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From:   Jann Horn <jannh@google.com>
Date:   Tue, 13 Mar 2018 15:00:11 -0700
Message-ID: <CAG48ez05MDH7Wf1_j3qgbrqjEh-GXPLQHfx-Va_pyzvT5hP=gw@mail.gmail.com>
Subject: Re: [RESEND RFC] translate_pid API
To:     Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.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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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.

By the way, a related concern: The use of global identifiers will
probably also negatively affect Checkpoint/Restore In Userspace?