Re: [PATCH ghak90 V9 11/13] audit: contid check descendancy and nesting

[Paul Moore]

On Sat, Jun 27, 2020 at 9:23 AM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>
> Require the target task to be a descendant of the container
> orchestrator/engine.
>
> You would only change the audit container ID from one set or inherited
> value to another if you were nesting containers.
>
> If changing the contid, the container orchestrator/engine must be a
> descendant and not same orchestrator as the one that set it so it is not
> possible to change the contid of another orchestrator's container.
>
> Since the task_is_descendant() function is used in YAMA and in audit,
> remove the duplication and pull the function into kernel/core/sched.c
>
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> ---
>  include/linux/sched.h    |  3 +++
>  kernel/audit.c           | 23 +++++++++++++++++++++--
>  kernel/sched/core.c      | 33 +++++++++++++++++++++++++++++++++
>  security/yama/yama_lsm.c | 33 ---------------------------------
>  4 files changed, 57 insertions(+), 35 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2213ac670386..06938d0b9e0c 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2047,4 +2047,7 @@ static inline void rseq_syscall(struct pt_regs *regs)
>
>  const struct cpumask *sched_trace_rd_span(struct root_domain *rd);
>
> +extern int task_is_descendant(struct task_struct *parent,
> +                             struct task_struct *child);
> +
>  #endif
> diff --git a/kernel/audit.c b/kernel/audit.c
> index a862721dfd9b..efa65ec01239 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -2713,6 +2713,20 @@ int audit_signal_info(int sig, struct task_struct *t)
>         return audit_signal_info_syscall(t);
>  }
>
> +static bool audit_contid_isnesting(struct task_struct *tsk)
> +{
> +       bool isowner = false;
> +       bool ownerisparent = false;
> +
> +       rcu_read_lock();
> +       if (tsk->audit && tsk->audit->cont) {
> +               isowner = current == tsk->audit->cont->owner;
> +               ownerisparent = task_is_descendant(tsk->audit->cont->owner, current);

I want to make sure I'm understanding this correctly and I keep
mentally tripping over something: it seems like for a given audit
container ID a task is either the owner or a descendent, there is no
third state, is that correct?

Assuming that is true, can the descendent check simply be a negative
owner check given they both have the same audit container ID?

> +       }
> +       rcu_read_unlock();
> +       return !isowner && ownerisparent;
> +}
> +
>  /*
>   * audit_set_contid - set current task's audit contid
>   * @task: target task
> @@ -2755,8 +2769,13 @@ int audit_set_contid(struct task_struct *task, u64 contid)
>                 rc = -EBUSY;
>                 goto unlock;
>         }
> -       /* if contid is already set, deny */
> -       if (audit_contid_set(task))
> +       /* if task is not descendant, block */
> +       if (task == current || !task_is_descendant(current, task)) {

I'm also still fuzzy on why we can't let a task set it's own audit
container ID, assuming it meets all the criteria established in patch
2/13.  It somewhat made sense when you were tracking inherited vs
explicitly set audit container IDs, but that doesn't appear to be the
case so far in this patchset, yes?

> +               rc = -EXDEV;

I'm fairly confident we had a discussion about not using all these
different error codes, but that may be a moot point given my next
comment.

> +               goto unlock;
> +       }
> +       /* only allow contid setting again if nesting */
> +       if (audit_contid_set(task) && !audit_contid_isnesting(task))
>                 rc = -EEXIST;

It seems like what we need in audit_set_contid() is a check to ensure
that the task being modified is only modified by the owner of the
audit container ID, yes?  If so, I would think we could do this quite
easily with the following, or similar logic, (NOTE: assumes both
current and tsk are properly setup):

  if ((current->audit->cont != tsk->audit->cont) ||
(current->audit->cont->owner != current))
    return -EACCESS;

This is somewhat independent of the above issue, but we may also want
to add to the capability check.  Patch 2 adds a
"capable(CAP_AUDIT_CONTROL)" which is good, but perhaps we also need a
"ns_capable(CAP_AUDIT_CONTROL)" to allow a given audit container ID
orchestrator/owner the ability to control which of it's descendants
can change their audit container ID, for example:

  if (!capable(CAP_AUDIT_CONTROL) ||
      !ns_capable(current->nsproxy->user_ns, CAP_AUDIT_CONTROL))
    return -EPERM;

--
paul moore
www.paul-moore.com