[libxslt] Fix handling of RVTs returned from nested EXSLT functions

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 8bd32f7753ac253a54279a0b6a88d15a57076bb0
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Tue Jan 16 18:59:02 2018 +0100

    Fix handling of RVTs returned from nested EXSLT functions
    
    Set the context variable to NULL when evaluating EXSLT functions.
    Fixes potential use-after-free errors or memory leaks.
    
    Fixes bug 792580. Thanks to Clemens Gutweiler for the report.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=792580

 libexslt/functions.c      |    4 ++++
 tests/docs/bug-209.xml    |    1 +
 tests/general/bug-209.out |    2 ++
 tests/general/bug-209.xsl |   21 +++++++++++++++++++++
 4 files changed, 28 insertions(+), 0 deletions(-)
---
diff --git a/libexslt/functions.c b/libexslt/functions.c
index 74dea1a..2b83ca3 100644
--- a/libexslt/functions.c
+++ b/libexslt/functions.c
@@ -292,6 +292,7 @@ exsltFuncFunctionFunction (xmlXPathParserContextPtr ctxt, int nargs) {
     exsltFuncFunctionData *func;
     xmlNodePtr paramNode, oldInsert, fake;
     int oldBase;
+    void *oldCtxtVar;
     xsltStackElemPtr params = NULL, param;
     xsltTransformContextPtr tctxt = xsltXPathGetTransformContext(ctxt);
     int i, notSet;
@@ -430,11 +431,14 @@ exsltFuncFunctionFunction (xmlXPathParserContextPtr ctxt, int nargs) {
     fake = xmlNewDocNode(tctxt->output, NULL,
                         (const xmlChar *)"fake", NULL);
     oldInsert = tctxt->insert;
+    oldCtxtVar = tctxt->contextVariable;
     tctxt->insert = fake;
+    tctxt->contextVariable = NULL;
     xsltApplyOneTemplate (tctxt, tctxt->node,
                          func->content, NULL, NULL);
     xsltLocalVariablePop(tctxt, tctxt->varsBase, -2);
     tctxt->insert = oldInsert;
+    tctxt->contextVariable = oldCtxtVar;
     tctxt->varsBase = oldBase; /* restore original scope */
     if (params != NULL)
        xsltFreeStackElemList(params);
diff --git a/tests/docs/bug-209.xml b/tests/docs/bug-209.xml
new file mode 100644
index 0000000..69d62f2
--- /dev/null
+++ b/tests/docs/bug-209.xml
@@ -0,0 +1 @@
+<doc/>
diff --git a/tests/general/bug-209.out b/tests/general/bug-209.out
new file mode 100644
index 0000000..e829790
--- /dev/null
+++ b/tests/general/bug-209.out
@@ -0,0 +1,2 @@
+<?xml version="1.0"?>
+<result/>
diff --git a/tests/general/bug-209.xsl b/tests/general/bug-209.xsl
new file mode 100644
index 0000000..fe69ac6
--- /dev/null
+++ b/tests/general/bug-209.xsl
@@ -0,0 +1,21 @@
+<xsl:stylesheet
+    version="1.0"
+    xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
+    xmlns:func="http://exslt.org/functions";
+    extension-element-prefixes="func">
+
+    <xsl:template match="/">
+        <xsl:variable name="v" select="func:a()" />
+        <xsl:copy-of select="$v"/>
+    </xsl:template>
+
+    <func:function name="func:a">
+        <func:result select="func:b()" />
+    </func:function>
+
+    <func:function name="func:b">
+        <func:result>
+            <result/>
+        </func:result>
+    </func:function>
+</xsl:stylesheet>