Re: [RESEND RFC] translate_pid API

[Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>]

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.

Thanks,
Nagarathnam.