On Fri, Nov 01, 2013 at 09:45:29AM +0100, Steffen Klassert wrote: > On Wed, Oct 30, 2013 at 12:07:11AM +0000, Patrick McHardy wrote: > > > > The problem is that the reassembled packet is referenced by the individual > > fragments, so we trigger the BUG_ON in pskb_expand_head(). In this > > particular case the case we BUG() on is actually OK, but I'm looking at > > a way we can fix this without special casing. Hope to have a patch for > > testing in the next hours. > > Just for the record. I'm observing similar, quite reproducable crashes when > receiving fragmented icmp echo request packets on an IPsec gateway with > nf_conntrack_ipv6. > > Since git commit 58a317f10 ("netfilter: ipv6: add IPv6 NAT support") > netfilter might insert a reassembled ipv6 packet with a shared skb and > local_df = 1 to the ok function. In case of xfrm, __xfrm6_output() > fragments the packet again and when adjusting the headroom later, we > crash because of a shared skb. > > I can fix it by checking for a shared skb in ip6_fragment() and do > slow path fragmentation then. But we never needed such a check in > ip6_fragment(), so it's maybe better to fix it in netfilter. So what seems to be happening is that this case in __ipv6_conntrack_in() triggers: /* Conntrack helpers need the entire reassembled packet in the * POST_ROUTING hook. In case of unconfirmed connections NAT * might reassign a helper, so the entire packet is also * required. */ ct = nf_ct_get(reasm, &ctinfo); if (ct != NULL && !nf_ct_is_untracked(ct)) { help = nfct_help(ct); if ((help && help->helper) || !nf_ct_is_confirmed(ct)) { nf_conntrack_get_reasm(reasm); NF_HOOK_THRESH(NFPROTO_IPV6, hooknum, reasm, (struct net_device *)in, (struct net_device *)out, okfn, NF_IP6_PRI_CONNTRACK + 1); Since this code is called while walking through the fragment chain, we have extra references to the reassembled skb. So I think what we need to do is to release the fragment chain before calling NF_HOOK_THRESH() and indicate this to nf_ct_frag6_output() so it will stop processing the chain immediately. I'll give it a try, will let you know when I have a patch for testing. -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html