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

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Date:	Wed, 3 Jun 2009 08:38:47 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	"Larry H." <research@...reption.com>, 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, Christoph Lameter wrote:

> On Wed, 3 Jun 2009, Linus Torvalds wrote:
> 
> > The point being that we do need to support mmap at zero. Not necessarily
> > universally, but it can't be some fixed "we don't allow that".
> 
> Hmmm... Depend on some capability? CAP_SYS_PTRACE may be something
> remotely related?

But as mentioned several times, we do have the system-wide setting in
'mmap_min_addr' (that then can be overridden by CAP_SYS_RAWIO, so in that 
sense a capability already exists).

It defaults to 64kB in at least the x86 defconfig files, but to 0 in the 
Kconfig defaults. Also, for some reason it has a "depends on SECURITY", 
which means that if you just default to the old-style unix security you'll 
lose it.

So there are several ways to disable it by mistake. I don't know what 
distros do.

		Linus
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