Re: [PATCH -tip v2 3/3] [BUGFIX] kprobes: Prohibit probing on func_ptr_is_kernel_text

[Masami Hiramatsu]

(2013/11/05 20:38), Masami Hiramatsu wrote:
> (2013/11/05 16:05), Ingo Molnar wrote:
>>
>> * Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
>>
>>> (2013/11/05 15:09), Ingo Molnar wrote:
>>>>
>>>> * Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>>>>
>>>>> On Fri, 01 Nov 2013 11:25:37 +0000
>>>>> Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
>>>>>
>>>>>> Prohibit probing on func_ptr_is_kernel_text().
>>>>>> Since the func_ptr_is_kernel_text() is called from
>>>>>> notifier_call_chain() which is called from int3 handler,
>>>>>> probing it may cause double int3 fault and kernel will
>>>>>> reboot.
>>>>>>
>>>>>> This happenes when the kernel built with CONFIG_DEBUG_NOTIFIERS=y.
>>>>>>
>>>>>> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
>>>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>>>> Cc: "Uwe Kleine-KÃnig" <u.kleine-koenig@xxxxxxxxxxxxxx>
>>>>>> Cc: Borislav Petkov <bp@xxxxxxx>
>>>>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>>>>> ---
>>>>>>  kernel/extable.c |    2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/kernel/extable.c b/kernel/extable.c
>>>>>> index 832cb28..022fb25 100644
>>>>>> --- a/kernel/extable.c
>>>>>> +++ b/kernel/extable.c
>>>>>> @@ -129,7 +129,7 @@ int kernel_text_address(unsigned long addr)
>>>>>>   * pointer is part of the kernel text, we need to do some
>>>>>>   * special dereferencing first.
>>>>>>   */
>>>>>> -int func_ptr_is_kernel_text(void *ptr)
>>>>>> +int nokprobe func_ptr_is_kernel_text(void *ptr)
>>>>>>  {
>>>>>>  	unsigned long addr;
>>>>>>  	addr = (unsigned long) dereference_function_descriptor(ptr);
>>>>>>
>>>>>
>>>>> One thing I worry about the "nokprobe" annotation, is that it moves the 
>>>>> location of the function out of local. This function no exists in the 
>>>>> section with its users. Same with the debug functions in the other 
>>>>> patch.
>>>>
>>>> Well, it's a bit like noinline, that changes the position of the function 
>>>> as well. So it's not true that 'noxyz' attributes don't affect function 
>>>> placement - they often don't, but some do.
>>>>
>>>> The more important aspect is that 'noprobe' makes it really, really 
>>>> apparent what the tag is about, at first sight.
>>>>
>>>> _How_ the 'non probing' is achived is an implementational detail when 
>>>> kprobes are enabled: right now it puts a function into a separate section, 
>>>> but we could just a much build a list of function names and check against 
>>>> it at probe insertion time.
>>>
>>> Actually, kprobes already has it -- kprobes_blacklist. Currently the 
>>> list is manually maintained in kprobes.c separated from the function 
>>> definition. [...]
>>
>> Yes, I meant a list that is built automatically from the 'noprobe' 
>> annotations.
> 
> Agreed. That makes maintenance work simple, and we can remove
> ".kprobes.text" section.
> 
>>> [...] I hope to build the list when the kernel build time if possible... 
>>> Would you have any idea to classify some annotated(but no side-effect) 
>>> functions?
>>
>> The macro magic I can think of would need to change the syntax of the 
>> function definition - for example that is how the SYSCALL_DEFINE*() macros 
>> work.
> 
> Would you mean something like the below macro? :)
> 
> NOKPROBE_SYMBOL(int, func_ptr_is_kernel_text)(void *ptr)
> 
> which is expanded as;
> 
> static struct nokprobe_entry __used
> __nokprobe_entry_func_ptr_is_kernel_text = {
>   .name = "func_ptr_is_kernel_text"
> };
> static struct kprobe_blacklist_entry __used
> __attribute__((section("_nokprobe_list")))
> __p_nokprobe_entry_func_ptr_is_kernel_text = &__nokprobe_entry_func_ptr_is_kernel_text;
> int func_ptr_is_kernel_text(void *ptr)
> 

By the way, this method can be done in C file, but not in asm file.
And there are many sensitive entries in the entry_*.S. I think we have
two options.

(A) keep the kprobes.text(or nokprobe.text) section for such assembler parts.
(B) Put a starter symbol and end symbol in such region and make a list
 of symbols between them in build time by using nm.

Anyway, until removing all __kprobes from kernel, we can not remove the
kprobes.text section. So, at the first step I just try to introduce
above macro and apply it to only the symbols in kprobes_blacklist.
After that, I'll try to classify the real unsafe entries and apply
the new macro. Eventually, I think I can remove all __kprobes.

Thank you,


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


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