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

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Date:	Sun, 7 Nov 2010 09:50:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Willy Tarreau <w@....eu>
Cc:	Marcus Meissner <meissner@...e.de>, security@...nel.org,
	mort@....com, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	fweisbec@...il.com, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, jason.wessel@...driver.com,
	tj@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Security] [PATCH] kernel: make /proc/kallsyms mode 400 to
 reduce ease of attacking

* Willy Tarreau <w@....eu> wrote:

> On Thu, Nov 04, 2010 at 10:51:57PM +0100, Ingo Molnar wrote:
> > > Quite honnestly, it's the worst idea I've ever read to protect the kernel. Kernel 
> > > version is needed at many places, when building some code which relies on presence 
> > > of syscall X or Y depending on a version, etc... [...]
> > 
> > Actually that's not true, since we have a kernel ABI, and since there's many 
> > backports of newer kernel features into older kernels that it's generally not
> > needed nor meaningful to know the kernel version for syscalls.
> > 
> > Returning -ENOSYS is the general standard we use to communicate syscall 
> > capabilities.
> > 
> > In fact using kernel version to switch around library functionality is a bug i'd 
> > argue.
> 
> I'm sorry Ingo, but I still don't agree. We've had several versions of epoll, 
> several (some even buggy) versions of splice() which cannot even be detected 
> without checking the kernel release. And those are just two that immediately come 
> to my mind. If we've been providing a version for the last 19 years, it surely had 
> some valid uses.

I'm sorry Willy, but you are mostly wrong - and there's no need to speculate here 
really. Just try the patch below :-)

If your claim that 'kernel version is needed at many places' is true then why am i 
seeing this on a pretty general distro box bootup:

 [root@...ebaran ~]# uname -a
 Linux aldebaran 2.6.99-tip-01574-g6ba54c9-dirty #1 SMP Sun Nov 7 10:24:38 CET 2010 x86_64 x86_64 x86_64 GNU/Linux

?

Yes, some user-space might be unhappy if we set the version _back_ to say 2.4.0, but 
we could (as the patch below) fuzz up the version information from unprivileged 
attackers easily.

_Future_ ABI breakages that necessiate a version check are clearly frowned upon, so 
this patch could even be considered a debugging feature: it makes it harder to 
create ABI incompatibilities (at least for unprivileged user-space).

So you can think of version fuzzing also as the ultimate ABI check.

( This is a real defensive measure - here's a reason why attackers try stealth
  remote fingerprinting of a target system first: they really want to avoid 
  detection and knowing the exact OS and version of a target tells them which 
  attacks can be tried with a higher chance of success. Same goes for local attacks 
  as well.

  And once we have _that_, version fuzzing, removing kallsyms is one of the many 
  measures we need to use to hide the true version of the kernel from unprivileged 
  user-space. )

Thanks,

	Ingo

Index: linux/Makefile
===================================================================
--- linux.orig/Makefile
+++ linux/Makefile
@@ -1,7 +1,7 @@
 VERSION = 2
 PATCHLEVEL = 6
-SUBLEVEL = 37
-EXTRAVERSION = -rc1
+SUBLEVEL = 99
+EXTRAVERSION =
 NAME = Flesh-Eating Bats with Fangs

 # *DOCUMENTATION*
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