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Message-ID: <CALCETrVeq7HA5KE10DqsL_R49V7g7Fw=3fZgeLb-20v+zitNYA@mail.gmail.com>
Date: Fri, 21 Apr 2017 23:45:29 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Casey Schaufler <casey@...aufler-ca.com>
Cc: Andy Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.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>,
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 5:13 PM, Casey Schaufler <casey@...aufler-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@...aufler-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@...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.
>>>>
>>>> - 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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