[libxslt] Check for integer overflow in xsltAddTextString

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 08ab2774b870de1c7b5a48693df75e8154addae5
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Jan 12 15:39:52 2017 +0100

    Check for integer overflow in xsltAddTextString
    
    Limit buffer size in xsltAddTextString to INT_MAX. The issue can be
    exploited to trigger an out of bounds write on 64-bit systems.
    
    Originally reported to Chromium:
    
    https://crbug.com/676623

 libxslt/transform.c     |   25 ++++++++++++++++++++++---
 libxslt/xsltInternals.h |    4 ++--
 2 files changed, 24 insertions(+), 5 deletions(-)
---
diff --git a/libxslt/transform.c b/libxslt/transform.c
index 519133f..02bff34 100644
--- a/libxslt/transform.c
+++ b/libxslt/transform.c
@@ -813,13 +813,32 @@ xsltAddTextString(xsltTransformContextPtr ctxt, xmlNodePtr target,
         return(target);
 
     if (ctxt->lasttext == target->content) {
+        int minSize;
 
-       if (ctxt->lasttuse + len >= ctxt->lasttsize) {
+        /* Check for integer overflow accounting for NUL terminator. */
+        if (len >= INT_MAX - ctxt->lasttuse) {
+            xsltTransformError(ctxt, NULL, target,
+                "xsltCopyText: text allocation failed\n");
+            return(NULL);
+        }
+        minSize = ctxt->lasttuse + len + 1;
+
+        if (ctxt->lasttsize < minSize) {
            xmlChar *newbuf;
            int size;
+            int extra;
+
+            /* Double buffer size but increase by at least 100 bytes. */
+            extra = minSize < 100 ? 100 : minSize;
+
+            /* Check for integer overflow. */
+            if (extra > INT_MAX - ctxt->lasttsize) {
+                size = INT_MAX;
+            }
+            else {
+                size = ctxt->lasttsize + extra;
+            }
 
-           size = ctxt->lasttsize + len + 100;
-           size *= 2;
            newbuf = (xmlChar *) xmlRealloc(target->content,size);
            if (newbuf == NULL) {
                xsltTransformError(ctxt, NULL, target,
diff --git a/libxslt/xsltInternals.h b/libxslt/xsltInternals.h
index 060b178..5ad1771 100644
--- a/libxslt/xsltInternals.h
+++ b/libxslt/xsltInternals.h
@@ -1754,8 +1754,8 @@ struct _xsltTransformContext {
      * Speed optimization when coalescing text nodes
      */
     const xmlChar  *lasttext;          /* last text node content */
-    unsigned int    lasttsize;         /* last text node size */
-    unsigned int    lasttuse;          /* last text node use */
+    int             lasttsize;         /* last text node size */
+    int             lasttuse;          /* last text node use */
     /*
      * Per Context Debugging
      */