linux-kernel - Re: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of attacking

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Date:	Thu, 4 Nov 2010 12:46:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Marcus Meissner <meissner@...e.de>
Cc:	linux-kernel@...r.kernel.org, jason.wessel@...driver.com,
	fweisbec@...il.com, tj@...nel.org, mort@....com, akpm@...l.org,
	security@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] kernel: make /proc/kallsyms mode 400 to reduce ease of
 attacking


* Marcus Meissner <meissner@...e.de> wrote:

> Hi,
> 
> Making /proc/kallsyms readable only for root makes it harder for attackers to 
> write generic kernel exploits by removing one source of knowledge where things are 
> in the kernel.

Cc:-ed Linus - i think he argued in favor of such a patch in the past.

I generally agree with such patches (i have written some myself), but there's a few 
questions with this one, which make this limited change ineffective and which make 
it harder to implement a fuller patch that makes it truly harder to figure out the 
precise kernel build:

 - The real security obstruction effect is very small from this measure alone: the 
   overwhelming majority of our users are running distro kernels, so the Symbol.map 
   file (and hence 99% of /proc/kallsyms content) is well-known - unless we also 
   restrict 'uname -r' from nonprivileged users-ace. Hiding that might make sense - 
   but the two should be in one patch really.

 - ( It will break a few tools that can be run as a plain user out of box - perf
     for example. "chmod a+r /proc/kallsyms" during bootup will work that around so
     it's not the end of the world. )

 - For self-built kernels it might make sense - but there's "chmod a-r
   /proc/kallsyms" during bootup one can do already.

 - There's the side-question of module symbols - those are dynamically allocated
   hence arguably per system. But module symbols make up only 1% on a typical 
   booted up full distro box.

So what does a distribution like Suse expect from this change alone? Those have 
public packages in rpms which can be downloaded by anyone, so it makes little sense 
to hide it - unless _all_ version information is hidden.

So i'd like to see a _full_ version info sandboxing patch that thinks through all 
the angles and restricts uname -r kernel version info as well, and makes dmesg 
unaccessible to users - and closes a few other information holes as well that give 
away the exact kernel version - _that_ together will make it hard to blindly attack 
a very specific kernel version.

But without actually declaring and achieving that sandboxing goal this security 
measure is just a feel-good thing really - and makes it harder to make more 
difficult steps down the road, like closing 'uname -r' ...

I fully expect Linus to overrule me on this, but hey, i had to try it and lay out my 
arguments :-)

Thanks,

	Ingo
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