net, sk_msg: Annotate lockless access to sk_prot on clone

sk_msg and ULP frameworks override protocol callbacks pointer in
sk->sk_prot, while tcp accesses it locklessly when cloning the listening
socket, that is with neither sk_lock nor sk_callback_lock held.

Once we enable use of listening sockets with sockmap (and hence sk_msg),
there will be shared access to sk->sk_prot if socket is getting cloned
while being inserted/deleted to/from the sockmap from another CPU:

Read side:

tcp_v4_rcv
  sk = __inet_lookup_skb(...)
  tcp_check_req(sk)
    inet_csk(sk)->icsk_af_ops->syn_recv_sock
      tcp_v4_syn_recv_sock
        tcp_create_openreq_child
          inet_csk_clone_lock
            sk_clone_lock
              READ_ONCE(sk->sk_prot)

Write side:

sock_map_ops->map_update_elem
  sock_map_update_elem
    sock_map_update_common
      sock_map_link_no_progs
        tcp_bpf_init
          tcp_bpf_update_sk_prot
            sk_psock_update_proto
              WRITE_ONCE(sk->sk_prot, ops)

sock_map_ops->map_delete_elem
  sock_map_delete_elem
    __sock_map_delete
     sock_map_unref
       sk_psock_put
         sk_psock_drop
           sk_psock_restore_proto
             tcp_update_ulp
               WRITE_ONCE(sk->sk_prot, proto)

Mark the shared access with READ_ONCE/WRITE_ONCE annotations.
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200218171023.844439-2-jakub@cloudflare.com

include/linux/skmsg.h

@@ -352,7 +352,8 @@ static inline void sk_psock_update_proto(struct sock *sk,
 	psock->saved_write_space = sk->sk_write_space;
 
 	psock->sk_proto = sk->sk_prot;
-	sk->sk_prot = ops;
+	/* Pairs with lockless read in sk_clone_lock() */
+	WRITE_ONCE(sk->sk_prot, ops);
 }
 
 static inline void sk_psock_restore_proto(struct sock *sk,

net/core/sock.c

@@ -1572,13 +1572,14 @@ static inline void sock_lock_init(struct sock *sk)
  */
 static void sock_copy(struct sock *nsk, const struct sock *osk)
 {
+	const struct proto *prot = READ_ONCE(osk->sk_prot);
 #ifdef CONFIG_SECURITY_NETWORK
 	void *sptr = nsk->sk_security;
 #endif
 	memcpy(nsk, osk, offsetof(struct sock, sk_dontcopy_begin));
 
 	memcpy(&nsk->sk_dontcopy_end, &osk->sk_dontcopy_end,
-	       osk->sk_prot->obj_size - offsetof(struct sock, sk_dontcopy_end));
+	       prot->obj_size - offsetof(struct sock, sk_dontcopy_end));
 
 #ifdef CONFIG_SECURITY_NETWORK
 	nsk->sk_security = sptr;
@@ -1792,16 +1793,17 @@ static void sk_init_common(struct sock *sk)
  */
 struct sock *sk_clone_lock(const struct sock *sk, const gfp_t priority)
 {
+	struct proto *prot = READ_ONCE(sk->sk_prot);
 	struct sock *newsk;
 	bool is_charged = true;
 
-	newsk = sk_prot_alloc(sk->sk_prot, priority, sk->sk_family);
+	newsk = sk_prot_alloc(prot, priority, sk->sk_family);
 	if (newsk != NULL) {
 		struct sk_filter *filter;
 
 		sock_copy(newsk, sk);
 
-		newsk->sk_prot_creator = sk->sk_prot;
+		newsk->sk_prot_creator = prot;
 
 		/* SANITY */
 		if (likely(newsk->sk_net_refcnt))

net/ipv4/tcp_bpf.c

@@ -645,8 +645,10 @@ static void tcp_bpf_reinit_sk_prot(struct sock *sk, struct sk_psock *psock)
 	/* Reinit occurs when program types change e.g. TCP_BPF_TX is removed
 	 * or added requiring sk_prot hook updates. We keep original saved
 	 * hooks in this case.
+	 *
+	 * Pairs with lockless read in sk_clone_lock().
 	 */
-	sk->sk_prot = &tcp_bpf_prots[family][config];
+	WRITE_ONCE(sk->sk_prot, &tcp_bpf_prots[family][config]);
 }
 
 static int tcp_bpf_assert_proto_ops(struct proto *ops)

net/ipv4/tcp_ulp.c

@@ -106,7 +106,8 @@ void tcp_update_ulp(struct sock *sk, struct proto *proto,
 
 	if (!icsk->icsk_ulp_ops) {
 		sk->sk_write_space = write_space;
-		sk->sk_prot = proto;
+		/* Pairs with lockless read in sk_clone_lock() */
+		WRITE_ONCE(sk->sk_prot, proto);
 		return;
 	}
 

net/tls/tls_main.c

@@ -742,7 +742,8 @@ static void tls_update(struct sock *sk, struct proto *p,
 		ctx->sk_write_space = write_space;
 		ctx->sk_proto = p;
 	} else {
-		sk->sk_prot = p;
+		/* Pairs with lockless read in sk_clone_lock(). */
+		WRITE_ONCE(sk->sk_prot, p);
 		sk->sk_write_space = write_space;
 	}
 }