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

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Message-ID: <CACT4Y+ZszHhFr7VTShCGMigOssihC92x1Z87=AxVBhEMmojFdw@mail.gmail.com>
Date:	Mon, 28 Sep 2015 12:33:09 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Borislav Petkov <bp@...en8.de>
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 12:23 PM, Borislav Petkov <bp@...en8.de> wrote:
> 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;


I have not checked, but I would expect that it is caller's
responsibility. There is generally no way to magically resurrect a
pointer to a freed object passed in.
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