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

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Message-ID: <CALCETrUueOx1tqj+Ru93KGpy2HHR-A_GQ6DrAppiomkPTtX7Lw@mail.gmail.com>
Date: Wed, 19 Apr 2017 19:41:44 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Andy Lutomirski <luto@...nel.org>, Djalal Harouni <tixxdz@...il.com>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, 
	LSM List <linux-security-module@...r.kernel.org>, 
	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>, 
	belakhdar abdeldjalil <zendyani@...il.com>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules
 autoload restriction

On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni <tixxdz@...il.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.

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.

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