sock_map: fix a NULL pointer dereference in sock_map_link_update_prog()

The following race condition could trigger a NULL pointer dereference:

sock_map_link_detach():		sock_map_link_update_prog():
   mutex_lock(&sockmap_mutex);
   ...
   sockmap_link->map = NULL;
   mutex_unlock(&sockmap_mutex);
   				   mutex_lock(&sockmap_mutex);
				   ...
				   sock_map_prog_link_lookup(sockmap_link->map);
				   mutex_unlock(&sockmap_mutex);
   <continue>

Fix it by adding a NULL pointer check. In this specific case, it makes
no sense to update a link which is being released.

Reported-by: Ruan Bonan <[email protected]>
Fixes: 699c23f ("bpf: Add bpf_link support for sk_msg and sk_skb progs")
Cc: Yonghong Song <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: Jakub Sitnicki <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>

net/core/sock_map.c

@@ -1760,6 +1760,10 @@ static int sock_map_link_update_prog(struct bpf_link *link,
 		ret = -EINVAL;
 		goto out;
 	}
+	if (!sockmap_link->map) {
+		ret = -ENOLINK;
+		goto out;
+	}
 
 	ret = sock_map_prog_link_lookup(sockmap_link->map, &pprog, &plink,
 					sockmap_link->attach_type);