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

[Naveen N. Rao]

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.

> 
> But that's entirely untested :)
> 
> 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.

Thanks,
Naveen