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Date: Fri, 21 Apr 2017 16:40:37 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...capital.net>
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 Fri, Apr 21, 2017 at 4:28 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>> 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.
>>
>> (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
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