netdev - Re: [PATCH v2] net: don't allow CAP_NET_ADMIN to load non-netdev kernel modules

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Date:	Thu, 24 Mar 2011 14:03:53 -0400
From:	Eric Paris <eparis@...hat.com>
To:	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
Cc:	Eric Paris <eparis@...isplace.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	linux-kernel@...r.kernel.org, mjt@....msk.ru, arnd@...db.de,
	mirqus@...il.com, netdev@...r.kernel.org,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>, kuznet@....inr.ac.ru,
	pekkas@...core.fi, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
	kaber@...sh.net, eric.dumazet@...il.com, therbert@...gle.com,
	xiaosuo@...il.com, jesse@...ira.com, kees.cook@...onical.com,
	eugene@...hat.com, dan.j.rosenberg@...il.com,
	akpm@...ux-foundation.org, Greg KH <greg@...ah.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	LSM List <linux-security-module@...r.kernel.org>,
	Daniel J Walsh <dwalsh@...hat.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v2] net: don't allow CAP_NET_ADMIN to load non-netdev
 kernel modules

On Thu, 2011-03-24 at 10:37 -0500, Serge E. Hallyn wrote:
> Quoting Eric Paris (eparis@...isplace.org):
> > On Tue, Mar 1, 2011 at 4:33 PM, Vasiliy Kulikov <segoon@...nwall.com> wrote:
> ...
> > This patch is causing a bit of a problem in Fedora.  The problem lies
> 
> Sorry, what exactly is the problem it is causing?  I gather it's
> spitting out printks?  What exactly do the printks say?  The patch
> included at bottom checks for CAP_NET_ADMIN before checking for
> CAP_SYS_MODULE, so these must be cases which historically always
> quietly failed, and are now hitting the 'pr_err' which this patch
> adds?

Not quite.  SELinux logs every time an operation is denied.  This patch
means that every time a module is requested which does not exist as
netdev-* we check CAP_SYS_MODULE.  SELinux does not allow CAP_SYS_MODULE
and thus we get SELinux complaining that tasks are trying to load
modules.  I do have one report from a user who claims this is breaking
his system, but I'm not sure I believe him as I have yet to see any
dmesg printk from the pr_err.

On my local system reproduce the SELinux denials on every boot as
something tries to autoload "reg", "wifi0", and "virbr0".  I have no
modules which match these.  Thus the first try for CAP_NET_ADMIN
+netdev-wifi0 fails.  We then hit the CAP_SYS_MODULE check which SELinux
rejects and puts up a huge warning that someone is trying to load code
into the kernel.  Big red flags.  Even in permissive, where the
capable(CAP_SYS_MODULE) passes, we won't hit the pr_err() since there is
not module for "wifi"

I think there are 3 possibilities:

Change SELinux policy so as to not complain when udev/NM/libvirt try to
check CAP_SYS_MODULE, but that's a bad idea, since if they every try to
use init_module(2) we won't get denials.

Change this callsite to a _noaudit check.  Which is better than above
but still not great since we wouldn't get a denial log if anybody had
tried to load xfs....

Figure out a way to stop the calls to "reg" "wifi0" and "virbr0" if they
don't exist.

I feel like the last one is the best way, but I don't know what a
solution could look like....

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