net: sock_copy() fixes

Commit e912b1142be8f1e2c71c71001dc992c6e5eb2ec1
(net: sk_prot_alloc() should not blindly overwrite memory)
took care of not zeroing whole new socket at allocation time.

sock_copy() is another spot where we should be very careful.
We should not set refcnt to a non null value, until
we are sure other fields are correctly setup, or
a lockless reader could catch this socket by mistake,
while not fully (re)initialized.

This patch puts sk_node & sk_refcnt to the very beginning
of struct sock to ease sock_copy() & sk_prot_alloc() job.

We add appropriate smp_wmb() before sk_refcnt initializations
to match our RCU requirements (changes to sock keys should
be committed to memory before sk_refcnt setting)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 37 insertions, 15 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 2c0da9239b95..950409dcec3d 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -104,15 +104,15 @@ struct net;
 
 /**
  *      struct sock_common - minimal network layer representation of sockets
+ *      @skc_node: main hash linkage for various protocol lookup tables
+ *      @skc_nulls_node: main hash linkage for UDP/UDP-Lite protocol
+ *      @skc_refcnt: reference count
+ *      @skc_hash: hash value used with various protocol lookup tables
  *      @skc_family: network address family
  *      @skc_state: Connection state
  *      @skc_reuse: %SO_REUSEADDR setting
  *      @skc_bound_dev_if: bound device index if != 0
- *      @skc_node: main hash linkage for various protocol lookup tables
- *      @skc_nulls_node: main hash linkage for UDP/UDP-Lite protocol
  *      @skc_bind_node: bind hash linkage for various protocol lookup tables
- *      @skc_refcnt: reference count
- *      @skc_hash: hash value used with various protocol lookup tables
  *      @skc_prot: protocol handlers inside a network family
  *      @skc_net: reference to the network namespace of this socket
  *
@@ -120,17 +120,21 @@ struct net;
  *      for struct sock and struct inet_timewait_sock.
  */
 struct sock_common {
-        unsigned short          skc_family;
-        volatile unsigned char  skc_state;
-        unsigned char           skc_reuse;
-        int                     skc_bound_dev_if;
+        /*
+         * first fields are not copied in sock_copy()
+         */
         union {
                 struct hlist_node       skc_node;
                 struct hlist_nulls_node skc_nulls_node;
         };
-        struct hlist_node       skc_bind_node;
         atomic_t                skc_refcnt;
+
         unsigned int            skc_hash;
+        unsigned short          skc_family;
+        volatile unsigned char  skc_state;
+        unsigned char           skc_reuse;
+        int                     skc_bound_dev_if;
+        struct hlist_node       skc_bind_node;
         struct proto            *skc_prot;
 #ifdef CONFIG_NET_NS
         struct net              *skc_net;
@@ -208,15 +212,17 @@ struct sock {
          * don't add nothing before this first member (__sk_common) --acme
          */
         struct sock_common      __sk_common;
+#define sk_node                 __sk_common.skc_node
+#define sk_nulls_node           __sk_common.skc_nulls_node
+#define sk_refcnt               __sk_common.skc_refcnt
+
+#define sk_copy_start           __sk_common.skc_hash
+#define sk_hash                 __sk_common.skc_hash
 #define sk_family               __sk_common.skc_family
 #define sk_state                __sk_common.skc_state
 #define sk_reuse                __sk_common.skc_reuse
 #define sk_bound_dev_if         __sk_common.skc_bound_dev_if
-#define sk_node                 __sk_common.skc_node
-#define sk_nulls_node           __sk_common.skc_nulls_node
 #define sk_bind_node            __sk_common.skc_bind_node
-#define sk_refcnt               __sk_common.skc_refcnt
-#define sk_hash                 __sk_common.skc_hash
 #define sk_prot                 __sk_common.skc_prot
 #define sk_net                  __sk_common.skc_net
         kmemcheck_bitfield_begin(flags);
diff --git a/net/core/sock.c b/net/core/sock.c
index ba5d2116aea1..d9eec153d531 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -919,13 +919,19 @@ static inline void sock_lock_init(struct sock *sk)
                         af_family_keys + sk->sk_family);
 }
 
+/*
+ * Copy all fields from osk to nsk but nsk->sk_refcnt must not change yet,
+ * even temporarly, because of RCU lookups. sk_node should also be left as is.
+ */
 static void sock_copy(struct sock *nsk, const struct sock *osk)
 {
 #ifdef CONFIG_SECURITY_NETWORK
         void *sptr = nsk->sk_security;
 #endif
-
-        memcpy(nsk, osk, osk->sk_prot->obj_size);
+        BUILD_BUG_ON(offsetof(struct sock, sk_copy_start) !=
+                     sizeof(osk->sk_node) + sizeof(osk->sk_refcnt));
+        memcpy(&nsk->sk_copy_start, &osk->sk_copy_start,
+               osk->sk_prot->obj_size - offsetof(struct sock, sk_copy_start));
 #ifdef CONFIG_SECURITY_NETWORK
         nsk->sk_security = sptr;
         security_sk_clone(osk, nsk);
@@ -1140,6 +1146,11 @@ struct sock *sk_clone(const struct sock *sk, const gfp_t priority)
 
                 newsk->sk_err      = 0;
                 newsk->sk_priority = 0;
+                /*
+                 * Before updating sk_refcnt, we must commit prior changes to memory
+                 * (Documentation/RCU/rculist_nulls.txt for details)
+                 */
+                smp_wmb();
                 atomic_set(&newsk->sk_refcnt, 2);
 
                 /*
@@ -1855,6 +1866,11 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 
         sk->sk_stamp = ktime_set(-1L, 0);
 
+        /*
+         * Before updating sk_refcnt, we must commit prior changes to memory
+         * (Documentation/RCU/rculist_nulls.txt for details)
+         */
+        smp_wmb();
         atomic_set(&sk->sk_refcnt, 1);
         atomic_set(&sk->sk_wmem_alloc, 1);
         atomic_set(&sk->sk_drops, 0);