[S390] zcrypt: add sanity check before copy_from_user()

It's not obvious that copy_from_user() is called with a sane length
parameter here. Even though it currently seems to be correct better
add a check to prevent stack corruption / exploits.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/crypto/zcrypt_api.c

@@ -393,10 +393,12 @@ static long zcrypt_rsa_crt(struct ica_rsa_modexpo_crt *crt)
 			 * u_mult_inv > 128 bytes.
 			 */
 			if (copied == 0) {
-				int len;
+				unsigned int len;
 				spin_unlock_bh(&zcrypt_device_lock);
 				/* len is max 256 / 2 - 120 = 8 */
 				len = crt->inputdatalength / 2 - 120;
+				if (len > sizeof(z1))
+					return -EFAULT;
 				z1 = z2 = z3 = 0;
 				if (copy_from_user(&z1, crt->np_prime, len) ||
 				    copy_from_user(&z2, crt->bp_key, len) ||