Subject: Re: [PATCH 05/19] netfilter: nf_conntrack_ipv6: improve fragmentation handling

From: Pablo Neira Ayuso <[email-masked]>
Date: Fri, 17 Aug 2012 15:36:08 +0200

On Thu, Aug 09, 2012 at 10:08:49PM +0200, [email-masked] wrote:
> From: Patrick McHardy <[email-masked]>
> 
> The IPv6 conntrack fragmentation currently has a couple of shortcomings.
> Fragmentes are collected in PREROUTING/OUTPUT, are defragmented, the
> defragmented packet is then passed to conntrack, the resulting conntrack
> information is attached to each original fragment and the fragments then
> continue their way through the stack.
> 
> Helper invocation occurs in the POSTROUTING hook, at which point only
> the original fragments are available. The result of this is that
> fragmented packets are never passed to helpers.
> 
> This patch improves the situation in the following way:
> 
> - If a reassembled packet belongs to a connection that has a helper
>   assigned, the reassembled packet is passed through the stack instead
>   of the original fragments.
> 
> - During defragmentation, the largest received fragment size is stored.
>   On output, the packet is refragmented if required. If the largest
>   received fragment size exceeds the outgoing MTU, a "packet too big"
>   message is generated, thus behaving as if the original fragments
>   were passed through the stack from an outside point of view.
> 
> - The ipv6_helper() hook function can't receive fragments anymore for
>   connections using a helper, so it is switched to use ipv6_skip_exthdr()
>   instead of the netfilter specific nf_ct_ipv6_skip_exthdr() and the
>   reassembled packets are passed to connection tracking helpers.
> 
> The result of this is that we can properly track fragmented packets, but
> still generate ICMPv6 Packet too big messages if we would have before.
> 
> This patch is also required as a precondition for IPv6 NAT, where NAT
> helpers might enlarge packets up to a point that they require
> fragmentation. In that case we can't generate Packet too big messages
> since the proper MTU can't be calculated in all cases (f.i. when
> changing textual representation of a variable amount of addresses),
> so the packet is transparently fragmented iff the original packet or
> fragments would have fit the outgoing MTU.
> 
> Signed-off-by: Patrick McHardy <[email-masked]>
> ---
>  include/linux/ipv6.h                           |    1 +
>  net/ipv6/ip6_output.c                          |    7 +++-
>  net/ipv6/netfilter/nf_conntrack_l3proto_ipv6.c |   37 ++++++++++++++++++------
>  net/ipv6/netfilter/nf_conntrack_reasm.c        |   19 ++++++++++--
>  4 files changed, 50 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
> index 879db26..0b94e91 100644
> --- a/include/linux/ipv6.h
> +++ b/include/linux/ipv6.h
> @@ -256,6 +256,7 @@ struct inet6_skb_parm {
>  #if defined(CONFIG_IPV6_MIP6) || defined(CONFIG_IPV6_MIP6_MODULE)
>  	__u16			dsthao;
>  #endif
> +	__u16			frag_max_size;
>  
>  #define IP6SKB_XFRM_TRANSFORMED	1
>  #define IP6SKB_FORWARDED	2
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 5b2d63e..a4f6263 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -493,7 +493,8 @@ int ip6_forward(struct sk_buff *skb)
>  	if (mtu < IPV6_MIN_MTU)
>  		mtu = IPV6_MIN_MTU;
>  
> -	if (skb->len > mtu && !skb_is_gso(skb)) {
> +	if ((!skb->local_df && skb->len > mtu && !skb_is_gso(skb)) ||
> +	    (IP6CB(skb)->frag_max_size && IP6CB(skb)->frag_max_size > mtu)) {
>  		/* Again, force OUTPUT device used as source address */
>  		skb->dev = dst->dev;
>  		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
> @@ -636,7 +637,9 @@ int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
>  	/* We must not fragment if the socket is set to force MTU discovery
>  	 * or if the skb it not generated by a local socket.
>  	 */
> -	if (unlikely(!skb->local_df && skb->len > mtu)) {
> +	if (unlikely(!skb->local_df && skb->len > mtu) ||
> +		     (IP6CB(skb)->frag_max_size &&
> +		      IP6CB(skb)->frag_max_size > mtu)) {
>  		if (skb->sk && dst_allfrag(skb_dst(skb)))
>  			sk_nocaps_add(skb->sk, NETIF_F_GSO_MASK);
>  
> diff --git a/net/ipv6/netfilter/nf_conntrack_l3proto_ipv6.c b/net/ipv6/netfilter/nf_conntrack_l3proto_ipv6.c
> index 4794f96..560d823 100644
> --- a/net/ipv6/netfilter/nf_conntrack_l3proto_ipv6.c
> +++ b/net/ipv6/netfilter/nf_conntrack_l3proto_ipv6.c
> @@ -153,10 +153,10 @@ static unsigned int ipv6_helper(unsigned int hooknum,
>  	const struct nf_conn_help *help;
>  	const struct nf_conntrack_helper *helper;
>  	enum ip_conntrack_info ctinfo;
> -	unsigned int ret, protoff;
> -	unsigned int extoff = (u8 *)(ipv6_hdr(skb) + 1) - skb->data;
> -	unsigned char pnum = ipv6_hdr(skb)->nexthdr;
> -
> +	unsigned int ret;
> +	__be16 frag_off;
> +	int protoff;
> +	u8 nexthdr;
>  
>  	/* This is where we call the helper: as the packet goes out. */
>  	ct = nf_ct_get(skb, &ctinfo);
> @@ -171,9 +171,10 @@ static unsigned int ipv6_helper(unsigned int hooknum,
>  	if (!helper)
>  		return NF_ACCEPT;
>  
> -	protoff = nf_ct_ipv6_skip_exthdr(skb, extoff, &pnum,
> -					 skb->len - extoff);
> -	if (protoff > skb->len || pnum == NEXTHDR_FRAGMENT) {
> +	nexthdr = ipv6_hdr(skb)->nexthdr;
> +	protoff = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr), &nexthdr,
> +				   &frag_off);
> +	if (protoff < 0 || (frag_off & ntohs(~0x7)) != 0) {
>  		pr_debug("proto header not found\n");
>  		return NF_ACCEPT;
>  	}
> @@ -199,9 +200,13 @@ static unsigned int ipv6_confirm(unsigned int hooknum,
>  static unsigned int __ipv6_conntrack_in(struct net *net,
>  					unsigned int hooknum,
>  					struct sk_buff *skb,
> +					const struct net_device *in,
> +					const struct net_device *out,
>  					int (*okfn)(struct sk_buff *))
>  {
>  	struct sk_buff *reasm = skb->nfct_reasm;
> +	struct nf_conn *ct;
> +	enum ip_conntrack_info ctinfo;
>  
>  	/* This packet is fragmented and has reassembled packet. */
>  	if (reasm) {
> @@ -213,6 +218,20 @@ static unsigned int __ipv6_conntrack_in(struct net *net,
>  			if (ret != NF_ACCEPT)
>  				return ret;
>  		}
> +
> +		/* Conntrack helpers need the entire reassembled packet in the
> +		 * POST_ROUTING hook.
> +		 */
> +		ct = nf_ct_get(reasm, &ctinfo);
> +		if (ct != NULL && test_bit(IPS_HELPER_BIT, &ct->status)) {

