Re: [PATCH net] ipv6: Prevent ipv6_find_hdr() from returning ENOENT for valid non-first fragments

Pablo Neira Ayuso
2015-01-08T21:08:35Z

On Thu, Jan 08, 2015 at 09:53:28PM +0100, Pablo Neira Ayuso wrote:
> On Wed, Jan 07, 2015 at 11:43:16AM +0100, Hannes Frederic Sowa wrote:
> > > >> @@ -250,9 +251,7 @@ int ipv6_find_hdr(const struct sk_buff *
> > > >>
> > > >> _frag_off = ntohs(*fp) & ~0x7;
> > > >> if (_frag_off) {
> > > >> - if (target < 0 &&
> > > >> - ((!ipv6_ext_hdr(hp->nexthdr)) ||
> > > >
> > > > This check assumes that the following headers cannot show up in the
> > > > fragmented part of the IPv6 packet:
> > > >
> > > > 12 bool ipv6_ext_hdr(u8 nexthdr)
> > > > 13 {
> > > > 14 /*
> > > > 15 * find out if nexthdr is an extension header or a protocol
> > > > 16 */
> > > > 17 return (nexthdr == NEXTHDR_HOP) ||
> > > > 18 (nexthdr == NEXTHDR_ROUTING) ||
> > > > 19 (nexthdr == NEXTHDR_FRAGMENT) ||
> > > > 20 (nexthdr == NEXTHDR_AUTH) ||
> > > > 21 (nexthdr == NEXTHDR_NONE) ||
> > > > 22 (nexthdr == NEXTHDR_DEST);
> > > >
> > > >> - hp->nexthdr == NEXTHDR_NONE)) {
> > > >> + if (target < 0) {
> > > >> if (fragoff)
> > > >> *fragoff = _frag_off;
> > > >> return hp->nexthdr;
> > > >> --
> > > >> To unsubscribe from this list: send the line "unsubscribe netdev" in
> > > >> the body of a message to majordomo@vger.kernel.org
> > > >> More majordomo info at http://vger.kernel.org/majordomo-info.html
> > >
> > > I think this is incorrect. Authentication header shows up in the
> > > fragmentable part of the original IPv6 packet. So, for the non-first
> > > fragments the next-header field value can be NEXTHDR_AUTH.
> >
> > Pablo's mail got me thinking again.
> >
> > In general, IPv6 extension headers can appear in any order and stacks
> > must be process them. Fragmentation adds a limitation, that some
> > extension headers do not make sense and don't have any effect if they
> > appear after a fragmentation header (HbH and ROUTING).
> >
> > Looking at the rest of the function we don't check for HBHHDR or RTHDR
> > following a fragmentation header either if we process the first fragment
> > (core stack only processes HBH if directly following the ipv6 header
> > anyway).
> >
> > So, in my opinion, it is safe to completely remove this check and it
> > would align if the rest of the extension processing logic. The callers
> > all seem fine with that.
> >
> > Pablo, what do you think?
>
> I'm afraid we cannot just get rid of that !ipv6_ext_hdr() check. The
> ipv6_find_hdr() function is designed to return the transport protocol.
> After the proposed change, it will return extension header numbers.
> This will break existing ip6tables rulesets since the `-p' option
> relies on this function to match the transport protocol.
>
> Note that the AH header is skipped (see code a bit below this
> problematic fragmentation handling) so the follow up header after the
> AH header is returned as the transport header.
>
> We can probably return the AH protocol number for non-1st fragments.
> However, that would be something new to ip6tables since nobody has
> ever seen packet matching `-p ah' rules.

# ip6tables -I INPUT -p ah
Warning: never matched protocol: ah. use extension match instead.