Re: [RFC PATCH ghak32 V2 01/13] audit: add container id

[Richard Guy Briggs]

On 2018-04-24 15:01, Paul Moore wrote:
> On Mon, Apr 23, 2018 at 10:02 PM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> > On 2018-04-23 19:15, Paul Moore wrote:
> >> On Sat, Apr 21, 2018 at 10:34 AM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> >> > On 2018-04-18 19:47, Paul Moore wrote:
> >> >> On Fri, Mar 16, 2018 at 5:00 AM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> >> >> > Implement the proc fs write to set the audit container ID of a process,
> >> >> > emitting an AUDIT_CONTAINER record to document the event.
> >> >> >
> >> >> > This is a write from the container orchestrator task to a proc entry of
> >> >> > the form /proc/PID/containerid where PID is the process ID of the newly
> >> >> > created task that is to become the first task in a container, or an
> >> >> > additional task added to a container.
> >> >> >
> >> >> > The write expects up to a u64 value (unset: 18446744073709551615).
> >> >> >
> >> >> > This will produce a record such as this:
> >> >> > type=CONTAINER msg=audit(1519903238.968:261): op=set pid=596 uid=0 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 auid=0 tty=pts0 ses=1 opid=596 old-contid=18446744073709551615 contid=123455 res=0
> >> >> >
> >> >> > The "op" field indicates an initial set.  The "pid" to "ses" fields are
> >> >> > the orchestrator while the "opid" field is the object's PID, the process
> >> >> > being "contained".  Old and new container ID values are given in the
> >> >> > "contid" fields, while res indicates its success.
> >> >> >
> >> >> > It is not permitted to self-set, unset or re-set the container ID.  A
> >> >> > child inherits its parent's container ID, but then can be set only once
> >> >> > after.
> >> >> >
> >> >> > See: https://github.com/linux-audit/audit-kernel/issues/32
> >> >> >
> >> >> > Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> >> >> > ---
> >> >> >  fs/proc/base.c             | 37 ++++++++++++++++++++
> >> >> >  include/linux/audit.h      | 16 +++++++++
> >> >> >  include/linux/init_task.h  |  4 ++-
> >> >> >  include/linux/sched.h      |  1 +
> >> >> >  include/uapi/linux/audit.h |  2 ++
> >> >> >  kernel/auditsc.c           | 84 ++++++++++++++++++++++++++++++++++++++++++++++
> >> >> >  6 files changed, 143 insertions(+), 1 deletion(-)
> 
> ...
> 
> >> >> >  /* audit_rule_data supports filter rules with both integer and string
> >> >> >   * fields.  It corresponds with AUDIT_ADD_RULE, AUDIT_DEL_RULE and
> >> >> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> >> >> > index 4e0a4ac..29c8482 100644
> >> >> > --- a/kernel/auditsc.c
> >> >> > +++ b/kernel/auditsc.c
> >> >> > @@ -2073,6 +2073,90 @@ int audit_set_loginuid(kuid_t loginuid)
> >> >> >         return rc;
> >> >> >  }
> >> >> >
> >> >> > +static int audit_set_containerid_perm(struct task_struct *task, u64 containerid)
> >> >> > +{
> >> >> > +       struct task_struct *parent;
> >> >> > +       u64 pcontainerid, ccontainerid;
> >> >> > +
> >> >> > +       /* Don't allow to set our own containerid */
> >> >> > +       if (current == task)
> >> >> > +               return -EPERM;
> >> >>
> >> >> Why not?  Is there some obvious security concern that I missing?
> >> >
> >> > We then lose the distinction in the AUDIT_CONTAINER record between the
> >> > initiating PID and the target PID.  This was outlined in the proposal.
> >>
> >> I just went back and reread the v3 proposal and I still don't see a
> >> good explanation of this.  Why is this bad?  What's the security
> >> concern?
> >
> > I don't remember, specifically.  Maybe this has been addressed by the
> > check for children/threads or identical parent container ID.  So, I'm
> > reluctantly willing to remove that check for now.
> 
> Okay.  For the record, if someone can explain to me why this
> restriction saves us from some terrible situation I'm all for leaving
> it.  I'm just opposed to restrictions without solid reasoning behind
> them.
> 
> >> > Having said that, I'm still not sure we have protected sufficiently from
> >> > a child turning around and setting it's parent's as yet unset or
> >> > inherited audit container ID.
> >>
> >> Yes, I believe we only want to let a task set the audit container for
> >> it's children (or itself/threads if we decide to allow that, see
> >> above).  There *has* to be a function to check to see if a task if a
> >> child of a given task ... right? ... although this is likely to be a
> >> pointer traversal and locking nightmare ... hmmm.
> >
> > Isn't that just (struct task_struct)parent == (struct
> > task_struct)child->parent (or ->real_parent)?
> >
> > And now that I say that, it is covered by the following patch's child
> > check, so as long as we keep that, we should be fine.
> 
> I was thinking of checking not just current's immediate children, but
> any of it's descendants as I believe that is what we want to limit,
> yes?  I just worry that it isn't really practical to perform that
> check.

