On 07/12/2011 01:00 PM, Jan Kiszka wrote:
> On 2011-07-12 12:59, Gilles Chanteperdrix wrote:
>> On 07/12/2011 09:22 AM, Jan Kiszka wrote:
>>> On 2011-07-12 08:41, Gilles Chanteperdrix wrote:
>>>> On 07/11/2011 10:12 PM, Jan Kiszka wrote:
>>>>> On 2011-07-11 22:09, Gilles Chanteperdrix wrote:
>>>>>> On 07/11/2011 10:06 PM, Jan Kiszka wrote:
>>>>>>> On 2011-07-11 22:02, Gilles Chanteperdrix wrote:
>>>>>>>> On 07/11/2011 09:59 PM, Jan Kiszka wrote:
>>>>>>>>> On 2011-07-11 21:51, Gilles Chanteperdrix wrote:
>>>>>>>>>> On 07/11/2011 09:16 PM, Jan Kiszka wrote:
>>>>>>>>>>> On 2011-07-11 21:10, Jan Kiszka wrote:
>>>>>>>>>>>> On 2011-07-11 20:53, Gilles Chanteperdrix wrote:
>>>>>>>>>>>>> On 07/08/2011 06:29 PM, GIT version control wrote:
>>>>>>>>>>>>>> @@ -2528,6 +2534,22 @@ static inline void 
>>>>>>>>>>>>>> do_taskexit_event(struct task_struct *p)
>>>>>>>>>>>>>>          magic = xnthread_get_magic(thread);
>>>>>>>>>>>>>>  
>>>>>>>>>>>>>>          xnlock_get_irqsave(&nklock, s);
>>>>>>>>>>>>>> +
>>>>>>>>>>>>>> +        gksched = thread->gksched;
>>>>>>>>>>>>>> +        if (gksched) {
>>>>>>>>>>>>>> +                xnlock_put_irqrestore(&nklock, s);
>>>>>>>>>>>>>
>>>>>>>>>>>>> Are we sure irqs are on here? Are you sure that what is needed is 
>>>>>>>>>>>>> not an
>>>>>>>>>>>>> xnlock_clear_irqon?
>>>>>>>>>>>>
>>>>>>>>>>>> We are in the context of do_exit. Not only IRQs are on, also 
>>>>>>>>>>>> preemption.
>>>>>>>>>>>> And surely no nklock is held.
>>>>>>>>>>>>
>>>>>>>>>>>>> Furthermore, I do not understand how we
>>>>>>>>>>>>> "synchronize" with the gatekeeper, how is the gatekeeper 
>>>>>>>>>>>>> garanteed to
>>>>>>>>>>>>> wait for this assignment?
>>>>>>>>>>>>
>>>>>>>>>>>> The gatekeeper holds the gksync token while it's active. We 
>>>>>>>>>>>> request it,
>>>>>>>>>>>> thus we wait for the gatekeeper to become idle again. While it is 
>>>>>>>>>>>> idle,
>>>>>>>>>>>> we reset the queued reference - but I just realized that this may 
>>>>>>>>>>>> tramp
>>>>>>>>>>>> on other tasks' values. I need to add a check that the value to be
>>>>>>>>>>>> null'ified is actually still ours.
>>>>>>>>>>>
>>>>>>>>>>> Thinking again, that's actually not a problem: gktarget is only 
>>>>>>>>>>> needed
>>>>>>>>>>> while gksync is zero - but then we won't get hold of it anyway and,
>>>>>>>>>>> thus, can't cause any damage.
>>>>>>>>>>
>>>>>>>>>> Well, you make it look like it does not work. From what I understand,
>>>>>>>>>> what you want is to set gktarget to null if a task being hardened is
>>>>>>>>>> destroyed. But by waiting for the semaphore, you actually wait for 
>>>>>>>>>> the
>>>>>>>>>> harden to be complete, so setting to NULL is useless. Or am I missing
>>>>>>>>>> something else?
>>>>>>>>>
>>>>>>>>> Setting to NULL is probably unneeded but still better than rely on the
>>>>>>>>> gatekeeper never waking up spuriously and then dereferencing a stale
>>>>>>>>> pointer.
>>>>>>>>>
>>>>>>>>> The key element of this fix is waitng on gksync, thus on the 
>>>>>>>>> completion
>>>>>>>>> of the non-RT part of the hardening. Actually, this part usually fails
>>>>>>>>> as the target task received a termination signal at this point.
>>>>>>>>
>>>>>>>> Yes, but since you wait on the completion of the hardening, the test
>>>>>>>> if (target &&...) in the gatekeeper code will always be true, because 
>>>>>>>> at
>>>>>>>> this point the cleanup code will still be waiting for the semaphore.
>>>>>>>
>>>>>>> Yes, except we will ever wake up the gatekeeper later on without an
>>>>>>> updated gktarget, ie. spuriously. Better safe than sorry, this is hairy
>>>>>>> code anyway (hopefully obsolete one day).
>>>>>>
>>>>>> The gatekeeper is not woken up by posting the semaphore, the gatekeeper
>>>>>> is woken up by the thread which is going to be hardened (and this thread
>>>>>> is the one which waits for the semaphore).
>>>>>
>>>>> All true. And what is the point?
>>>>
>>>> The point being, would not something like this patch be sufficient?
>>>>
>>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c
>>>> index 01f4200..4742c02 100644
>>>> --- a/ksrc/nucleus/shadow.c
>>>> +++ b/ksrc/nucleus/shadow.c
>>>> @@ -2527,6 +2527,18 @@ static inline void do_taskexit_event(struct
>>>> task_struct *p)
>>>>    magic = xnthread_get_magic(thread);
>>>>
>>>>    xnlock_get_irqsave(&nklock, s);
>>>> +  if (xnthread_test_info(thread, XNATOMIC)) {
>>>> +          struct xnsched *gksched = xnpod_sched_slot(task_cpu(p));
>>>
>>> That's not reliable, the task might have been migrated by Linux in the
>>> meantime. We must use the stored gksched.
>>>
>>>> +          xnlock_put_irqrestore(&nklock, s);
>>>> +
>>>> +          /* Thread is in flight to primary mode, wait for the
>>>> +             gatekeeper to be done with it. */
>>>> +          down(&gksched->gksync);
>>>> +          up(&gksched->gksync);
>>>> +
>>>> +          xnlock_get_irqsave(&nklock, s);
>>>> +  }
>>>> +
>>>>    /* Prevent wakeup call from xnshadow_unmap(). */
>>>>    xnshadow_thrptd(p) = NULL;
>>>>    xnthread_archtcb(thread)->user_task = NULL;
>>>>
>>>
>>> Again, setting gktarget to NULL and testing for NULL is simply safer,
>>> and I see no gain in skipping that. But if you prefer the
>>> micro-optimization, I'll drop it.
>>
>> Could not we use an info bit instead of adding a pointer?
>>
> 
> "That's not reliable, the task might have been migrated by Linux in the
> meantime. We must use the stored gksched."

I mean add another info bit to mean that the task is queued for wakeup
by the gatekeeper.

XNGKQ, or something.

-- 
                                                                Gilles.

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