Re: [PATCH 1/2] LSM: Do not apply mmap_min_addr check to PROT_NONE

From: Gregory Sahanovitch
Date: Fri Oct 28 2011 - 09:36:38 EST

> 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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