linux-kernel - Re: mmap_min_addr/SECURITY_DEFAULT_MMAP_MIN

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Date:	Mon, 23 Jun 2008 07:04:35 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	"Dr. David Alan Gilbert" <linux@...blig.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: mmap_min_addr/SECURITY_DEFAULT_MMAP_MIN_ADDR suggested values

On Mon, 23 Jun 2008 14:53:37 +0100
"Dr. David Alan Gilbert" <linux@...blig.org> wrote:

> Hi,
>   The config help for SECURITY_DEFAULT_MMAP_MIN_ADDR suggests 65536
> as a 'reasonable' value for x86 and the original mmap_min_addr
> patches suggested that 'something like 64k' was a safe value that
> wouldn't affect most programs.
> 
>   Where does this 64k value come from?  A number of distros seem
> to have followed this advice and have it set to 64k; but is there
> really any likely benefit of having it larger than PAGE_SIZE say?

there's a few things in the kernel that are bigger than 4K (or rather,
lead to pointers beyond 4K) so it's not all that bad advice.

> 
> I ask because I have an ancient program that maps stuff at 8k; the
> general advice of stuff on the net seems to be to set this limit
> to 0 if people have problems (I'm just lowering it to 4k),
> but I was thinking perhaps using a lower default value would be more
> secure since less people would take the easy answer and just turn the
> feature off altogether.

interesting... what does the program do?
(applications making assumptions on where they can mmap stuff... that's
not really valid. Realistically, the only safe way to use MAP_FIXED is
on memory that you got from mmap before.. think about it: what if
glibc happened to mmap something there first?)

So I'm quite curious what/why this app is doing this

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