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Subject: Re: [RESEND RFC] translate_pid API
To:     Jann Horn <jannh@google.com>
Cc:     kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Nagarajan.Muthukrishnan@oracle.com,
        Prakash Sangappa <prakash.sangappa@oracle.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Eugene Syromiatnikov <esyr@redhat.com>, xemul@parallels.com
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From:   Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>
Message-ID: <65625f8c-d701-7407-3999-6ca30a59c236@oracle.com>
Date:   Tue, 13 Mar 2018 16:52:40 -0700
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On 03/13/2018 04:10 PM, Jann Horn wrote:
> On Tue, Mar 13, 2018 at 3:45 PM, Nagarathnam Muthusamy
> <nagarathnam.muthusamy@oracle.com> wrote:
>> On 03/13/2018 03:00 PM, Jann Horn wrote:
>>> On Tue, Mar 13, 2018 at 2:44 PM, Nagarathnam Muthusamy
>>> <nagarathnam.muthusamy@oracle.com> wrote:
>>>> On 03/13/2018 02:28 PM, Jann Horn wrote:
>>>>> On Tue, Mar 13, 2018 at 2:20 PM, Nagarathnam Muthusamy
>>>>> <nagarathnam.muthusamy@oracle.com> wrote:
>>>>>> On 03/13/2018 01:47 PM, Jann Horn wrote:
>>>>>>> On Mon, Mar 12, 2018 at 10:18 AM,  <nagarathnam.muthusamy@oracle.com>
>>>>>>> wrote:
> [...]
>>>>>>>> + */
>>>>>>>> +SYSCALL_DEFINE3(translate_pid, pid_t, pid, u64, source,
>>>>>>>> +               u64, target)
>>>>>>>> +{
>>>>>>>> +       struct pid_namespace *source_ns = NULL, *target_ns = NULL;
>>>>>>>> +       struct pid *struct_pid;
>>>>>>>> +       struct pid_namespace *ph;
>>>>>>>> +       struct hlist_bl_head *shead = NULL;
>>>>>>>> +       struct hlist_bl_head *thead = NULL;
>>>>>>>> +       struct hlist_bl_node *dup_node;
>>>>>>>> +       pid_t result;
>>>>>>>> +
>>>>>>>> +       if (!source) {
>>>>>>>> +               source_ns = &init_pid_ns;
>>>>>>>> +       } else {
>>>>>>>> +               shead = pid_ns_hash_head(pid_ns_hash, source);
>>>>>>>> +               hlist_bl_lock(shead);
>>>>>>>> +               hlist_bl_for_each_entry(ph, dup_node, shead, node) {
>>>>>>>> +                       if (source == ph->ns.ns_id) {
>>>>>>>> +                               source_ns = ph;
>>>>>>>> +                               break;
>>>>>>>> +                       }
>>>>>>>> +               }
>>>>>>>> +               if (!source_ns) {
>>>>>>>> +                       hlist_bl_unlock(shead);
>>>>>>>> +                       return -EINVAL;
>>>>>>>> +               }
>>>>>>>> +       }
>>>>>>>> +       if (!ptrace_may_access(source_ns->child_reaper,
>>>>>>>> +                              PTRACE_MODE_READ_FSCREDS)) {
>>>>>>> AFAICS this proposal breaks the visibility restrictions that
>>>>>>> namespaces normally create. If there are two namespaces-based
>>>>>>> containers that use the same UID range, I don't think they should be
>>>>>>> able to learn information about each other, such as which PIDs are in
>>>>>>> use in the other container; but as far as I can tell, your proposal
>>>>>>> makes it possible to do that (unless an LSM or so is interfering). I
>>>>>>> would prefer it if this API required visibility of the targeted PID
>>>>>>> namespaces in the caller's PID namespace.
>>>>>>
>>>>>> I am trying to simulate the same access restrictions allowed
>>>>>> on a process's /proc/<pid>/ns/pid file. If the translator has
>>>>>> access to /proc/<pid>/ns/pid file of both source and destination
>>>>>> namespaces, shouldn't it be allowed to translate the pid between
>>>>>> them?
>>>>> But the translator doesn't actually need to have access to those
>>>>> procfs files, right?
>>>> I thought it should have access to those procfs files to satisfy the
>>>> visibility constraint that targeted PID namespaces should be visible
>>>> in caller's PID namespace and ptrace_may_access checks that
>>>> constraint.
>>> If there are two containers that use the same UID range,
>>> ptrace_may_access() checks from a process in one container on a
>>> process in another container can pass. Normally, you just can't even
>>> reach the ptrace_may_access() checks because you can't reference
>>> processes in another container in any way.
>>
>> If there is no way to reference the process in another container,
>> there is no way to get to the /proc/<pid>/ns/pidns_id file which
>> exports the ID of that container right? So, a translator has to
>> first guess the container ID then try translate. Even after translation,
>> unless the translator has proper privileges, I believe it cant do
>> anything with just the pid right?
> Well, yes to both. You'd have to guess the ID of the container, and
> you wouldn't be able to do much with it, apart from finding valid PIDs
> and their mapping between namespaces.
>
>>> By the way, a related concern: The use of global identifiers will
>>> probably also negatively affect Checkpoint/Restore In Userspace?
>> Will look into this. Can you point me to the specifics of the
>> usecase which could be negatively affected?
> AFAICS you won't be able to reliably recreate namespace IDs when a
> process is checkpointed and resumed, meaning that checkpoint/resume
> won't work on processes that use these namespace IDs.
I agree. When the process is resumed, the namespace IDs
might be obsolete.

Thanks,
Nagarathnam.