Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction

From: Kees Cook <keescook@chromium.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Djalal Harouni <tixxdz@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	LSM List <linux-security-module@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Dongsu Park <dpark@posteo.net>,
	Casey Schaufler <casey@schaufler-ca.com>,
	James Morris <james.l.morris@oracle.com>,
	Paul Moore <paul@paul-moore.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jonathan Corbet <corbet@lwn.net>, Jessica Yu <jeyu@redhat.com>,
	Rusty Russell <rusty@rustcorp.com.au>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	belakhdar abdeldjalil <zendyani@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction
Date: Fri, 21 Apr 2017 16:19:05 -0700	[thread overview]
Message-ID: <CAGXu5jJBp6VHvDP9182hJgvzS2sBUbUFpmZZ+196K2FcEHUxzw@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUueOx1tqj+Ru93KGpy2HHR-A_GQ6DrAppiomkPTtX7Lw@mail.gmail.com>

On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski <luto@kernel.org> wrote:
> On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski <luto@kernel.org> wrote:
>>> On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni <tixxdz@gmail.com> wrote:
>>>> +/* Sets task's modules_autoload */
>>>> +static inline int task_set_modules_autoload(struct task_struct *task,
>>>> +                                           unsigned long value)
>>>> +{
>>>> +       if (value > MODULES_AUTOLOAD_DISABLED)
>>>> +               return -EINVAL;
>>>> +       else if (task->modules_autoload > value)
>>>> +               return -EPERM;
>>>> +       else if (task->modules_autoload < value)
>>>> +               task->modules_autoload = value;
>>>> +
>>>> +       return 0;
>>>> +}
>>>
>>> This needs to be more locked down.  Otherwise someone could set this
>>> and then run a setuid program.  Admittedly, it would be quite odd if
>>> this particular thing causes a problem, but the issue exists
>>> nonetheless.
>>
>> Eeeh, I don't agree this needs to be changed. APIs provided by modules
>> are different than the existing privilege-manipulation syscalls this
>> concern stems from. Applications are already forced to deal with
>> things being missing like this in the face of it simply not being
>> built into the kernel.
>>
>> Having to hide this behind nnp seems like it'd reduce its utility...
>>
>
> I think that adding an inherited boolean to task_struct that can be
> set by unprivileged tasks and passed to privileged tasks is a terrible
> precedent.  Ideally someone would try to find all the existing things
> like this and kill them off.

(Tristate, not boolean, but yeah.)

I see two others besides seccomp and nnp:

PR_MCE_KILL
PR_SET_THP_DISABLE

I really don't think this needs nnp protection.

> I agree that I don't see how one would exploit this particular
> feature, but I still think I dislike the approach.  This is a slippery
> slope to adding a boolean for perf_event_open(), unshare(), etc, and
> we should solve these for real rather than half-arsing them IMO.

I disagree (obviously); this would be protecting the entire module
autoload attack surface. That's hardly a specific control, and it's a
demonstrably needed flag.

-Kees

-- 
Kees Cook
Pixel Security