Re: [RESEND RFC] translate_pid API

[Nagarathnam Muthusamy]

On 03/13/2018 03:00 PM, Jann Horn wrote:
> On Tue, Mar 13, 2018 at 2:44 PM, Nagarathnam Muthusamy
> <nagarathnam.muthusamy@xxxxxxxxxx> wrote:
> > On 03/13/2018 02:28 PM, Jann Horn wrote:
> > > On Tue, Mar 13, 2018 at 2:20 PM, Nagarathnam Muthusamy
> > > <nagarathnam.muthusamy@xxxxxxxxxx> wrote:
> > > > On 03/13/2018 01:47 PM, Jann Horn wrote:
> > > > > On Mon, Mar 12, 2018 at 10:18 AM,  <nagarathnam.muthusamy@xxxxxxxxxx>
> > > > > 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.