Re: [PATCH v2 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support

Christophe Leroy
2019-12-12T09:38:49Z

Le 12/12/2019 à 08:42, Balbir Singh a écrit :
>
>
> On 12/12/19 1:24 am, Daniel Axtens wrote:
>> Hi Balbir,
>>
>>>>>> +Discontiguous memory can occur when you have a machine with memory spread
>>>>>> +across multiple nodes. For example, on a Talos II with 64GB of RAM:
>>>>>> +
>>>>>> + - 32GB runs from 0x0 to 0x0000_0008_0000_0000,
>>>>>> + - then there's a gap,
>>>>>> + - then the final 32GB runs from 0x0000_2000_0000_0000 to 0x0000_2008_0000_0000
>>>>>> +
>>>>>> +This can create _significant_ issues:
>>>>>> +
>>>>>> + - If we try to treat the machine as having 64GB of _contiguous_ RAM, we would
>>>>>> + assume that ran from 0x0 to 0x0000_0010_0000_0000. We'd then reserve the
>>>>>> + last 1/8th - 0x0000_000e_0000_0000 to 0x0000_0010_0000_0000 as the shadow
>>>>>> + region. But when we try to access any of that, we'll try to access pages
>>>>>> + that are not physically present.
>>>>>> +
>>>>>
>>>>> If we reserved memory for KASAN from each node (discontig region), we might survive
>>>>> this no? May be we need NUMA aware KASAN? That might be a generic change, just thinking
>>>>> out loud.
>>>>
>>>> The challenge is that - AIUI - in inline instrumentation, the compiler
>>>> doesn't generate calls to things like __asan_loadN and
>>>> __asan_storeN. Instead it uses -fasan-shadow-offset to compute the
>>>> checks, and only calls the __asan_report* family of functions if it
>>>> detects an issue. This also matches what I can observe with objdump
>>>> across outline and inline instrumentation settings.
>>>>
>>>> This means that for this sort of thing to work we would need to either
>>>> drop back to out-of-line calls, or teach the compiler how to use a
>>>> nonlinear, NUMA aware mem-to-shadow mapping.
>>>
>>> Yes, out of line is expensive, but seems to work well for all use cases.
>>
>> I'm not sure this is true. Looking at scripts/Makefile.kasan, allocas,
>> stacks and globals will only be instrumented if you can provide
>> KASAN_SHADOW_OFFSET. In the case you're proposing, we can't provide a
>> static offset. I _think_ this is a compiler limitation, where some of
>> those instrumentations only work/make sense with a static offset, but
>> perhaps that's not right? Dmitry and Andrey, can you shed some light on
>> this?
>>
>
> From what I can read, everything should still be supported, the info page
> for gcc states that globals, stack asan should be enabled by default.
> allocas may have limited meaning if stack-protector is turned on (no?)

Where do you read that ?

As far as I can see, there is not much details about
-fsanitize=kernel-address and -fasan-shadow-offset=number in GCC doc
(https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html)

[...]

>>
>
> I think I got CONFIG_PHYS_MEM_SIZE_FOR_KASN wrong, honestly I don't get why
> we need this size? The size is in MB and the default is 0.
>
> Why does the powerpc port of KASAN need the SIZE to be explicitly specified?
>

AFAICS, it is explained in details in Daniel's commit log. That's
because on book3s64, KVM requires KASAN to also work when MMU is off.

The 0 default is for when CONFIG_KASAN is not selected, in order to
avoid a forest of #ifdefs in the code.

Christophe