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Date: Mon, 28 Sep 2015 12:23:14 +0200
From: Borislav Petkov <bp@alien8.de>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, luto@kernel.org, dvlasenk@redhat.com,
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        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Sasha Levin <sasha.levin@oracle.com>, Andi Kleen <ak@linux.intel.com>,
        kasan-dev <kasan-dev@googlegroups.com>
Subject: Re: [PATCH] arch/x86: fix out-of-bounds in get_wchan()
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On Mon, Sep 28, 2015 at 11:49:18AM +0200, Dmitry Vyukov wrote:
> Original code did:
> 
>      if (p->thread.sp < stack || p->thread.sp >= stack+THREAD_SIZE)
>            return 0;
>      fp = *(u64 *)(p->thread.sp);
> 
> p->thread.sp can change concurrently.
> So we could check that p->thread.sp is within stack bounds, but then
> dereference another value (which is already outside of bounds).

Right, we do deref it. I realized that after hitting "Send" :\

Which begs another, probably also stupid, question:

What guarantees the task won't disappear after we've checked p?

I.e., after this:

        if (!p || p == current || p->state == TASK_RUNNING)
                return 0;

Thanks.

-- 
Regards/Gruss,
    Boris.

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