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From: Kees Cook <keescook@chromium.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>,
	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 <zen>
Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction
Date: Fri, 21 Apr 2017 16:40:37 -0700	[thread overview]
Message-ID: <CAGXu5jJTdL7To2JsQUXyg6B7Xbb0kow6sXH-L+teVi88=gm7MQ@mail.gmail.com> (raw)
In-Reply-To: <CALCETrVZ2KH4s2uGW2J3zUQ8RNsrq=N7pJP=YDqd=+cp6fJW=g@mail.gmail.com>

On Fri, Apr 21, 2017 at 4:28 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook <keescook@chromium.org> wrote:
>> 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
>
> Well, that's interesting.  That should presumably be reset on setuid
> exec or something.
>
>> PR_SET_THP_DISABLE
>
> Um.  At least that's just a performance issue.
>
>>
>> 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.
>>
>
> The list is just going to get longer.  We should probably have controls for:
>
>  - Use of perf.  Unclear how fine grained they should be.

This can already be "given up" by a process by using seccomp. The
system-wide setting is what's missing here, and that's a whole other
thread already even though basically every distro has implemented the
= 3 sysctl knob level.

>  - Creation of new user namespaces.  Possibly also use of things like
> iptables without global privilege.

This is another one that can be controlled by seccomp. The system-wide
setting already exists in /proc/sys/user/max_user_namespaces.

>  - Ability to look up tasks owned by different uids (or maybe other
> tasks *at all*) by pid/tid.  Conceptually, this is easy.  The API is
> the only hard part, I think.

The attack surface here is relatively small compared to the other examples.

>  - Ability to bind ports, maybe?

seccomp and maybe a sysctl? I'd have to look at that more carefully,
but again, this isn't a comparable attack-surface/confinement issue.

> My point is that all of these need some way to handle configuration
> and inheritance, and I don't think that a bunch of per-task prctls is
> the right way.  As just an example, saying that interactive users can
> autoload modules but other users can't, or that certain systemd
> services can, etc, might be nice.  Linus already complained that he
> (i.e. user "torvalds" or whatever) should be able to profile the
> kernel but that other uids should not be able to.
>
> I personally like my implicit_rights idea, and it might be interesting
> to prototype it.

I don't like blocking a needed feature behind a large super-feature
that doesn't exist yet. We'd be able to refactor this code into using
such a thing in the future, so I'd prefer to move ahead with this
since it would stop actual exploits.

-Kees

-- 
Kees Cook
Pixel Security

  reply	other threads:[~2017-04-21 23:40 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-19 22:20 [PATCH v3 0/2] modules:capabilities: automatic module loading restrictions Djalal Harouni
2017-04-19 22:20 ` [PATCH v3 1/2] modules:capabilities: automatic module loading restriction Djalal Harouni
2017-04-19 23:16   ` Andy Lutomirski
2017-04-20  2:22   ` Ben Hutchings
2017-04-20 12:44     ` [kernel-hardening] " Djalal Harouni
2017-04-20 15:02       ` Ben Hutchings
     [not found]         ` <1492700543.31767.23.camel-/+tVBieCtBitmTQ+vhA3Yw@public.gmane.org>
2017-04-20 20:39           ` [kernel-hardening] " Djalal Harouni
     [not found]             ` <CAEiveUdFL53XyQpacmN6f8F28M0bLQDcetpRXJjrJ10vDmQi8Q-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-20 21:28               ` Kees Cook
2017-04-19 22:20 ` [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction Djalal Harouni
2017-04-19 23:15   ` Andy Lutomirski
2017-04-19 23:43     ` Kees Cook
2017-04-20  2:41       ` Andy Lutomirski
     [not found]         ` <CALCETrUueOx1tqj+Ru93KGpy2HHR-A_GQ6DrAppiomkPTtX7Lw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-21 23:19           ` Kees Cook
2017-04-21 23:28             ` Andy Lutomirski
2017-04-21 23:40               ` Kees Cook [this message]
2017-04-21 23:51                 ` Andy Lutomirski
2017-04-22  0:12                   ` Djalal Harouni
     [not found]                     ` <CAEiveUcx8fwQgXdLPeMNsTjX2KPhQKH__a-XzcHko_1aCmh4sg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-22  1:19                       ` Djalal Harouni
2017-04-22  6:51                       ` Andy Lutomirski
     [not found]                         ` <CALCETrUT73CcPQx2T=1zWbOUhw9r-c_YqXw5-KTwxgWPgXuTwA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-22 19:29                           ` Kees Cook
     [not found]                             ` <CAGXu5jLV+WZyj+xnxVFkFEgEthNt6eXdcSgHT-=85mJ1ECZ1Rw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-24 14:25                               ` Djalal Harouni
2017-04-24 18:02                                 ` Kees Cook
     [not found]                                   ` <CAGXu5jL_-cxidy_O4ORaN0iX9o7=hsi3DYTRvQs5w5363Z+MVg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-04-24 18:35                                     ` Djalal Harouni
2017-04-21 23:52               ` Casey Schaufler
2017-04-22  0:00                 ` Andy Lutomirski
2017-04-22  0:13                   ` Casey Schaufler
2017-04-22  6:45                     ` Andy Lutomirski
2017-04-22 12:17               ` Djalal Harouni
     [not found]                 ` <CAEiveUdbQcfn1xC5xWMv91vL_uR1MGTvARqw-E4GDTMUZ6t=bA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-05-04 13:07                   ` Djalal Harouni
2017-05-04 14:58                     ` Serge E. Hallyn
2017-05-05 13:06                       ` Djalal Harouni
2017-05-05 16:18                     ` Andy Lutomirski
2017-04-20  1:57   ` kbuild test robot
     [not found]   ` <1492640420-27345-3-git-send-email-tixxdz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2017-04-19 22:38     ` Djalal Harouni
2017-04-24  4:29     ` Rusty Russell
2017-04-26  9:06       ` Djalal Harouni
2017-04-27  2:07         ` Rusty Russell
     [not found]           ` <87k266hacq.fsf-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
2017-04-27 13:16             ` Djalal Harouni

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