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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: <df6ad381-db14-3ded-c724-a80565b549b2@oracle.com>
Date:   Tue, 13 Mar 2018 15:45:45 -0700
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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:
>>>>>> Resending the RFC with participants of previous discussions
>>>>>> in the list.
>>>>>>
>>>>>> Following patch which is a variation of a solution discussed
>>>>>> in https://lwn.net/Articles/736330/ provides the users of
>>>>>> pid namespace, the functionality of pid translation between
>>>>>> namespaces using a namespace identifier. The topic of
>>>>>> pid translation has been discussed in the community few times
>>>>>> but there has always been a resistance to adding new solution
>>>>>> for this problem.
>>>>>> I will outline the planned usecase of pid namespace by oracle
>>>>>> database and explain why any of the existing solution cannot
>>>>>> be used to solve their problem.
>>>>>>
>>>>>> Consider a system in which several PID namespaces with multiple
>>>>>> nested levels exists in parallel with monitor processes managing
>>>>>> all the namespaces. PID translation is required for controlling
>>>>>> and accessing information about the processes by the monitors
>>>>>> and other processes down the hierarchy of namespaces. Controlling
>>>>>> primarily involves sending signals or using ptrace by a process in
>>>>>> parent namespace on any of the processes in its child namespace.
>>>>>> Accessing information deals with the reading /proc/<pid>/* files
>>>>>> of processes in child namespace. None of the processes have
>>>>>> root/CAP_SYS_ADMIN privileges.
>>>>> How are you dealing with PID reuse?
>>>>
>>>> We have a monitor process which keeps track of the aliveness of
>>>> important processes. When a process dies, monitor makes a note of
>>>> it and hence detects if pid is reused.
>>> How do you do that in a race-free manner?
>>
>> AFAIK, the monitor runs periodically to check the aliveness of the processes
>> and this period is too short for pids to recycle. I will get back with more
>> information
>> on this if any other mechanisms are in place.
>>
>>
>>>
>>>>>> + */
>>>>>> +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?

>
> 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?

Thanks,
Nagarathnam.