linux-kernel - Re: [PATCH 1/2] LSM: Do not apply mmap_min_addr check to PROT

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Date:	Fri, 28 Oct 2011 15:36:33 +0200
From:	Gregory Sahanovitch <gsahanovitch@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Morris <jmorris@...ei.org>,
	Eric Paris <eparis@...isplace.org>,
	Stephen Smalley <sds@...ho.nsa.gov>, selinux@...ho.nsa.gov,
	John Johansen <john.johansen@...onical.com>,
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] LSM: Do not apply mmap_min_addr check to PROT_NONE

> It's exactly the case that I did mention: an application's own attempt to
> ensure robustness by doing a PROT_NONE mmap of the [0,0x10000) region.  An
> application cannot presume that this region is already precluded from being
> used by any non-MAP_FIXED mmap across all systems and configurations, so
> it's defensive coding to explicitly block it off with a PROT_NONE mapping.

I don't see a realistic threat model in the example you give.

Since mmap_min_addr is used to prevent a *malicious* process from
maping the zero page and then taking advantage of a user-pointer
dereference in the *kernel code*, I do not see what you gain by
guaranteeing that the application *that you control* would never
exploit such a vulnerability?

Sorry if I'm being thick, but it would be helpful to me if you clarify.

-- 
- Greg
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