The child check I'm talking about prevents setting a task's audit
container ID if it *has* any children or threads, so if it has children
it is automatically disqualified and its grandchildren are irrelevant.

> >> >> I ask because I suppose it might be possible for some container
> >> >> runtime to do a fork, setup some of the environment and them exec the
> >> >> container (before you answer the obvious "namespaces!" please remember
> >> >> we're not trying to define containers).
> >> >
> >> > I don't think namespaces have any bearing on this concern since none are
> >> > required.
> >> >
> >> >> > +       /* Don't allow the containerid to be unset */
> >> >> > +       if (!cid_valid(containerid))
> >> >> > +               return -EINVAL;
> >> >> > +       /* if we don't have caps, reject */
> >> >> > +       if (!capable(CAP_AUDIT_CONTROL))
> >> >> > +               return -EPERM;
> >> >> > +       /* if containerid is unset, allow */
> >> >> > +       if (!audit_containerid_set(task))
> >> >> > +               return 0;
> >> >> > +       /* it is already set, and not inherited from the parent, reject */
> >> >> > +       ccontainerid = audit_get_containerid(task);
> >> >> > +       rcu_read_lock();
> >> >> > +       parent = rcu_dereference(task->real_parent);
> >> >> > +       rcu_read_unlock();
> >> >> > +       task_lock(parent);
> >> >> > +       pcontainerid = audit_get_containerid(parent);
> >> >> > +       task_unlock(parent);
> >> >> > +       if (ccontainerid != pcontainerid)
> >> >> > +               return -EPERM;
> >> >> > +       return 0;
> 
> I'm looking at the parent checks again and I wonder if the logic above
> is what we really want.  Maybe it is, but I'm not sure.
> 
> Things I'm wondering about:
> 
> * "ccontainerid" and "containerid" are too close in name, I kept
> confusing myself when looking at this code.  Please change one.  Bonus
> points if it is shorter.

Would c_containerid and p_containerid be ok?  child_cid and parent_cid?
I'd really like it to have the same root as the parameter handed in so
teh code is easier to follow.  It would be nice to have that across
caller to local, but that's challenging.

I've been tempted to use contid or even cid everywhere instead of
containerid.  Perhaps the longer name doesn't bother me because I
like its uniqueness and I learned touch-typing in grade 9 and I like
100+ character wide terminals?  ;-)

> * What if the orchestrator wants to move the task to a new container?
> Right now it looks like you can only do that once, then then the
> task's audit container ID will no longer be the same as real_parent

A task's audit container ID can be unset or inherited, and then set
only once.  After that, if you want it moved to a new container you
can't and your only option is to spawn another peer to that task or a
child of it and set that new task's audit container ID.

Currently, the method of detecting if its audit container ID has been
set (rather than inherited) was to check its parent's audit container
ID.  The only reason to change this might be if the audit container ID
were not inheritable, but then we lose the accountability of a task
spawning another process and being able to leave its child's audit
container ID unset and unaccountable to any existing container.  I think
the relationship to the parent is crucial, and if something wants to
change audit container ID it can, by spawning childrent and leaving a
trail of container IDs in its parent processes.  (So what if a parent
dies?)

> ... or does the orchestrator change that?  *Can* the orchestrator
> change real_parent (I suspect the answer is "no")?

I don't think the orchestrator is able to change real_parent.  I've
forgotten why there is a ->parent and ->real_parent and how they can
change.  One is for the wait signal.  I don't remember the purpose of
the other.

If the parent dies before the child, the child will be re-parented on
its grandparent if the parent doesn't hang around zombified, if I
understand correctly.  If anything, a parent dying would likely further
restrict the ability to set a task's audit container ID because a parent
with an identical ID could vanish.

> * I think the key is the relationship between current and task, not
> between task and task->real_parent.  I believe what we really care
> about is that task is a descendant of current.  We might also want to
> allow current to change the audit container ID if it holds
> CAP_AUDIT_CONTROL, regardless of it's relationship with task.

