sctp: fail if no bound addresses can be used for a given scope

Currently, if you bind the socket to something like:
servaddr.sin6_family = AF_INET6;
servaddr.sin6_port = htons(0);
servaddr.sin6_scope_id = 0;
inet_pton(AF_INET6, "::1", &servaddr.sin6_addr);

And then request a connect to:
connaddr.sin6_family = AF_INET6;
connaddr.sin6_port = htons(20000);
connaddr.sin6_scope_id = if_nametoindex("lo");
inet_pton(AF_INET6, "fe88::1", &connaddr.sin6_addr);

What the stack does is:
- bind the socket
- create a new asoc
- to handle the connect
- copy the addresses that can be used for the given scope
- try to connect

But the copy returns 0 addresses, and the effect is that it ends up
trying to connect as if the socket wasn't bound, which is not the
desired behavior. This unexpected behavior also allows KASLR leaks
through SCTP diag interface.

The fix here then is, if when trying to copy the addresses that can
be used for the scope used in connect() it returns 0 addresses, bail
out. This is what TCP does with a similar reproducer.

Reported-by: Pietro Borrello <[e-mail]>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Marcelo Ricardo Leitner <[e-mail]>
Reviewed-by: Xin Long <[e-mail]>
Link: https://lore.kernel.org/r/9fcd182[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sctp/bind_addr.c

···
 		}
 	}
 
+	/* If somehow no addresses were found that can be used with this
+	 * scope, it's an error.
+	 */
+	if (list_empty(&dest->address_list))
+		error = -ENETUNREACH;
+
 out:
 	if (error)
 		sctp_bind_addr_clean(dest);