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