linux-kernel - Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm

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Date:	Tue, 29 Nov 2011 12:40:57 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm_struct entires

On Tue, Nov 29, 2011 at 12:37 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, Nov 30, 2011 at 12:29:51AM +0400, Cyrill Gorcunov wrote:
> ...
>> >
>> > arg3 needs to be significantly more carefully validated. find_vma() doesn't
>> > say that vm_start <= addr, only that vm_end > addr. This effectively
>> > bypasses all the vma checks (mmap_min_addr, max process size, etc), with
>> > some pretty crazy side-effects, I think.
>> >
>>
>> Yes, I know it needs some more testing, but apart from vma bounds (yup,
>> good point with find_vma, I'll fix) I thought about what else should be
>> checked? I think VMA prototype should be checked to fit "code", "data"
>> templates, ie code should be at least readable and execytable, but what
>> about data and stack and brk, should stack be executable? That is the
>> point where I've got a bit confused and though putting RFC out might be
>> a good idea to collect opinions.
>
> On the other hands these fields are set up by elf hanlder code, which
> does mmap these areas, so we have to check that particular member
> belongs to existing VMA and never cross user-space area, and together
> with root-only approach would not it be enough? I'm sure missing something
> that is why I'm asking.

Right, if you verify that the addresses are actually inside valid
userspace vmas, that is likely to be right, though there are probably
other things I haven't thought of. The trouble is avoiding vdso, stack
guard page, vsyscall, and anything else that isn't meant for the mm to
have direct access to.

-- 
Kees Cook
ChromeOS Security
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