linux-kernel - Re: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE

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Date:	Wed, 3 Jun 2009 11:41:12 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Stephen Smalley <sds@...ho.nsa.gov>
cc:	"Larry H." <research@...reption.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	pageexec@...email.hu
Subject: Re: Security fix for remapping of page 0 (was [PATCH] Change
 ZERO_SIZE_PTR to point at unmapped space)

On Wed, 3 Jun 2009, Stephen Smalley wrote:

> > If one remaps page 0 then the kernel checks for NULL pointers of various
> > flavors are bypassed and this may be exploited in various creative ways
> > to transfer data from kernel space to user space.
> >
> > Fix this by not allowing the remapping of page 0. Return -EINVAL if
> > such a mapping is attempted.
>
> You can already prevent unauthorized processes from mapping low memory
> via the existing mmap_min_addr setting, configurable via
> SECURITY_DEFAULT_MMAP_MIN_ADDR or /proc/sys/vm/mmap_min_addr.  Then
> cap_file_mmap() or selinux_file_mmap() will apply a check when a process
> attempts to map memory below that address.

mmap_min_addr depends on CONFIG_SECURITY which establishes various
strangely complex "security models".

The system needs to be secure by default.


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