QTextMarkdownImporter: fix use after free; add fuzz-generated tests

It was possible to end up with a dangling pointer in m_listStack.
This is now avoided by using QPointer and doing nullptr checks before
accessing any QTextList pointer stored there.

We have 2 specimens of garbage that caused crashes before; now they don't.
But only fuzz20450 triggered the dangling pointer in the list stack.
The crash caused by fuzz20580 was fixed by updating md4c from upstream:
4b0fc030777cd541604f5ebaaad47a2b76d61ff9

Change-Id: I8e1eca23b281256a03aea0f55e9ae20f1bdd2a38
Reviewed-by: Robert Loehning <robert.loehning@qt.io>

2 files changed, 6 insertions, 3 deletions

diff --git a/src/gui/text/qtextmarkdownimporter.cpp b/src/gui/text/qtextmarkdownimporter.cpp
index 7e18a10895..5e75e7816b 100644
--- a/src/gui/text/qtextmarkdownimporter.cpp
+++ b/src/gui/text/qtextmarkdownimporter.cpp
@@ -577,7 +577,10 @@ void QTextMarkdownImporter::insertBlock()
     QTextBlockFormat blockFormat;
     if (!m_listStack.isEmpty() && !m_needsInsertList && m_listItem) {
         QTextList *list = m_listStack.top();
-        blockFormat = list->item(list->count() - 1).blockFormat();
+        if (list)
+            blockFormat = list->item(list->count() - 1).blockFormat();
+        else
+            qWarning() << "attempted to insert into a list that no longer exists";
     }
     if (m_blockQuoteDepth) {
         blockFormat.setProperty(QTextFormat::BlockQuoteLevel, m_blockQuoteDepth);
@@ -607,7 +610,7 @@ void QTextMarkdownImporter::insertBlock()
     }
     if (m_needsInsertList) {
         m_listStack.push(m_cursor->createList(m_listFormat));
-    } else if (!m_listStack.isEmpty() && m_listItem) {
+    } else if (!m_listStack.isEmpty() && m_listItem && m_listStack.top()) {
         m_listStack.top()->add(m_cursor->block());
     }
     m_needsInsertList = false;
diff --git a/src/gui/text/qtextmarkdownimporter_p.h b/src/gui/text/qtextmarkdownimporter_p.h
index f450da5eb3..e3b4bcd0f2 100644
--- a/src/gui/text/qtextmarkdownimporter_p.h
+++ b/src/gui/text/qtextmarkdownimporter_p.h
@@ -113,7 +113,7 @@ private:
 #endif
     QString m_blockCodeLanguage;
     QVector<int> m_nonEmptyTableCells; // in the current row
-    QStack<QTextList *> m_listStack;
+    QStack<QPointer<QTextList>> m_listStack;
     QStack<QTextCharFormat> m_spanFormatStack;
     QFont m_monoFont;
     QPalette m_palette;