[Patch net-next] ipv6: fix a potential NULL deref

[Cong Wang <amwang@redhat.com>]

In ipv6_del_addr():

                if (rt != net->ipv6.ip6_null_entry &&
                    addrconf_is_prefix_route(rt)) {
                        if (onlink == 0) {
                                ip6_del_rt(rt);
                                rt = NULL;
                        } else if (!(rt->rt6i_flags & RTF_EXPIRES)) {
                                rt6_set_expires(rt, expires);
                        }
                }
                dst_release(&rt->dst);

obviously rt could be NULL'd before dst_release(), so
we have to check if rt is NULL before calling it.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>

---
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 8f0b12a..c467dbb 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -951,7 +951,8 @@ static void ipv6_del_addr(struct inet6_ifaddr *ifp)
 				rt6_set_expires(rt, expires);
 			}
 		}
-		dst_release(&rt->dst);
+		if (rt)
+			dst_release(&rt->dst);
 	}
 
 	/* clean up prefsrc entries */