linux-kernel - Re: [PATCH v3 1/2] modules:capabilities: automatic module loading restriction

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Date:   Thu, 20 Apr 2017 03:22:22 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Djalal Harouni <tixxdz@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>, serge@...lyn.com,
        kernel-hardening@...ts.openwall.com,
        linux-security-module@...r.kernel.org
Cc:     Linux API <linux-api@...r.kernel.org>,
        Dongsu Park <dpark@...teo.net>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <james.l.morris@...cle.com>,
        Paul Moore <paul@...l-moore.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>, Jessica Yu <jeyu@...hat.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, zendyani@...il.com,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/2] modules:capabilities: automatic module loading
 restriction

On Thu, 2017-04-20 at 00:20 +0200, Djalal Harouni wrote:
[...] 
> +modules_autoload:
> +
> +A sysctl to control if modules auto-load feature is allowed or not.
> +This sysctl complements "modules_disabled" which is for all module
> +operations where this flag applies only to automatic module loading.
> +Automatic module loading happens when programs request a kernel feature
> +that is implemented by an unloaded module, the kernel automatically
> +runs the program pointed by "modprobe" sysctl in order to load the
> +corresponding module.
> +
> +When modules_autoload is set to (0), the default, there are no
> +restrictions.
> +
> +When modules_autoload is set to (1), processes must have CAP_SYS_MODULE
> +to be able to trigger a module auto-load operation, or CAP_NET_ADMIN
> +for modules with a 'netdev-%s' alias.
> +
> +When modules_autoload is set to (2), automatic module loading is
> +disabled for all. Once set, this value can not be changed.

I would expect a parameter 'modules_autoload' to be a boolean, so this
behaviour would be surprising.

What is the point of mode 2?  Why would someone want to set
modules_disabled=0 and modules_autoload=2?

[...]
> --- a/kernel/module.c
> +++ b/kernel/module.c
[...]
> +static int modules_autoload_privileged_access(const char *name)
> +{
> +	if (capable(CAP_SYS_MODULE))
> +		return 0;
> +	else if (name && strstr(name, "netdev-") && capable(CAP_NET_ADMIN))
[...]

We want a prefix match, so use strncmp() not strstr().

Ben.

-- 
Ben Hutchings
It is easier to change the specification to fit the program than vice
versa.


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