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

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Message-ID: <20101104135802.GA31416@elte.hu>
Date:	Thu, 4 Nov 2010 14:58:02 +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:

> On Thu, Nov 04, 2010 at 12:46:48PM +0100, Ingo Molnar wrote:
> > 
> > * 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.
> 
> Of course. System.map and others also need to turn to mode 400.

That is not what I meant, at all.

It's not the System.map _on the system_.

It's the SuSE or Fedora kernel rpm package with a System.map in it, which
package the attacker can download from a hundred mirrors on the internet,
based on 'uname -r' output.

You cannot obfuscate the System.map of a distro kernel without obfuscating all 
identification info. (Note that even the pure size of the System.map might tell a 
kernel rpm version from another ...)

	Ingo
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