Message-ID: <20190329120403.GA30213@hmswarspite.think-freely.org>
Date:   Fri, 29 Mar 2019 10:50:17 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        netfilter-devel@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Simo Sorce <simo@...hat.com>, netdev@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Eric Paris <eparis@...isplace.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH ghak90 V5 09/10] audit: add support for containerid to
 network namespaces

On Thu, Mar 28, 2019 at 05:40:23PM -0400, Richard Guy Briggs wrote:
> On 2019-03-28 11:46, Paul Moore wrote:
> > On Wed, Mar 27, 2019 at 9:12 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > >
> > > On 2019-03-27 23:42, Ondrej Mosnacek wrote:
> > > > On Fri, Mar 15, 2019 at 7:35 PM Richard Guy Briggs <rgb@...hat.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@...hat.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.
> 
> Ok, from the above it isn't clear to me if you are happy with the
> current code or would prefer any changes, or from below that you still
> need to work it through to make a pronouncement.  It sounds to me you
> would be ok with *either* spinlock *or* refcount_t, but don't see the
> need for both.
> 
I'll reiterate I think we should keep the refcount type just as it is, not for
safetys sake, but for readability and convienience.

Because the refcount currently only is used on add and delete operations
(implying it is only read in paths where its also modified), we need to
guarantee atomicity against multiple parallel writes.  We already have that
guarantee because every path in which we call
refcount_set/refcount_inc/refcount_dec_and_test occurs under the protection of
the list spin lock, and so from that standpoint we don't need the additional
guarantees offered by the refcount_t type.

However, if it were to be converted to an int type, we would have to replace the
refcount_dec_and_test call with this:
if (x == 0)
   warn_on_underflow
   return
x -= 1;
if (x == 0)
   preform_operations_to_free_list_entry

I find refcount_dec_and_test to be far easier to read and maintain, and you
still have to do all of this under the protection of a spin lock, to protect
against multiple writes.  And if you ever find that you are adding a pure read
side query of the refcount, you would need to hold the spin lock there as well,
instead of just using the available refcount_read api call

Yeah, you would save a few cycles if you didn't use an atomic type here, but
we're only talking about paths from user space making system calls executing
here (no high volume per packet receive paths or anything), and these paths have
already taken a few locks (the list lock, the task lock, etc), so eliminating
this one atomic isn't going to amount to anything.  Lets leave it as it is and
buy ourselves the extra code readability.

Neil

> > 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
> 
> - RGB
> 
> --
> Richard Guy Briggs <rgb@...hat.com>
> 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
> 
> 

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