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

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2013/11/05 16:05), Ingo Molnar wrote:
> 
> * Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> 
>> (2013/11/05 15:09), Ingo Molnar wrote:
>>>
>>> * Steven Rostedt <rostedt@goodmis.org> wrote:
>>>
>>>> On Fri, 01 Nov 2013 11:25:37 +0000
>>>> Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> 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@hitachi.com>
>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>>> Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
>>>>> Cc: Borislav Petkov <bp@suse.de>
>>>>> Cc: Ingo Molnar <mingo@kernel.org>
>>>>> ---
>>>>>  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)

Hmm, this looks worth to try.

Thank you,

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