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SubjectRe: [PATCH v2 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support
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On 12/11/19 5:24 PM, 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?
>

There is no code in the kernel is poisoning/unpoisoning
redzones/variables on stack. It's because it's always done by the compiler, it inserts
some code in prologue/epilogue of every function.
So compiler needs to know the shadow offset which will be used to poison/unpoison
stack frames.

There is no such kind of limitation on globals instrumentation. The only reason globals
instrumentation depends on -fasan-shadow-offset is because there was some bug related to
globals in old gcc version which didn't support -fasan-shadow-offset.


If you want stack instrumentation with not standard mem-to-shadow mapping, the options are:
1. Patch compiler to make it possible the poisoning/unpoisonig of stack frames via function calls.
2. Use out-line instrumentation and do whatever mem-to-shadow mapping you want, but keep all kernel
stacks in some special place for which standard mem-to-shadow mapping (addr >>3 +offset)
works.


> Also, as it currently stands, the speed difference between inline and
> outline is approximately 2x, and given that we'd like to run this
> full-time in syzkaller I think there is value in trading off speed for
> some limitations.
>

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