ipv6: add rcu grace period before freeing fib6_node

commit c5cff8561d2d0006e972bd114afd51f082fee77c upstream.

We currently keep rt->rt6i_node pointing to the fib6_node for the route.
And some functions make use of this pointer to dereference the fib6_node
from rt structure, e.g. rt6_check(). However, as there is neither
refcount nor rcu taken when dereferencing rt->rt6i_node, it could
potentially cause crashes as rt->rt6i_node could be set to NULL by other
CPUs when doing a route deletion.
This patch introduces an rcu grace period before freeing fib6_node and
makes sure the functions that dereference it takes rcu_read_lock().

Note: there is no "Fixes" tag because this bug was there in a very
early stage.

Change-Id: Ibde30a1b3719427404cda47846de62f56ea8080d
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

include/net/ip6_fib.h

@@ -66,6 +66,7 @@ struct fib6_node {
 	__u16			fn_flags;
 	__u32			fn_sernum;
 	struct rt6_info		*rr_ptr;
+	struct rcu_head		rcu;
 };
 
 #ifndef CONFIG_IPV6_SUBTREES
@@ -196,9 +197,36 @@ static inline void rt6_set_from(struct rt6_info *rt, struct rt6_info *from)
 	rt->dst.from = new;
 }
 
+/* Function to safely get fn->sernum for passed in rt
+ * and store result in passed in cookie.
+ * Return true if we can get cookie safely
+ * Return false if not
+ */
+static inline bool rt6_get_cookie_safe(const struct rt6_info *rt,
+				       u32 *cookie)
+{
+	struct fib6_node *fn;
+	bool status = false;
+
+	rcu_read_lock();
+	fn = rcu_dereference(rt->rt6i_node);
+
+	if (fn) {
+		*cookie = fn->fn_sernum;
+		status = true;
+	}
+
+	rcu_read_unlock();
+	return status;
+}
+
 static inline u32 rt6_get_cookie(const struct rt6_info *rt)
 {
-	return rt->rt6i_node ? rt->rt6i_node->fn_sernum : 0;
+	u32 cookie = 0;
+
+	rt6_get_cookie_safe(rt, &cookie);
+
+	return cookie;
 }
 
 static inline void ip6_rt_put(struct rt6_info *rt)

net/ipv6/ip6_fib.c

@@ -156,11 +156,23 @@ static __inline__ struct fib6_node * node_alloc(void)
 	return fn;
 }
 
-static __inline__ void node_free(struct fib6_node * fn)
+static void node_free_immediate(struct fib6_node *fn)
 {
 	kmem_cache_free(fib6_node_kmem, fn);
 }
 
+static void node_free_rcu(struct rcu_head *head)
+{
+	struct fib6_node *fn = container_of(head, struct fib6_node, rcu);
+
+	kmem_cache_free(fib6_node_kmem, fn);
+}
+
+static void node_free(struct fib6_node *fn)
+{
+	call_rcu(&fn->rcu, node_free_rcu);
+}
+
 static __inline__ void rt6_release(struct rt6_info *rt)
 {
 	if (atomic_dec_and_test(&rt->rt6i_ref))
@@ -562,9 +574,9 @@ insert_above:
 
 		if (!in || !ln) {
 			if (in)
-				node_free(in);
+				node_free_immediate(in);
 			if (ln)
-				node_free(ln);
+				node_free_immediate(ln);
 			return ERR_PTR(-ENOMEM);
 		}
 
@@ -902,7 +914,7 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info)
 				   root, and then (in st_failure) stale node
 				   in main tree.
 				 */
-				node_free(sfn);
+				node_free_immediate(sfn);
 				err = PTR_ERR(sn);
 				goto st_failure;
 			}

net/ipv6/route.c

@@ -1072,6 +1072,7 @@ struct dst_entry *ip6_blackhole_route(struct net *net, struct dst_entry *dst_ori
 static struct dst_entry *ip6_dst_check(struct dst_entry *dst, u32 cookie)
 {
 	struct rt6_info *rt;
+	u32 rt_cookie;
 
 	rt = (struct rt6_info *) dst;
 
@@ -1082,7 +1083,7 @@ static struct dst_entry *ip6_dst_check(struct dst_entry *dst, u32 cookie)
 	if (rt->rt6i_genid != rt_genid(dev_net(rt->dst.dev)))
 		return NULL;
 
-	if (!rt->rt6i_node || (rt->rt6i_node->fn_sernum != cookie))
+	if (!rt6_get_cookie_safe(rt, &rt_cookie) || rt_cookie != cookie)
 		return NULL;
 
 	if (rt6_check_expired(rt))
@@ -1121,8 +1122,14 @@ static void ip6_link_failure(struct sk_buff *skb)
 			dst_hold(&rt->dst);
 			if (ip6_del_rt(rt))
 				dst_free(&rt->dst);
-		} else if (rt->rt6i_node && (rt->rt6i_flags & RTF_DEFAULT)) {
-			rt->rt6i_node->fn_sernum = -1;
+		} else {
+			struct fib6_node *fn;
+
+			rcu_read_lock();
+			fn = rcu_dereference(rt->rt6i_node);
+			if (fn && (rt->rt6i_flags & RTF_DEFAULT))
+				fn->fn_sernum = -1;
+			rcu_read_unlock();
 		}
 	}
 }