[PATCH 1/2] powerpc: kprobes: blacklist exception handlers

[Michael Ellerman]

"Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> writes:

> Excerpts from Michael Ellerman's message of April 20, 2017 12:03:
>> "Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> writes:
>> 
>>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>>> index 71286dfd76a0..59159337a097 100644
>>> --- a/arch/powerpc/kernel/kprobes.c
>>> +++ b/arch/powerpc/kernel/kprobes.c
>>> @@ -112,6 +113,14 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>>>  	return addr;
>>>  }
>>>  
>>> +bool arch_within_kprobe_blacklist(unsigned long addr)
>>> +{
>>> +	return  (addr >= (unsigned long)__kprobes_text_start &&
>>> +		 addr < (unsigned long)__kprobes_text_end) ||
>>> +		(addr >= (unsigned long)_stext &&
>>> +		 addr < (unsigned long)__head_end);
>>> +}
>> 
>> This isn't quite right when the kernel is relocated.
>> 
>> _stext and __head_end will be updated to point to the relocated copy of
>> the kernel, eg:
>> 
>> # grep -e _stext /proc/kallsyms 
>> c000000002000000 T _stext
>> 
>> So you probably also want something like:
>> 
>>   if (_stext != PAGE_OFFSET &&
>>       addr >= PAGE_OFFSET &&
>>       addr < (PAGE_OFFSET + (__head_end - _stext)))
>>       return true;
>
> Ah, so that's for ensuring we don't allow probing at the real exception 
> vectors, which get copied down from _stext. In that case, we are covered 
> by the test for kernel_text_address() in check_kprobe_address_safe(). We 
> only allow probing from _stext to _etext.

OK good. I was thinking of is_kernel_addr() which just checks it's >
PAGE_OFFSET, but of course it needs to be a text address also.

>> You can test the relocatable case by enabling CONFIG_RELOCATABLE_TEST.
>
> Done, thanks. This is working as expected (without the need for the 
> changes above).  I am not allowed to probe at the real exception vectors 
> (and PAGE_OFFSET) as well as between _stext and __head_end.

Great.

cheers