linux-kernel - Re: [PATCHv2 3/6] x86/arch_prctl/vdso: add ARCH_MAP_VDSO

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Date:	Tue, 12 Jul 2016 16:14:46 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dmitry Safonov <dsafonov@...tuozzo.com>,
	Michal Hocko <mhocko@...e.com>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dmitry Safonov <0x7f454c46@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Ingo Molnar <mingo@...hat.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>, xemul@...tuozzo.com,
	Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCHv2 3/6] x86/arch_prctl/vdso: add ARCH_MAP_VDSO_*

On 07/11, Andy Lutomirski wrote:
>
> On Mon, Jul 11, 2016 at 11:26 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Do we really care? I mean, the kernel can't crash or something like this,
> > just the old vdso mapping can faultin the "wrong" page from the new
> > vdso_image, right?
>
> That makes me nervous.  IMO a mapping should have well-defined
> semantics.

Perhaps. but map_vdso() will be special anyway, it also changes ->vdso.

For example, if a 32-bit application calls prctl(ARCH_MAP_VDSO) from a
signal handler and we unmap the old vdso mapping, it will crash later
trying to call the (unmapped) restorer == kernel_rt_sigreturn.

> If nothing else, could be really messy if the list of
> pages were wrong.

I do not see anything really wrong, but I can easily miss something.

And don't get me wrong, I agree that any cleanup (say, associate vdso
image with vma) makes sense.

> My real concern is DoS: I doubt that __install_special_mapping gets
> all the accounting right.

Yes, and if it was not clear I fully agree. Even if we forget about the
accounting, I feel that special mappings must not be abused by userspace.

> > So it seems that we should do this by hand somehow. But in fact, what
> > I actually think right now is that I am totally confused and got lost ;)
>
> I'm starting to wonder if we should finally suck it up and give
> special mappings a non-NULL vm_file so we can track them properly.
> Oleg, weren't you thinking of doing that for some other reason?

Yes, uprobes. Currently we can't probe vdso page(s).

Oleg.