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

[Christophe Leroy]

On 02/17/2020 03:38 PM, Christophe Leroy 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 at 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 at 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 at 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.
> 
> 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.
> 

What about something like 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


Christophe