linux: Fix sockopt copyout

The Linux getsockopt did not check the size of the provided buffer when
copying out the value, leading to buffer overflows (e.g., for TCP_INFO).

Fix is to use the smaller of the option value size and the provided
buffer.

MFC after:	1 month
Relnotes:	yes
Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D55881

sys/compat/linux/linux_socket.c

@@ -2316,10 +2316,21 @@ linux_sockopt_copyout(struct thread *td, void *val, socklen_t len,
     struct linux_getsockopt_args *args)
 {
 	int error;
+	l_int loptlen;
+	socklen_t optlen;
 
-	error = copyout(val, PTRIN(args->optval), len);
-	if (error == 0)
-		error = copyout(&len, PTRIN(args->optlen), sizeof(len));
+	error = copyin(PTRIN(args->optlen), &loptlen, sizeof(loptlen));
+	if (error != 0)
+		return (error);
+	if (loptlen < 0)
+		return (EINVAL);
+
+	optlen = (socklen_t)loptlen;
+	error = copyout(val, PTRIN(args->optval), min(len, optlen));
+	if (error == 0) {
+		loptlen = (l_int)len;
+		error = copyout(&loptlen, PTRIN(args->optlen), sizeof(loptlen));
+	}
 	return (error);
 }