Possible NULL dereference caused by -stable commit ef81bb40bf15f350fe865f31fa42f1082772a576

Hi all:

A possible NULL dereference were noticed by the stable commit ef81bb40bf15f350fe865f31fa42f1082772a576 (which is a backport of 87c48fa3b4630905f98268dde838ee43626a060c). The case happens when bridge froward a packet from guest to a physical nic, at this time no route is attached to the skb which may lead a NULL dereference in ipv6_select_ident(). -Next version have this check so it is fine. The following patch may be used to avoid this but may also lead the ip identification predicable, and this defect is also exist -next version when no route because we still depends on a global variable to generate the identification. Any thought on this?

Thanks.

index 4ea6e21..414e2f4 100644
@@ -622,7 +622,9 @@ static u32 __ipv6_select_ident(const struct in6_addr *addr)
 
 void ipv6_select_ident(struct frag_hdr *fhdr, struct rt6_info *rt)
 {
-       fhdr->identification = htonl(__ipv6_select_ident(&rt->rt6i_dst.addr));
+       const struct in6_addr addr = IN6ADDR_ANY_INIT;
+       fhdr->identification =
+               htonl(__ipv6_select_ident(rt ? &rt->rt6i_dst.addr : &addr));
 }
 
 int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))

1) Discussion on current kernel :

All we need here is not the route but inet_peer, so that inet_getid() can be called on it.

If no route is given to ipv6_select_ident(), at least we can try to get inet_peer, and release it before exiting from ipv6_select_ident()

2) On stable kernel, we already use an array (ipv6_fragmentation_id[FID_HASH_SZ];) to make less predictables fragments ids.

So we could get dst addr from the packet to be forwarded instead of the (possibly NULL) route.

From: Eric Dumazet <[email protected]>
Date: Wed, 28 Sep 2011 14:09:09 +0200

It's not exactly that simple, you have to parse all hop-by-hop options to get the correct "destination" address.

From: Eric Dumazet <[email protected]>
Date: Wed, 28 Sep 2011 14:09:09 +0200

Ok, after some auditing, there is only one call site of ipv6_select_ident() that can happen with a NULL route and that is udp6_ufo_fragment().

ipv6_gso_segment() already walks the extension headers via ipv6_gso_pull_exthdrs() so maybe we can calculate the true destination address there and get that passed down somehow into the fragment ID selection for an inetpeer lookup.