[PATCH net 1/4] ipvs: do not use dest after ip_vs_dest_put in LBLC

[Julian Anastasov <ja@xxxxxx>]

commit c2a4ffb70eef39 ("ipvs: convert lblc scheduler to rcu")
allows RCU readers to use dest after calling ip_vs_dest_put().
In the corner case it can race with ip_vs_dest_trash_expire()
which can release the dest while it is being returned to the
RCU readers as scheduling result.

To fix the problem do not allow en->dest to be replaced and
defer the ip_vs_dest_put() call by using RCU callback. Now
en->dest does not need to be RCU pointer.

Signed-off-by: Julian Anastasov <ja@xxxxxx>
---
 net/netfilter/ipvs/ip_vs_lblc.c |   68 ++++++++++++++++++---------------------
 1 files changed, 31 insertions(+), 37 deletions(-)

diff --git a/net/netfilter/ipvs/ip_vs_lblc.c b/net/netfilter/ipvs/ip_vs_lblc.c
index 1383b0e..b39b0e9 100644
--- a/net/netfilter/ipvs/ip_vs_lblc.c
+++ b/net/netfilter/ipvs/ip_vs_lblc.c
@@ -93,7 +93,7 @@ struct ip_vs_lblc_entry {
        struct hlist_node       list;
        int                     af;             /* address family */
        union nf_inet_addr      addr;           /* destination IP address */
-       struct ip_vs_dest __rcu *dest;          /* real server (cache) */
+       struct ip_vs_dest       *dest;          /* real server (cache) */
        unsigned long           lastuse;        /* last used time */
        struct rcu_head         rcu_head;
 };
@@ -130,20 +130,21 @@ static struct ctl_table vs_vars_table[] = {
 };
 #endif
 
-static inline void ip_vs_lblc_free(struct ip_vs_lblc_entry *en)
+static void ip_vs_lblc_rcu_free(struct rcu_head *head)
 {
-       struct ip_vs_dest *dest;
+       struct ip_vs_lblc_entry *en = container_of(head,
+                                                  struct ip_vs_lblc_entry,
+                                                  rcu_head);
 
-       hlist_del_rcu(&en->list);
-       /*
-        * We don't kfree dest because it is referred either by its service
-        * or the trash dest list.
-        */
-       dest = rcu_dereference_protected(en->dest, 1);
-       ip_vs_dest_put(dest);
-       kfree_rcu(en, rcu_head);
+       ip_vs_dest_put(en->dest);
+       kfree(en);
 }
 
+static inline void ip_vs_lblc_del(struct ip_vs_lblc_entry *en)
+{
+       hlist_del_rcu(&en->list);
+       call_rcu(&en->rcu_head, ip_vs_lblc_rcu_free);
+}
 
 /*
  *     Returns hash value for IPVS LBLC entry
@@ -203,30 +204,23 @@ ip_vs_lblc_new(struct ip_vs_lblc_table *tbl, const union 
nf_inet_addr *daddr,
        struct ip_vs_lblc_entry *en;
 
        en = ip_vs_lblc_get(dest->af, tbl, daddr);
-       if (!en) {
-               en = kmalloc(sizeof(*en), GFP_ATOMIC);
-               if (!en)
-                       return NULL;
-
-               en->af = dest->af;
-               ip_vs_addr_copy(dest->af, &en->addr, daddr);
-               en->lastuse = jiffies;
+       if (en) {
+               if (en->dest == dest)
+                       return en;
+               ip_vs_lblc_del(en);
+       }
+       en = kmalloc(sizeof(*en), GFP_ATOMIC);
+       if (!en)
+               return NULL;
 
-               ip_vs_dest_hold(dest);
-               RCU_INIT_POINTER(en->dest, dest);
+       en->af = dest->af;
+       ip_vs_addr_copy(dest->af, &en->addr, daddr);
+       en->lastuse = jiffies;
 
-               ip_vs_lblc_hash(tbl, en);
-       } else {
-               struct ip_vs_dest *old_dest;
+       ip_vs_dest_hold(dest);
+       en->dest = dest;
 
-               old_dest = rcu_dereference_protected(en->dest, 1);
-               if (old_dest != dest) {
-                       ip_vs_dest_put(old_dest);
-                       ip_vs_dest_hold(dest);
-                       /* No ordering constraints for refcnt */
-                       RCU_INIT_POINTER(en->dest, dest);
-               }
-       }
+       ip_vs_lblc_hash(tbl, en);
 
        return en;
 }
@@ -246,7 +240,7 @@ static void ip_vs_lblc_flush(struct ip_vs_service *svc)
        tbl->dead = 1;
        for (i=0; i<IP_VS_LBLC_TAB_SIZE; i++) {
                hlist_for_each_entry_safe(en, next, &tbl->bucket[i], list) {
-                       ip_vs_lblc_free(en);
+                       ip_vs_lblc_del(en);
                        atomic_dec(&tbl->entries);
                }
        }
@@ -281,7 +275,7 @@ static inline void ip_vs_lblc_full_check(struct 
ip_vs_service *svc)
                                        sysctl_lblc_expiration(svc)))
                                continue;
 
-                       ip_vs_lblc_free(en);
+                       ip_vs_lblc_del(en);
                        atomic_dec(&tbl->entries);
                }
                spin_unlock(&svc->sched_lock);
@@ -335,7 +329,7 @@ static void ip_vs_lblc_check_expire(unsigned long data)
                        if (time_before(now, en->lastuse + ENTRY_TIMEOUT))
                                continue;
 
-                       ip_vs_lblc_free(en);
+                       ip_vs_lblc_del(en);
                        atomic_dec(&tbl->entries);
                        goal--;
                }
@@ -511,7 +505,7 @@ ip_vs_lblc_schedule(struct ip_vs_service *svc, const struct 
sk_buff *skb,
                 * free up entries from the trash at any time.
                 */
 
-               dest = rcu_dereference(en->dest);
+               dest = en->dest;
                if ((dest->flags & IP_VS_DEST_F_AVAILABLE) &&
                    atomic_read(&dest->weight) > 0 && !is_overloaded(dest, svc))
                        goto out;
@@ -631,7 +625,7 @@ static void __exit ip_vs_lblc_cleanup(void)
 {
        unregister_ip_vs_scheduler(&ip_vs_lblc_scheduler);
        unregister_pernet_subsys(&ip_vs_lblc_ops);
-       synchronize_rcu();
+       rcu_barrier();
 }
 
 
-- 
1.7.3.4

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