linux-kernel - Re: [PATCH] arch/x86: fix out-of-bounds in get

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Message-ID: <20150928102314.GB3556@pd.tnic>
Date:	Mon, 28 Sep 2015 12:23:14 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, luto@...nel.org,
	dvlasenk@...hat.com, "x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andi Kleen <ak@...ux.intel.com>,
	kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH] arch/x86: fix out-of-bounds in get_wchan()

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.

ECO tip #101: Trim your mails when you reply.
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