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

[Kees Cook]

On Fri, Apr 21, 2017 at 4:28 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>> On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>> On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>>> On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni <tixxdz@xxxxxxxxx> 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