powerpc/rtas_flash: fix a potential buffer overflow

Since snprintf() returns the possible output size instead of the
actual output size, the available flash_msg length returned by
get_validate_flash_msg may exceed the given buffer limit when
simple_read_from_buffer calls copy_to_user

Signed-off-by: Yi Zhuang <zhuangyi1@huawei.com>

arch/powerpc/kernel/rtas_flash.c

@@ -473,6 +473,10 @@ static int get_validate_flash_msg(struct rtas_validate_flash_t *args_buf,
 		    (args_buf->update_results == VALIDATE_TMP_UPDATE))
 			n += snprintf(msg + n, msglen - n, "%s\n",
 					args_buf->buf);
+			if (n >= msglen) {
+				n = msglen;
+				printk(KERN_ERR "FLASH: msg too long.\n");
+			}
 	} else {
 		n = sprintf(msg, "%d\n", args_buf->status);
 	}