linux-kernel - Re: mmap_min_addr and your local LSM (ok, just SELinux)

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Message-ID: <20090728155622.GO7316@outflux.net>
Date:	Tue, 28 Jul 2009 08:56:22 -0700
From:	Kees Cook <kees@...ntu.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	James Morris <jmorris@...ei.org>,
	James Carter <jwcart2@...ho.nsa.gov>,
	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	Stephen Smalley <sds@...ho.nsa.gov>, spender@...ecurity.net,
	Daniel J Walsh <dwalsh@...hat.com>, cl@...ux-foundation.org,
	Arjan van de Ven <arjan@...radead.org>,
	Chad Sellers <csellers@...sys.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	mingo@...e.hu
Subject: Re: mmap_min_addr and your local LSM (ok, just SELinux)

On Tue, Jul 28, 2009 at 11:21:29AM +0200, Andi Kleen wrote:
> Alan Cox <alan@...rguk.ukuu.org.uk> writes:
> 
> > A dumb question perhaps, but while addling my brain over the tty layer I
> > was wondering if for the specific case of jump through NULL (which seems
> > to be the most common but by no means only problem case that gets
> > exploited) is there any reason we can't set a default breakpoint for
> 
> You mean a hardware breakpoint? Hardware break points are a precious
> scarce resource. The people who rely on them would be likely
> unhappy if you take one way from them.

Could the page table flags be used to mask this region?  i.e. force
PROT_NONE (with the "desired" flags stored elsewhere) and in the segv
handler check if it is kernel or user space, and then fix-up the flags and
continue if it's userspace?  (I really don't know the internals on this,
but it would need to restore PROT_NONE on task-switch or something...)

-Kees

-- 
Kees Cook
Ubuntu Security Team
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