Currently, a process with CAP_AUDIT_CONTROL can set the audit container
ID of any task that hasn't got children or threads, isn't itself, and
its audit container ID is inherited or unset.  This was to try to
prevent games with parents and children scratching each other's backs.

I would feel more comfortable if only descendants were settable, so
adding that restriction sounds like a good idea to me other than the
tree-climbing excercise and overhead involved.

> >> >> > +static void audit_log_set_containerid(struct task_struct *task, u64 oldcontainerid,
> >> >> > +                                     u64 containerid, int rc)
> >> >> > +{
> >> >> > +       struct audit_buffer *ab;
> >> >> > +       uid_t uid;
> >> >> > +       struct tty_struct *tty;
> >> >> > +
> >> >> > +       if (!audit_enabled)
> >> >> > +               return;
> >> >> > +
> >> >> > +       ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_CONTAINER);
> >> >> > +       if (!ab)
> >> >> > +               return;
> >> >> > +
> >> >> > +       uid = from_kuid(&init_user_ns, task_uid(current));
> >> >> > +       tty = audit_get_tty(current);
> >> >> > +
> >> >> > +       audit_log_format(ab, "op=set pid=%d uid=%u", task_tgid_nr(current), uid);
> >> >> > +       audit_log_task_context(ab);
> >> >> > +       audit_log_format(ab, " auid=%u tty=%s ses=%u opid=%d old-contid=%llu contid=%llu res=%d",
> >> >> > +                        from_kuid(&init_user_ns, audit_get_loginuid(current)),
> >> >> > +                        tty ? tty_name(tty) : "(none)", audit_get_sessionid(current),
> >> >> > +                        task_tgid_nr(task), oldcontainerid, containerid, !rc);
> >> >> > +
> >> >> > +       audit_put_tty(tty);
> >> >> > +       audit_log_end(ab);
> >> >> > +}
> >> >> > +
> >> >> > +/**
> >> >> > + * audit_set_containerid - set current task's audit_context containerid
> >> >> > + * @containerid: containerid value
> >> >> > + *
> >> >> > + * Returns 0 on success, -EPERM on permission failure.
> >> >> > + *
> >> >> > + * Called (set) from fs/proc/base.c::proc_containerid_write().
> >> >> > + */
> >> >> > +int audit_set_containerid(struct task_struct *task, u64 containerid)
> >> >> > +{
> >> >> > +       u64 oldcontainerid;
> >> >> > +       int rc;
> >> >> > +
> >> >> > +       oldcontainerid = audit_get_containerid(task);
> >> >> > +
> >> >> > +       rc = audit_set_containerid_perm(task, containerid);
> >> >> > +       if (!rc) {
> >> >> > +               task_lock(task);
> >> >> > +               task->containerid = containerid;
> >> >> > +               task_unlock(task);
> >> >> > +       }
> >> >> > +
> >> >> > +       audit_log_set_containerid(task, oldcontainerid, containerid, rc);
> >> >> > +       return rc;
> >> >>
> >> >> Why are audit_set_containerid_perm() and audit_log_containerid()
> >> >> separate functions?
> >> >
> >> > (I assume you mean audit_log_set_containerid()?)
> >>
> >> Yep.  My fingers got tired typing in that function name and decided a
> >> shortcut was necessary.
> >>
> >> > It seemed clearer that all the permission checking was in one function
> >> > and its return code could be used to report the outcome when logging the
> >> > (attempted) action.  This is the same structure as audit_set_loginuid()
> >> > and it made sense.
> >>
> >> When possible I really like it when the permission checks are in the
> >> same function as the code which does the work; it's less likely to get
> >> abused that way (you have to willfully bypass the access checks).  The
> >> exceptions might be if you wanted to reuse the access control code, or
> >> insert a modular access mechanism (e.g. LSMs).
> >
> > I don't follow how it could be abused.  The return code from the perm
> > check gates setting the value and is used in the success field in the
> > log.
> 
> If the permission checks are in the same function body as the code
> which does the work you have to either split the function, or rewrite
> it, if you want to bypass the permission checks.  It may be more of a
> style issue than an actual safety issue, but the comments about
> single-use functions in the same scope is the tie breaker.

Perhaps I'm just being quite dense, but I just don't follow what the
problem is and how you suggest fixing it.  A bunch of gotos to a label
such as "out:" to log the refused action?  That seems messy and
unstructured.

> paul moore

- RGB

--
Richard Guy Briggs <rgb@xxxxxxxxxx>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635