Re: [PATCH RFC] Restrictions on module loading

[Kees Cook <kees.cook@canonical.com>]

On Sun, Nov 07, 2010 at 08:23:59PM -0500, Dan Rosenberg wrote:
> A significant portion of kernel vulnerabilities do not affect core code,
> but rather individual modules.  Unfortunately, there is no global kernel
> setting to restrict unprivileged users from triggering the automatic
> loading of kernel modules, for example by creating a socket using a
> packet family that is compiled as a module and not already loaded.  On
> most distributions, this creates a significant attack surface, and
> requires maintenance of blacklists and other inelegant solutions to a
> general problem.
> 
> The below patch replaces the existing "modules_disable" sysctl with a
> finer-grained "modules_restrict" sysctl.  By default, this is set at 0,
> which results in no deviation from normal module loading behavior.  When
> set to 1, unprivileged users cannot trigger the automatic loading of
> modules.  This behavior is based on grsecurity's GRKERNSEC_MODHARDEN
> setting.  The current check is against current_uid(), since several
> distributions explicitly remove CAP_SYS_MODULE from root processes, some
> of which incidentally cause (and rely on) the automatic loading of
> modules.  I expect this to be a point of discussion.
> 
> When set to 2, modules may not be loaded or unloaded by anyone, and the
> sysctl may not be changed from that point forward.  This is designed to
> provide protection against kernel module rootkits.
> 
> Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com>

Acked-by: Kees Cook <kees.cook@canonical.com>

This looks great to me. There will be a small amount of pain for people
that are already using modules_disabled=1, but I think the audience is so
small that it won't be a problem to switch to modules_restrict=2.

-Kees

-- 
Kees Cook
Ubuntu Security Team