Re: [PATCH v4] seccomp: add ptrace options for suspend/resume

[Kees Cook]

On Fri, Jun 12, 2015 at 4:27 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Jun 10, 2015 at 1:18 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Wed, Jun 10, 2015 at 10:20 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Wed, Jun 10, 2015 at 9:31 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>>>> On 06/09, Andy Lutomirski wrote:
>>>>>
>>>>> On Tue, Jun 9, 2015 at 5:49 PM, Tycho Andersen
>>>>> >
>>>>> > @@ -556,6 +556,15 @@ static int ptrace_setoptions(struct task_struct *child, unsigned long data)
>>>>> >         if (data & ~(unsigned long)PTRACE_O_MASK)
>>>>> >                 return -EINVAL;
>>>>> >
>>>>> > +       if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {
>>>>
>>>> Well, we should do this if
>>>>
>>>>                         (data & O_SUSPEND) && !(flags & O_SUSPEND)
>>>>
>>>> or at least if
>>>>
>>>>                         (data ^ flags) & O_SUSPEND
>>>>
>>>>
>>>>> > +               if (!config_enabled(CONFIG_CHECKPOINT_RESTORE) ||
>>>>> > +                   !config_enabled(CONFIG_SECCOMP))
>>>>> > +                       return -EINVAL;
>>>>> > +
>>>>> > +               if (!capable(CAP_SYS_ADMIN))
>>>>> > +                       return -EPERM;
>>>>>
>>>>> I tend to think that we should also require that current not be using
>>>>> seccomp.  Otherwise, in principle, there's a seccomp bypass for
>>>>> privileged-but-seccomped programs.
>>>>
>>>> Andy, I simply can't understand why do we need any security check at all.
>>>>
>>>> OK, yes, in theory we can have a seccomped CAP_SYS_ADMIN process, seccomp
>>>> doesn't filter ptrace, you hack that process and force it to attach to
>>>> another CAP_SYS_ADMIN/seccomped process, etc, etc... Looks too paranoid
>>>> to me.
>>>
>>> I've sometimes considered having privileged processes I write fork and
>>> seccomp their child.  Of course, if you're allowing ptrace through
>>> your seccomp filter, you open a giant can of worms, but I think we
>>> should take the more paranoid approach to start and relax it later as
>>> needed.  After all, for the intended use of this patch, stuff will
>>> break regardless of what we do if the ptracer is itself seccomped.
>>>
>>> I could be convinced that if the ptracer is outside seccomp then we
>>> shouldn't need the CAP_SYS_ADMIN check.  That would at least make this
>>> work in a user namespace.
>>
>> But not if that namespace is running under a manager that has added a
>> seccomp filter to do things like drop finit_module, as lxc does.
>
> In that case, criu isn't going to handle seccomp right regardless of
> what our security check is, so I think we can safely deal with the
> security aspects of that case once we figure out the functionality
> part.
>
> IOW, I think I still like the direct "you must not be seccomped in
> order to suspend seccomp" rule.

Adding that restriction would be fine by me.

-Kees

-- 
Kees Cook
Chrome OS Security
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