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

* Willy Tarreau <w@1wt.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@aldebaran ~]# 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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