efi_loader: Add size checks to efi_create_indexed_name()

Although the function description states the caller must provide a
sufficient buffer, it's better to have in function checks that the
destination buffer can hold the intended value.

So let's add an extra argument with the buffer size and check that
before doing any copying.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

lib/efi_loader/efi_string.c

@@ -23,13 +23,19 @@
  * Return: A pointer to the next position after the created string
  *	   in @buffer, or NULL otherwise
  */
-u16 *efi_create_indexed_name(u16 *buffer, const char *name, unsigned int index)
+u16 *efi_create_indexed_name(u16 *buffer, size_t buffer_size, const char *name,
+			     unsigned int index)
 {
 	u16 *p = buffer;
 	char index_buf[5];
+	size_t size;
 
+	size = (utf8_utf16_strlen(name) * sizeof(u16) +
+		sizeof(index_buf) * sizeof(u16));
+	if (buffer_size < size)
+		return NULL;
 	utf8_utf16_strcpy(&p, name);
-	sprintf(index_buf, "%04X", index);
+	snprintf(index_buf, sizeof(index_buf), "%04X", index);
 	utf8_utf16_strcpy(&p, index_buf);
 
 	return p;