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

[Masami Hiramatsu]

(2013/11/05 11:00), Steven Rostedt 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.
> 
> Now these may be a slow path where we really don't care, but if the
> nokprobe expands this can cause issues.
> 
> The "nokprobe" works differently than "notrace" as "notrace" is just an
> attribute that tells gcc not to add mcount to it. The "nokprobe"
> actually moves the function into a different section.

Well, in that case, I can put it in the opt-out type blacklist(kprobe_blacklist). :)
Hmm, I think if I can list nokprobe functions up at build time, we can almost
remove the .kprobes.text (Note that some of entry functions in asm still require it.)

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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