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

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Date:   Wed, 24 Aug 2016 09:11:58 -0700
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support

On Wed, Aug 24, 2016 at 3:03 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...nel.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