[IPVS]: Close race conditions on ip_vs_conn_tab list modification

In an smp system, it is possible for an connection timer to expire, calling
ip_vs_conn_expire while the connection table is being flushed, before
ct_write_lock_bh is acquired.

Since the list iterator loop in ip_vs_con_flush releases and re-acquires the
spinlock (even though it doesn't re-enable softirqs), it is possible for the
expiration function to modify the connection list, while it is being traversed
in ip_vs_conn_flush.

The result is that the next pointer gets set to NULL, and subsequently
dereferenced, resulting in an oops.

Signed-off-by: Neil Horman <[e-mail]>
Acked-by: JulianAnastasov
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ipvs/ip_vs_conn.c

···
 {
 	if (del_timer(&cp->timer))
 		mod_timer(&cp->timer, jiffies);
-	__ip_vs_conn_put(cp);
 }
 
 
···
 {
 	int idx;
 	struct ip_vs_conn *cp;
-	struct ip_vs_conn *ct;
 
 	/*
 	 * Randomly scan 1/32 of the whole table every second
···
 					continue;
 			}
 
-			/*
-			 * Drop the entry, and drop its ct if not referenced
-			 */
-			atomic_inc(&cp->refcnt);
-			ct_write_unlock(hash);
-
-			if ((ct = cp->control))
-				atomic_inc(&ct->refcnt);
 			IP_VS_DBG(4, "del connection\n");
 			ip_vs_conn_expire_now(cp);
-			if (ct) {
+			if (cp->control) {
 				IP_VS_DBG(4, "del conn template\n");
-				ip_vs_conn_expire_now(ct);
+				ip_vs_conn_expire_now(cp->control);
 			}
-			ct_write_lock(hash);
 		}
 		ct_write_unlock(hash);
 	}
···
 {
 	int idx;
 	struct ip_vs_conn *cp;
-	struct ip_vs_conn *ct;
 
   flush_again:
 	for (idx=0; idx<IP_VS_CONN_TAB_SIZE; idx++) {
···
 		ct_write_lock_bh(idx);
 
 		list_for_each_entry(cp, &ip_vs_conn_tab[idx], c_list) {
-			atomic_inc(&cp->refcnt);
-			ct_write_unlock(idx);
 
-			if ((ct = cp->control))
-				atomic_inc(&ct->refcnt);
 			IP_VS_DBG(4, "del connection\n");
 			ip_vs_conn_expire_now(cp);
-			if (ct) {
+			if (cp->control) {
 				IP_VS_DBG(4, "del conn template\n");
-				ip_vs_conn_expire_now(ct);
+				ip_vs_conn_expire_now(cp->control);
 			}
-			ct_write_lock(idx);
 		}
 		ct_write_unlock_bh(idx);
 	}