linux-kernel - Re: [PATCH v3] kernel: add kcov code coverage

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Message-ID: <20160118215201.GD14531@node.shutemov.name>
Date:	Mon, 18 Jan 2016 23:52:01 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Drysdale <drysdale@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Tavis Ormandy <taviso@...gle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: Re: [PATCH v3] kernel: add kcov code coverage

On Mon, Jan 18, 2016 at 08:28:04PM +0100, Dmitry Vyukov wrote:
> On Mon, Jan 18, 2016 at 1:42 PM, Kirill A. Shutemov
> <kirill@...temov.name> wrote:
> > On Thu, Jan 14, 2016 at 03:22:21PM +0100, Dmitry Vyukov wrote:
> >> +             area = t->kcov_area;
> >> +             /* The first u32 is number of subsequent PCs. */
> >> +             pos = READ_ONCE(area[0]) + 1;
> >> +             if (likely(pos < t->kcov_size)) {
> >> +                     area[pos] = (u32)_RET_IP_;
> >
> > If area[0] is UINT32_MAX the condition will be true and you'll make
> > area[0] temporary set to (u32)_RET_IP_. That's may be fun. :)
> 
> Well, if user wants to corrupt this region, he is free to do so. But
> it wont't corrupt kernel, right?

Right. But it's easy enough to avoid overflow too:
	"if (likely(pos && pos < t->kcov_size)"

> >> +
> >> +     page = vmalloc_to_page(kcov->area + off);
> >> +     get_page(page);
> >> +     vmf->page = page;
> >> +     return 0;
> >> +}
> >
> > BTW, since all pages pre-allocated, you don't really need fault handler --
> > just setup all pages table enties during mmap. This would shorten warm up
> > period for userspace. See vm_insert_page().
> 
> Done in v4
> I don't need to get_page() before vm_insert_page(), right?

Right. vm_insert_page() will take the referenece for you.


-- 
 Kirill A. Shutemov