From d62cecf193d6bf3b16fe91d725f4514161f602c3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Wed, 3 Mar 2021 11:31:52 +0200
Subject: [PATCH] matroskademux: Initialize track context out parameter to NULL
 before parsing

Various error return paths don't set it to NULL and callers are only
checking if the pointer is NULL. As it's allocated on the stack this
usually contains random stack memory, and more often than not the memory
of a previously parsed track.

This then causes all kinds of memory corruptions further down the line.

Thanks to Natalie Silvanovich for reporting.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/issues/858

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/904>
---
 gst/matroska/matroska-demux.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/gst/matroska/matroska-demux.c b/gst/matroska/matroska-demux.c
index b2cd9b5d4..4eb3d2a9f 100644
--- a/gst/matroska/matroska-demux.c
+++ b/gst/matroska/matroska-demux.c
@@ -660,6 +660,8 @@ gst_matroska_demux_parse_stream (GstMatroskaDemux * demux, GstEbmlRead * ebml,
 
   DEBUG_ELEMENT_START (demux, ebml, "TrackEntry");
 
+  *dest_context = NULL;
+
   /* start with the master */
   if ((ret = gst_ebml_read_master (ebml, &id)) != GST_FLOW_OK) {
     DEBUG_ELEMENT_STOP (demux, ebml, "TrackEntry", ret);
-- 
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