tools: use cryptographically safe RNG

The PRNG implementing the random() function only has 2^31 states and
therefore is unsafe to use for cryptography. Use arc4random() instead.

Fixes: cc34f04efd63 ("tools: image-host.c: use random instead of rand")
Addresses-Coverity-ID: 312953 Calling risky function
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

tools/image-host.c

@@ -364,33 +364,46 @@ static int fit_image_read_key_iv_data(const char *keydir, const char *key_iv_nam
 	return ret;
 }
 
-static int get_random_data(void *data, int size)
+/**
+ * get_random_data() - fill buffer with random data
+ *
+ * There is no common cryptographically safe function in Linux and BSD.
+ * Hence directly access the /dev/urandom PRNG.
+ *
+ * @data:	buffer to fill
+ * @size:	buffer size
+ */
+static int get_random_data(void *data, size_t size)
 {
-	unsigned char *tmp = data;
+	int fd;
-	struct timespec date;
+	int ret;
-	int i, ret;
 
-	if (!tmp) {
+	fd = open("/dev/urandom", O_RDONLY);
-		fprintf(stderr, "%s: pointer data is NULL\n", __func__);
+	if (fd < 0) {
+		perror("Failed to open /dev/urandom");
+		return -1;
+	}
+
+	while (size) {
+		ssize_t count;
+
+		count = read(fd, data, size);
+		if (count < 0) {
+			if (errno == EINTR) {
+				continue;
+			} else {
+				perror("Failed to read from /dev/urandom");
 				ret = -1;
 				goto out;
 			}
-
-	ret = clock_gettime(CLOCK_MONOTONIC, &date);
-	if (ret) {
-		fprintf(stderr, "%s: clock_gettime has failed (%s)\n", __func__,
-			strerror(errno));
-		goto out;
 		}
-
+		data += count;
-	srandom(date.tv_nsec);
+		size -= count;
-
-	for (i = 0; i < size; i++) {
-		*tmp = random() & 0xff;
-		tmp++;
 	}
+	ret = 0;
+out:
+	close(fd);
 
- out:
 	return ret;
 }