Re: [PATCH v2] audit: use proper refcount locking on audit_sock

[Richard Guy Briggs <rgb@redhat.com>]

On 2016-12-12 15:58, Cong Wang wrote:
> On Mon, Dec 12, 2016 at 2:03 AM, Richard Guy Briggs <rgb@redhat.com> wrote:
> > Resetting audit_sock appears to be racy.
> >
> > audit_sock was being copied and dereferenced without using a refcount on
> > the source sock.
> >
> > Bump the refcount on the underlying sock when we store a refrence in
> > audit_sock and release it when we reset audit_sock.  audit_sock
> > modification needs the audit_cmd_mutex.
> >
> > See: https://lkml.org/lkml/2016/11/26/232
> >
> > Thanks to Eric Dumazet <edumazet@google.com> and Cong Wang
> > <xiyou.wangcong@gmail.com> on ideas how to fix it.
> >
> > Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
> > ---
> > There has been a lot of change in the audit code that is about to go
> > upstream to address audit queue issues.  This patch is based on the
> > source tree: git://git.infradead.org/users/pcmoore/audit#next
> > ---
> >  kernel/audit.c |   34 ++++++++++++++++++++++++++++------
> >  1 files changed, 28 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index f20eee0..439f7f3 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c
> > @@ -452,7 +452,9 @@ static void auditd_reset(void)
> >         struct sk_buff *skb;
> >
> >         /* break the connection */
> > +       sock_put(audit_sock);
> 
> Why audit_sock can't be NULL here?

Fixed.

> >         audit_pid = 0;
> > +       audit_nlk_portid = 0;
> >         audit_sock = NULL;
> >
> >         /* flush all of the retry queue to the hold queue */
> > @@ -478,6 +480,12 @@ static int kauditd_send_unicast_skb(struct sk_buff *skb)
> >         if (rc >= 0) {
> >                 consume_skb(skb);
> >                 rc = 0;
> > +       } else {
> > +               if (rc & (-ENOMEM|-EPERM|-ECONNREFUSED)) {
> 
> Are these errno's bits??

No, I've fixed this silly error.

> > +                       mutex_lock(&audit_cmd_mutex);
> > +                       auditd_reset();
> > +                       mutex_unlock(&audit_cmd_mutex);
> > +               }
> >         }
> >
> >         return rc;
> > @@ -579,7 +587,9 @@ static int kauditd_thread(void *dummy)
> >
> >                                 auditd = 0;
> >                                 if (AUDITD_BAD(rc, reschedule)) {
> > +                                       mutex_lock(&audit_cmd_mutex);
> >                                         auditd_reset();
> > +                                       mutex_unlock(&audit_cmd_mutex);
> >                                         reschedule = 0;
> >                                 }
> >                         } else
> > @@ -594,7 +604,9 @@ static int kauditd_thread(void *dummy)
> >                                 auditd = 0;
> >                                 if (AUDITD_BAD(rc, reschedule)) {
> >                                         kauditd_hold_skb(skb);
> > +                                       mutex_lock(&audit_cmd_mutex);
> >                                         auditd_reset();
> > +                                       mutex_unlock(&audit_cmd_mutex);
> >                                         reschedule = 0;
> >                                 } else
> >                                         /* temporary problem (we hope), queue
> > @@ -623,7 +635,9 @@ quick_loop:
> >                                 if (rc) {
> >                                         auditd = 0;
> >                                         if (AUDITD_BAD(rc, reschedule)) {
> > +                                               mutex_lock(&audit_cmd_mutex);
> >                                                 auditd_reset();
> > +                                               mutex_unlock(&audit_cmd_mutex);
> >                                                 reschedule = 0;
> >                                         }
> >
> > @@ -1004,17 +1018,22 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
> >                                 return -EACCES;
> >                         }
> >                         if (audit_pid && new_pid &&
> > -                           audit_replace(requesting_pid) != -ECONNREFUSED) {
> > +                           (audit_replace(requesting_pid) & (-ECONNREFUSED|-EPERM|-ENOMEM))) {
> >                                 audit_log_config_change("audit_pid", new_pid, audit_pid, 0);
> >                                 return -EEXIST;
> >                         }
> >                         if (audit_enabled != AUDIT_OFF)
> >                                 audit_log_config_change("audit_pid", new_pid, audit_pid, 1);
> > -                       audit_pid = new_pid;
> > -                       audit_nlk_portid = NETLINK_CB(skb).portid;
> > -                       audit_sock = skb->sk;
> > -                       if (!new_pid)
> > +                       if (new_pid) {
> > +                               if (audit_sock)
> > +                                       sock_put(audit_sock);
> > +                               audit_pid = new_pid;
> > +                               audit_nlk_portid = NETLINK_CB(skb).portid;
> > +                               sock_hold(skb->sk);
> 
> Why refcnt is still needed here? I need it because I removed the code
> in net exit code path.

Because there is a chance that auditd exits abnormally and no message is
send from the kauditd thread to discover it has gone.

> > +                               audit_sock = skb->sk;
> > +                       } else {
> >                                 auditd_reset();
> > +                       }
> >                         wake_up_interruptible(&kauditd_wait);
> >                 }
> >                 if (s.mask & AUDIT_STATUS_RATE_LIMIT) {
> > @@ -1283,8 +1302,11 @@ static void __net_exit audit_net_exit(struct net *net)
> >  {
> >         struct audit_net *aunet = net_generic(net, audit_net_id);
> >         struct sock *sock = aunet->nlsk;
> > -       if (sock == audit_sock)
> > +       if (sock == audit_sock) {
> > +               mutex_lock(&audit_cmd_mutex);
> 
> You need to put the if check inside the mutex too. Again, this could be
> removed if you use refcnt.

Ok, right, fixed.

That last patch was a bit of a mess!  Thanks for your patience in
checking it...

> >                 auditd_reset();
> > +               mutex_unlock(&audit_cmd_mutex);
> > +       }
> >
> >         RCU_INIT_POINTER(aunet->nlsk, NULL);
> >         synchronize_net();
> > --
> > 1.7.1
> >

- RGB

--
Richard Guy Briggs <rgb@redhat.com>
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635