Re: [PATCH ghak90 V5 09/10] audit: add support for containerid to network namespaces

[Paul Moore <paul@paul-moore.com>]

On Wed, Mar 27, 2019 at 9:12 PM Richard Guy Briggs <rgb@redhat.com> wrote:
>
> On 2019-03-27 23:42, Ondrej Mosnacek wrote:
> > On Fri, Mar 15, 2019 at 7:35 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > Audit events could happen in a network namespace outside of a task
> > > context due to packets received from the net that trigger an auditing
> > > rule prior to being associated with a running task.  The network
> > > namespace could be in use by multiple containers by association to the
> > > tasks in that network namespace.  We still want a way to attribute
> > > these events to any potential containers.  Keep a list per network
> > > namespace to track these audit container identifiiers.
> > >
> > > Add/increment the audit container identifier on:
> > > - initial setting of the audit container identifier via /proc
> > > - clone/fork call that inherits an audit container identifier
> > > - unshare call that inherits an audit container identifier
> > > - setns call that inherits an audit container identifier
> > > Delete/decrement the audit container identifier on:
> > > - an inherited audit container identifier dropped when child set
> > > - process exit
> > > - unshare call that drops a net namespace
> > > - setns call that drops a net namespace
> > >
> > > See: https://github.com/linux-audit/audit-kernel/issues/92
> > > See: https://github.com/linux-audit/audit-testsuite/issues/64
> > > See: https://github.com/linux-audit/audit-kernel/wiki/RFE-Audit-Container-ID
> > > Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
> > > ---
> > >  include/linux/audit.h | 19 ++++++++++++
> > >  kernel/audit.c        | 86 +++++++++++++++++++++++++++++++++++++++++++++++++--
> > >  kernel/nsproxy.c      |  4 +++
> > >  3 files changed, 106 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > > index fa19fa408931..70255c2dfb9f 100644
> > > --- a/include/linux/audit.h
> > > +++ b/include/linux/audit.h
> > > @@ -27,6 +27,7 @@
> > >  #include <linux/ptrace.h>
> > >  #include <linux/namei.h>  /* LOOKUP_* */
> > >  #include <uapi/linux/audit.h>
> > > +#include <linux/refcount.h>
> > >
> > >  #define AUDIT_INO_UNSET ((unsigned long)-1)
> > >  #define AUDIT_DEV_UNSET ((dev_t)-1)
> > > @@ -99,6 +100,13 @@ struct audit_task_info {
> > >
> > >  extern struct audit_task_info init_struct_audit;
> > >
> > > +struct audit_contid {
> > > +       struct list_head        list;
> > > +       u64                     id;
> > > +       refcount_t              refcount;
> >
> > Hm, since we only ever touch the refcount under a spinlock, I wonder
> > if we could just make it a regular unsigned int (we don't need the
> > atomicity guarantees). OTOH, refcount_t comes with some extra overflow
> > checking, so it's probably better to leave it as is...
>
> Since the update is done using rcu-safe methods, do we even need the
> spin_lock?  Neil?  Paul?

As discussed, the refcount field is protected against simultaneous
writes by the spinlock that protects additions/removals from the list
as a whole so I don't believe the refcount_t atomicity is critical in
this regard.

Where it gets tricky, and I can't say I'm 100% confident on my answer
here, is if refcount was a regular int and we wanted to access it
outside of a spinlock (to be clear, it doesn't look like this patch
currently does this).  With RCU, if refcount was a regular int
(unsigned or otherwise), I believe it would be possible for different
threads of execution to potentially see different values of refcount
(assuming one thread was adding/removing from the list).  Using a
refcount_t would protect against this, alternatively, taking the
spinlock should also protect against this.

As we all know, RCU can be tricky at times, so I may be off on the
above; if I am, please provide an explanation so I (and likely others
as well) can learn a little bit more. :)

-- 
paul moore
www.paul-moore.com