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

[Andy Lutomirski]

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.

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

 - 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.

 - Ability to bind ports, maybe?

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.