>
>>> BTW, it seems that other paths have initialized hlist by
>>> INIT_HLIST_NODE(). However, actually there is no need to
>>> init a node for adding on a hlist again. Just from the viewpoint
>>> of maintaining the code, coding style should have coherence and
>>> it's better to init by INIT_HLIST_NODE().
>>>
>>> (In this function, a node deleted from free_instances hlist is
>>>  always added on a hlist again. So maybe it's enough to use
>>>  hlist_del_init() instead of hlist_del() at least here.)
>>>
>>> Anyway, this should fix the problem, and should be an urgent fix.
>>> Thanks!
>>>
>>>
>>>> ---
>>>>  kernel/kprobes.c |   10 +++++++++-
>>>>  1 files changed, 9 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
>>>> index 5240d75..69d0ca9 100644
>>>> --- a/kernel/kprobes.c
>>>> +++ b/kernel/kprobes.c
>>>> @@ -971,8 +971,16 @@ static int __kprobes pre_handler_kretprobe(struct
>>>> kprobe *p,
>>>>               ri->rp = rp;
>>>>               ri->task = current;
>>>>
>>>> -             if (rp->entry_handler && rp->entry_handler(ri, regs))
>>>> +             if (rp->entry_handler && rp->entry_handler(ri, regs)) {
>>>> +                     /*
>>>> +                      * Restore the instance to the free list
>>>> +                      * if it is not needed any more.
>>>> +                      */
>>>> +                     spin_lock_irqsave(&rp->lock, flags);
>>>> +                     hlist_add_head(&ri->hlist, &rp->free_instances);
>>>> +                     spin_unlock_irqrestore(&rp->lock, flags);
>>>>                       return 0;
>>>> +             }
>>>>
>>>>               arch_prepare_kretprobe(ri, regs);
>>>> ----
>>>>
>>>> I'm not at all positive that this is the right fix, but at least in our
>>>> environment it seems to help. Here is some background info:
>>>>
>>>> We have implemented a kernel module that implements capability check
>>>> tracing by adding a kretprobe in the "capable" function. Every time a
>>>> capability check is made, it gathers some data about the process being
>>>> checked, the capability number and the result of the check. If the check
>>>> comes with current->mm == NULL, it's disregarded by the tracer to avoid
>>>> unnecessary overhead, and the entry_handler returns value 1.
>>>>
>>>> Normally this works fine, but this week we noticed that if the module is
>>>> compiled in and activated in an early phase in the boot it doesn't work
>>>> at all. It appeared that our entry_handler was called as many times as
>>>> was set in the maxactive field of the registered probe, and every time
>>>> it returned 1 because current->mm was NULL in all of the calls. Then no
>>>> more callbacks were made. When the probe was de-registered, the nmissed
>>>> counter had a large value (>6000), and after registering it again the
>>>> probing started to work.
>>>>
>>>> This made me suspect a resource leak, and as far as I can see there
>>>> indeed is one in kprobe.c::pre_handler_kretprobe. The fix above solved
>>>> the problem and seems not to have any undesired side effects.
>>>>
>>>> We are using kernel version 2.6.32, but it seems to me that the same
>>>> problem exists in more current kernels judging by a quick look.
>>>>
>>>> Why the problem manifests itself only if the tracing is enabled early in
>>>> the boot I cannot say. Could it be that if the entry_handler returns 0
>>>> at least once before the free list has been exhausted, it resets the
>>>> situation somehow? Or is it that after some point after userspace
>>>> initialization current->mm is never NULL?
>>>>
>>>> The capability tracing module itself is ment for upstream, but I have
>>>> been told its code is not kernel quality (not enough comments) and the
>>>> implementation lacks obvious features so we have not dared to offer it
>>>> yet. It is used for defining profiles for daemon processes currently
>>>> running as root by checking what capabilities they actually need and
>>>> then assigning them only those, in the context of the MSSF security
>>>> framework project:
>>>>
>>>>
>>> http://conference2010.meego.com/session/mobile-simplified-security-framework-overview
>>>>
>>>> In case you are interested I'm happy to make the code and documentation
>>>> available at the forum of your choice.
>>>>
>>>> Yours sincerely,
>>>> Juhani Mäkelä
>>>> Nixu OPEN - https://www.nixuopen.org


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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