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Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support

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On Wed, Aug 24, 2016 at 3:03 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Andy Lutomirski <luto@kernel.org> wrote:
>
>> +config VMAP_STACK
>> +     default y
>> +     bool "Use a virtually-mapped stack"
>> +     depends on HAVE_ARCH_VMAP_STACK && !KASAN
>> +     ---help---
>> +       Enable this if you want the use virtually-mapped kernel stacks
>> +       with guard pages.  This causes kernel stack overflows to be
>> +       caught immediately rather than causing difficult-to-diagnose
>> +       corruption.
>> +
>> +       This is presently incompatible with KASAN because KASAN expects
>> +       the stack to map directly to the KASAN shadow map using a formula
>> +       that is incorrect if the stack is in vmalloc space.
>
> Btw., is this KASAN limitation fundamental?
>
> As x86 is going to enable this feature by default, this probably limits KASAN
> utility rather significantly.


No, it's not fundamental.

KASAN has shadow for vmalloc range, but currently we map a single
read-only zero page there (which means "this memory is good"). Stack
instrumentation tries to write to that read-only page, which causes
crash.

Andrey proposed that we can map some real writable pages in the shadow
range when we allocate a vmalloc-ed stack:
https://groups.google.com/d/msg/kasan-dev/0YxqFs9r0V8/OKoGHQL8BAAJ

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[PATCH v6 1/3] fork: Add generic vmalloced stack support Andy Lutomirski <luto@kernel.org> - 2016-08-11 11:40 +0200
  Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support Michal Hocko <mhocko@kernel.org> - 2016-08-15 14:00 +0200
  Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support Ingo Molnar <mingo@kernel.org> - 2016-08-24 12:20 +0200
    Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support Dmitry Vyukov <dvyukov@google.com> - 2016-08-24 18:20 +0200
  [tip:x86/asm] fork: Add generic vmalloced stack support tip-bot for Andy Lutomirski <tipbot@zytor.com> - 2016-08-24 15:10 +0200
  Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support Josh Cartwright <joshc@ni.com> - 2016-08-24 19:20 +0200
    Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support Andy Lutomirski <luto@amacapital.net> - 2016-08-31 00:10 +0200
      Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support Josh Cartwright <joshc@ni.com> - 2016-08-31 00:30 +0200

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