Fix nullptr deref with XPath logic ops

If the XPath stack is corrupted, for example by a misbehaving extension
function, the "and" and "or" XPath operators could dereference NULL
pointers. Check that the XPath stack isn't empty and optimize the
logic operators slightly.

Closes: https://gitlab.gnome.org/GNOME/libxml2/issues/5

Also see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901817
https://bugzilla.redhat.com/show_bug.cgi?id=1595985

This is CVE-2018-14404.

Thanks to Guy Inbar for the report.

1 files changed, 4 insertions, 6 deletions

diff --git a/xpath.c b/xpath.c
index 3fae0bf4..5e3bb9ff 100644
--- a/xpath.c
+++ b/xpath.c
@@ -13234,9 +13234,8 @@ xmlXPathCompOpEval(xmlXPathParserContextPtr ctxt, xmlXPathStepOpPtr op)
 		return(0);
 	    }
             xmlXPathBooleanFunction(ctxt, 1);
-            arg1 = valuePop(ctxt);
-            arg1->boolval &= arg2->boolval;
-            valuePush(ctxt, arg1);
+            if (ctxt->value != NULL)
+                ctxt->value->boolval &= arg2->boolval;
 	    xmlXPathReleaseObject(ctxt->context, arg2);
             return (total);
         case XPATH_OP_OR:
@@ -13252,9 +13251,8 @@ xmlXPathCompOpEval(xmlXPathParserContextPtr ctxt, xmlXPathStepOpPtr op)
 		return(0);
 	    }
             xmlXPathBooleanFunction(ctxt, 1);
-            arg1 = valuePop(ctxt);
-            arg1->boolval |= arg2->boolval;
-            valuePush(ctxt, arg1);
+            if (ctxt->value != NULL)
+                ctxt->value->boolval |= arg2->boolval;
 	    xmlXPathReleaseObject(ctxt->context, arg2);
             return (total);
         case XPATH_OP_EQUAL: