Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 18/02/2020 à 01:44, Masami Hiramatsu a écrit :
> On Mon, 17 Feb 2020 16:38:50 +0100
> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
> 
>>
>>
>> Le 17/02/2020 à 11:27, Masami Hiramatsu a écrit :
>>> On Mon, 17 Feb 2020 10:03:22 +0100
>>> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
>>>
>>>>
>>>>
>>>> Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit :
>>>>> On Sat, 15 Feb 2020 11:28:49 +0100
>>>>> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Le 14/02/2020 à 14:54, Masami Hiramatsu a écrit :
>>>>>>> Hi,
>>>>>>>
>>>>>>> On Fri, 14 Feb 2020 12:47:49 +0000 (UTC)
>>>>>>> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
>>>>>>>
>>>>>>>> When a program check exception happens while MMU translation is
>>>>>>>> disabled, following Oops happens in kprobe_handler() in the following
>>>>>>>> test:
>>>>>>>>
>>>>>>>> 		} else if (*addr != BREAKPOINT_INSTRUCTION) {
>>>>>>>
>>>>>>> Thanks for the report and patch. I'm not so sure about powerpc implementation
>>>>>>> but at where the MMU translation is disabled, can the handler work correctly?
>>>>>>> (And where did you put the probe on?)
>>>>>>>
>>>>>>> Your fix may fix this Oops, but if the handler needs special care, it is an
>>>>>>> option to blacklist such place (if possible).
>>>>>>
>>>>>> I guess that's another story. Here we are not talking about a place
>>>>>> where kprobe has been illegitimately activated, but a place where there
>>>>>> is a valid trap, which generated a valid 'program check exception'. And
>>>>>> kprobe was off at that time.
>>>>>
>>>>> Ah, I got it. It is not a kprobe breakpoint, but to check that correctly,
>>>>> it has to know the address where the breakpoint happens. OK.
>>>>>
>>>>>>
>>>>>> As any 'program check exception' due to a trap (ie a BUG_ON, a WARN_ON,
>>>>>> a debugger breakpoint, a perf breakpoint, etc...) calls
>>>>>> kprobe_handler(), kprobe_handler() must be prepared to handle the case
>>>>>> where the MMU translation is disabled, even if probes are not supposed
>>>>>> to be set for functions running with MMU translation disabled.
>>>>>
>>>>> Can't we check the MMU is disabled there (as same as checking the exception
>>>>> happened in user space or not)?
>>>>>
>>>>
>>>> What do you mean by 'there' ? At the entry of kprobe_handler() ?
>>>>
>>>> That's what my patch does, it checks whether MMU is disabled or not. If
>>>> it is, it converts the address to a virtual address.
>>>>
>>>> Do you mean kprobe_handler() should bail out early as it does when the
>>>> trap happens in user mode ?
>>>
>>> Yes, that is what I meant.
>>>
>>>> Of course we can do that, I don't know
>>>> enough about kprobe to know if kprobe_handler() should manage events
>>>> that happened in real-mode or just ignore them. But I tested adding an
>>>> event on a function that runs in real-mode, and it (now) works.
>>>>
>>>> So, what should we do really ?
>>>
>>> I'm not sure how the powerpc kernel runs in real mode.
>>> But clearly, at least kprobe event can not handle that case because
>>> it tries to access memory by probe_kernel_read(). Unless that function
>>> correctly handles the address translation, I want to prohibit kprobes
>>> on such address.
>>>
>>> So what I would like to see is, something like below.
>>>
>>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>>> index 2d27ec4feee4..4771be152416 100644
>>> --- a/arch/powerpc/kernel/kprobes.c
>>> +++ b/arch/powerpc/kernel/kprobes.c
>>> @@ -261,7 +261,7 @@ int kprobe_handler(struct pt_regs *regs)
>>>           unsigned int *addr = (unsigned int *)regs->nip;
>>>           struct kprobe_ctlblk *kcb;
>>>    
>>> -       if (user_mode(regs))
>>> +       if (user_mode(regs) || !(regs->msr & MSR_IR))
>>>                   return 0;
>>>    
>>>           /*
>>>
>>>
>>
>> With this instead change of my patch, I get an Oops everytime a kprobe
>> event occurs in real-mode.
>>
>> This is because kprobe_handler() is now saying 'this trap doesn't belong
>> to me' for a trap that has been installed by it.
> 
> Hmm, on powerpc, kprobes is allowed to probe on the code which runs
> in the real mode? I think we should also prohibit it by blacklisting.
> (It is easy to add blacklist by NOKPROBE_SYMBOL(func))

Yes, I see a lot of them tagged with _ASM_NOKPROBE_SYMBOL() on PPC64, 
but none on PPC32. I suppose that's missing and have to be added. 
Nevertheless, if one symbol has been forgotten in the blacklist, I think 
it is a problem if it generate Oopses.

> Or, some parts are possble to run under both real mode and kernel mode?

I don't think so, at least on PPC32

> 
>>
>> So the 'program check' exception handler doesn't find the owner of the
>> trap hence generate an Oops.
>>
>> Even if we don't want kprobe() to proceed with the event entirely
>> (allthough it works at least for simple events), I'd expect it to fail
>> gracefully.
> 
> Agreed. I thought it was easy to identify real mode code. But if it is
> hard, we should apply your first patch and also skip user handlers
> if we are in the real mode (and increment missed count).

user handlers are already skipped.

What do you think about my latest proposal below ? If a trap is 
encoutered in real mode, if checks if the matching virtual address 
corresponds to a valid kprobe. If it is, it skips it. If not, it returns 
0 to tell "it's no me". You are also talking about incrementing the 
missed count. Who do we do that ?



@@ -264,6 +265,13 @@ int kprobe_handler(struct pt_regs *regs)
      if (user_mode(regs))
          return 0;

+    if (!(regs->msr & MSR_IR)) {
+        if (!get_kprobe(phys_to_virt(regs->nip)))
+            return 0;
+        regs->nip += 4;
+        return 1;
+    }
+
      /*
       * We don't want to be preempted for the entire
       * duration of kprobe processing


> 
> BTW, can the emulater handle the real mode code correctly?

I don't know, how do I test that ?

Christophe