udp: introduce sk_for_each_rcu_safenext()

Corey Minyard found a race added in commit 271b72c7fa82c2c7a795bc16896149933110672d
(udp: RCU handling for Unicast packets.)

 "If the socket is moved from one list to another list in-between the
 time the hash is calculated and the next field is accessed, and the
 socket has moved to the end of the new list, the traversal will not
 complete properly on the list it should have, since the socket will
 be on the end of the new list and there's not a way to tell it's on a
 new list and restart the list traversal.  I think that this can be
 solved by pre-fetching the "next" field (with proper barriers) before
 checking the hash."

This patch corrects this problem, introducing a new
sk_for_each_rcu_safenext() macro.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 23 insertions, 6 deletions

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index e649bd3f2c97..3ba2998b22ba 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -383,5 +383,22 @@ static inline void hlist_add_after_rcu(struct hlist_node *prev,
                 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); \
                 pos = rcu_dereference(pos->next))
 
+/**
+ * hlist_for_each_entry_rcu_safenext - iterate over rcu list of given type
+ * @tpos:       the type * to use as a loop cursor.
+ * @pos:        the &struct hlist_node to use as a loop cursor.
+ * @head:       the head for your list.
+ * @member:     the name of the hlist_node within the struct.
+ * @next:       the &struct hlist_node to use as a next cursor
+ *
+ * Special version of hlist_for_each_entry_rcu that make sure
+ * each next pointer is fetched before each iteration.
+ */
+#define hlist_for_each_entry_rcu_safenext(tpos, pos, head, member, next) \
+        for (pos = rcu_dereference((head)->first);                       \
+                pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) && \
+                ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); \
+                pos = rcu_dereference(next))
+
 #endif  /* __KERNEL__ */
 #endif
diff --git a/include/net/sock.h b/include/net/sock.h
index 0bea25db5471..a4f6d3fc0470 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -419,8 +419,8 @@ static __inline__ void sk_add_bind_node(struct sock *sk,
 
 #define sk_for_each(__sk, node, list) \
         hlist_for_each_entry(__sk, node, list, sk_node)
-#define sk_for_each_rcu(__sk, node, list) \
-        hlist_for_each_entry_rcu(__sk, node, list, sk_node)
+#define sk_for_each_rcu_safenext(__sk, node, list, next) \
+        hlist_for_each_entry_rcu_safenext(__sk, node, list, sk_node, next)
 #define sk_for_each_from(__sk, node) \
         if (__sk && ({ node = &(__sk)->sk_node; 1; })) \
                 hlist_for_each_entry_from(__sk, node, sk_node)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index ced820318f94..c3ecec8a9e1c 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -256,7 +256,7 @@ static struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
                 int dif, struct udp_table *udptable)
 {
         struct sock *sk, *result;
-        struct hlist_node *node;
+        struct hlist_node *node, *next;
         unsigned short hnum = ntohs(dport);
         unsigned int hash = udp_hashfn(net, hnum);
         struct udp_hslot *hslot = &udptable->hash[hash];
@@ -266,7 +266,7 @@ static struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
 begin:
         result = NULL;
         badness = -1;
-        sk_for_each_rcu(sk, node, &hslot->head) {
+        sk_for_each_rcu_safenext(sk, node, &hslot->head, next) {
                 /*
                  * lockless reader, and SLAB_DESTROY_BY_RCU items:
                  * We must check this item was not moved to another chain
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 1d9790e43dfc..32d914db6c4f 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -98,7 +98,7 @@ static struct sock *__udp6_lib_lookup(struct net *net,
                                       int dif, struct udp_table *udptable)
 {
         struct sock *sk, *result;
-        struct hlist_node *node;
+        struct hlist_node *node, *next;
         unsigned short hnum = ntohs(dport);
         unsigned int hash = udp_hashfn(net, hnum);
         struct udp_hslot *hslot = &udptable->hash[hash];
@@ -108,7 +108,7 @@ static struct sock *__udp6_lib_lookup(struct net *net,
 begin:
         result = NULL;
         badness = -1;
-        sk_for_each_rcu(sk, node, &hslot->head) {
+        sk_for_each_rcu_safenext(sk, node, &hslot->head, next) {
                 /*
                  * lockless reader, and SLAB_DESTROY_BY_RCU items:
                  * We must check this item was not moved to another chain