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

[Andy Lutomirski]

On Fri, Apr 21, 2017 at 5:13 PM, Casey Schaufler <casey at schaufler-ca.com> wrote:
> On 4/21/2017 5:00 PM, Andy Lutomirski wrote:
>> On Fri, Apr 21, 2017 at 4:52 PM, Casey Schaufler <casey at schaufler-ca.com> wrote:
>>> On 4/21/2017 4:28 PM, Andy Lutomirski wrote:
>>>> On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook <keescook at chromium.org> wrote:
>>>>> On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski <luto at kernel.org> wrote:
>>>>>> On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook <keescook at chromium.org> wrote:
>>>>>>> On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski <luto at kernel.org> wrote:
>>>>>>>> On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni <tixxdz at 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.
>>>>
>>>>  - 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?
>>> One of my longer term (i.e. after stacking) projects
>>> is to create sensible access control on ports. Why shouldn't
>>> they have owners and mode bits (or ACLs, if you prefer)
>>> or real names. I kind of think we should be able to eliminate
>>> the need for dbus without resorting to kdbus.
>> My implicit_rights concept gives any type of access control you can
>> use on inodes because they *are* inodes.  So you get ACLs, etc.
>>
>> Brief summary for those who didn't read my old email: We add a new
>> kind of filesystem object called a "right".  It's a special kind of
>> socket inode that can't be bound or connected but is instead created
>> by a new syscall.  It has a name, so "port:1234" might be a name of a
>> right.
>>
>> To use an implicit right, you do whatever syscall you would do
>> normally.  The kernel looks for a right object at
>> /dev/implicit_rights/<name>.  If that object exists, is a right of the
>> correct type (i.e. the right's name matches <name>) and you have
>> execute access, you win.  Otherwise you lose.
>>
>> To avoid breaking existing distros, for things like modules_autoload,
>> you would set a sysctl
>> /proc/sys/kernel/required_implicit_rights/modules_autoload=1.  With
>> that set, to autoload a module without CAP_SYS_MODULE, you need the
>> /dev/implicit_rights/modules_autoload.
>
> Sounds good.
>
>>> So I don't like the idea of treating that as a special case.
>>> I'd rather see ports controlled properly. (Of course, the
>>> SELinux crowd will point out they have this handled, but I
>>> remain unconvinced of the overall solution)
>> Agreed.  But I think we should address all of these things together.
>
> What I don't want is to have to buy into a hundred things I
> don't want in order to get the one thing I do. A General mechanism
> is dandy, but I don't want to have to write a gazillion policy
> lines for features I don't want in order to get a simple control.
> The problem with SELinux is not the effort required to protect
> what you care about, it's the effort required to do everything else.
>

The point is to make it super simple.  chown, chmod and, if you want
to get fancy, setfacl.  You'll need a mkright tool, but that's
trivial.
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