rxrpc: Improve setsockopt() handling of malformed user input

copy_from_sockptr() does not return negative value on error; instead, it
reports the number of bytes that failed to copy. Since it's deprecated,
switch to copy_safe_from_sockptr().

Note: Keeping the `optlen != sizeof(unsigned int)` check as
copy_safe_from_sockptr() by itself would also accept
optlen > sizeof(unsigned int). Which would allow a more lenient handling
of inputs.

Fixes: 17926a7 ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: mmhal

net/rxrpc/af_rxrpc.c

@@ -707,9 +707,10 @@ static int rxrpc_setsockopt(struct socket *sock, int level, int optname,
 			ret = -EISCONN;
 			if (rx->sk.sk_state != RXRPC_UNBOUND)
 				goto error;
-			ret = copy_from_sockptr(&min_sec_level, optval,
-				       sizeof(unsigned int));
-			if (ret < 0)
+			ret = copy_safe_from_sockptr(&min_sec_level,
+						     sizeof(min_sec_level),
+						     optval, optlen);
+			if (ret)
 				goto error;
 			ret = -EINVAL;
 			if (min_sec_level > RXRPC_SECURITY_MAX)