Subject: Re: [kernel-hardening] [PATCH v4 next 0/3] modules: automatic module loading restrictions

From: Solar Designer <solar-cxoSlKxDwOJWk0Htik3J/[email-masked]>
Date: Mon, 22 May 2017 18:43:23 +0200

On Mon, May 22, 2017 at 03:49:15PM +0200, Djalal Harouni wrote:
> On Mon, May 22, 2017 at 2:08 PM, Solar Designer <solar-cxoSlKxDwOJWk0Htik3J/[email-masked]> wrote:
> > On Mon, May 22, 2017 at 01:57:03PM +0200, Djalal Harouni wrote:
> >> *) When modules_autoload_mode is set to (2), automatic module loading is
> >> disabled for all. Once set, this value can not be changed.
> >
> > What purpose does this securelevel-like property ("Once set, this value
> > can not be changed.") serve here?  I think this mode 2 is needed, but
> > without this extra property, which is bypassable by e.g. explicitly
> > loaded kernel modules anyway (and that's OK).
> 
> My reasoning about "Once set, this value can not be changed" is mainly for:
> 
> If you have some systems where modules are not updated for any given
> reason, then the only one who will be able to load a module is an
> administrator, basically this is a shortcut for:
> 
> * Apps/services can run with CAP_NET_ADMIN but they are not allowed to
> auto-load 'netdev' modules.
> 
> * Explicitly loading modules can be guarded by seccomp filters *per*
> app, so even if these apps have
>   CAP_SYS_MODULE they won't be able to explicitly load modules, one
> has to remount some sysctl /proc/ entries read-only here and remove
> CAP_SYS_ADMIN for all apps anyway.
> 
> This mainly serves the purpose of these systems that do not receive
> updates, if I don't want to expose those kernel interfaces what should
> I do ? then if I want to unload old versions and replace them with new
> ones what operation should be allowed ? and only real root of the
> system can do it. Hence, the "Once set, this value can not be changed"
> is more of a shortcut, also the idea was put in my mind based on how
> "modules_disabled" is disabled forever, and some other interfaces. I
> would say: it is easy to handle a transition from 1) "hey this system
> is still up to date, some features should be exposed" to 2) "this
> system is not up to date anymore, only root should expose some
> features..."
> 
> Hmm, I am not sure if this answers your question ? :-)

This answers my question, but in a way that I summarize as "there's no
good reason to include this securelevel-like property".

> I definitively don't want to fall into "modules_disabled" trap where
> is it too strict! "Once set, this value can not be changed" means for
> some users do not set it otherwise the system is unusable...
> 
> Maybe an extra "4" mode for that ? better get it right.

I think you should simply exclude this property from mode 2.

The module autoloading restrictions aren't meant to reduce root's
powers; they're only meant to protect processes from shooting themselves
and the system in the foot inadvertently (confused deputy).

modules_disabled may be different in that respect, although with the
rest of the kernel lacking securelevel-like support the point is moot.

We had working securelevel in 2.0.34 through 2.0.40 inclusive, but
we've lost it in 2.1+ with cap-bound apparently never becoming as
complete a replacement for it and having been lost/broken further in
2.6.25+.  I regret this, but that's a different story.  Like I say,
module autoloading doesn't even fit in with those restrictions - it's
about a totally different threat model.

Alexander