Re: [xml] C14N issue with digital signature due to pointer comparison

[Frank Gross <fg 4js com>]

Hi,

 Actually, I have an additional layer to emulate DOM compliant API over 
libxml. In that layer, I have a global list of xmlNsPtr for all 
namespaces encountered by the nodes (and referenced in node->ns) of a 
single XML document. And for each namespace definition (xmlns:xxx) set 
on a node, a new xmlNsPtr is created and referenced in node->nsDef.  So 
during canonicalization process, when a node looks for a parent 
namespace definition via xmlSearchNs, it can happen that the xmlNsPtr 
reference from the node is different from the parent node->nsDef , even 
if the href and prefix value are the same. Then of course simple pointer 
comparison doesn't work. So I was asking myself if such situation cannot 
happen to other people, and maybe change the pointer comparison because 
depending on how people build and manipulate the document, the test is 
not always accurate.


Frank


Le 20/03/2014 03:16, Aleksey Sanin a écrit :
> How do you manipulate the XML tree? If you are using "official" LibXML2
> function then I believe this code should work just fine unless there is
> a bug in the strings dictionary.
>
> Aleksey
>
> On 3/12/14, 10:28 AM, Frank Gross wrote:
> > Hi,
> >
> >    I'm getting some trouble to verify a XML signature because the
> > xmlC14NProcessNamespacesAxis() function does a xmlNsPtr pointer
> > comparison to decide whether a sub node belongs to the same default
> > namespace as an ancestor. But if the sub node has been manipulated by
> > program (in my case) to point to another xmlNsPtr with same values, the
> > canonicalization process breaks. Wouldn't it be better to check the
> > namespace href values instead as in following patch ?
> >
> > diff --git a/lib-xmlsoft-libxml2/src/c14n.c
> > b/lib-xmlsoft-libxml2/src/c14n.c
> > index 9c3cad2..f73e709 100644
> > --- a/lib-xmlsoft-libxml2/src/c14n.c
> > +++ b/lib-xmlsoft-libxml2/src/c14n.c
> > @@ -623,7 +623,7 @@ xmlC14NProcessNamespacesAxis(xmlC14NCtxPtr ctx,
> > xmlNodePtr cur, int visible)
> >       for(ns = n->nsDef; ns != NULL; ns = ns->next) {
> >           tmp = xmlSearchNs(cur->doc, cur, ns->prefix);
> >
> > -        if((tmp == ns) && !xmlC14NIsXmlNs(ns) && xmlC14NIsVisible(ctx,
> > ns, cur)) {
> > +        if((xmlStrEqual(tmp->href,ns->href)) &&
> > (xmlStrEqual(tmp->prefix,ns->prefix)) && !xmlC14NIsXmlNs(ns) &&
> > xmlC14NIsVisible(ctx, ns, cur)) {
> >           already_rendered = xmlC14NVisibleNsStackFind(ctx->ns_rendered,
> > ns);
> >           if(visible) {
> >                   xmlC14NVisibleNsStackAdd(ctx->ns_rendered, ns, cur);
> >
> > Regards,
> > Frank

--
Frank GROSS
Software Engineer - Web Services
Four J's Development Tools - http://www.4js.com