bpf, sockmap: fix leak in bpf_tcp_sendmsg wait for mem path

In bpf_tcp_sendmsg() the sk_alloc_sg() may fail. In the case of
ENOMEM, it may also mean that we've partially filled the scatterlist
entries with pages. Later jumping to sk_stream_wait_memory()
we could further fail with an error for several reasons, however
we miss to call free_start_sg() if the local sk_msg_buff was used.

Fixes: 4f738adba30a ("bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/sockmap.c

@@ -1048,7 +1048,7 @@ static int bpf_tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 	timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
 
 	while (msg_data_left(msg)) {
-		struct sk_msg_buff *m;
+		struct sk_msg_buff *m = NULL;
 		bool enospc = false;
 		int copy;
 
@@ -1116,8 +1116,11 @@ wait_for_sndbuf:
 		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 wait_for_memory:
 		err = sk_stream_wait_memory(sk, &timeo);
-		if (err)
+		if (err) {
+			if (m && m != psock->cork)
+				free_start_sg(sk, m);
 			goto out_err;
+		}
 	}
 out_err:
 	if (err < 0)