Two things regarding the line above:

- I think this also need to check for !nf_ct_is_untracked(ct)

- IPS_HELPER_BIT is only set if the CT target is used to attach
  helpers. I know, this behaviour may seem confusing, but I didn't
  find any better way to avoid that NAT removes the helper
  explicitly attached via CT.

  So basically now that status bit means: "this helper has been attached
  via CT".

  Setting it inconditionally in __nf_ct_try_assign_helper would break
  the magic auto-assign helper code.

  On the other hand, the automatic helper assignment is scheduled to
  be removed (well, it would still take at least one 1.5/2 years
  before we do so). At that time, we'll be able to say that all
  conntrack with IPS_HELPER really has one helper. But now I think that
  you'll have to use for nfct_help instead to check if that ct has a
  helper.

> +			nf_conntrack_get_reasm(skb);
> +			NF_HOOK_THRESH(NFPROTO_IPV6, hooknum, reasm,
> +				       (struct net_device *)in,
> +				       (struct net_device *)out,
> +				       okfn, NF_IP6_PRI_CONNTRACK + 1);
> +			return NF_DROP_ERR(-ECANCELED);
> +		}
> +
>  		nf_conntrack_get(reasm->nfct);
>  		skb->nfct = reasm->nfct;
>  		skb->nfctinfo = reasm->nfctinfo;
> @@ -228,7 +247,7 @@ static unsigned int ipv6_conntrack_in(unsigned int hooknum,
>  				      const struct net_device *out,
>  				      int (*okfn)(struct sk_buff *))
>  {
> -	return __ipv6_conntrack_in(dev_net(in), hooknum, skb, okfn);
> +	return __ipv6_conntrack_in(dev_net(in), hooknum, skb, in, out, okfn);
>  }
>  
>  static unsigned int ipv6_conntrack_local(unsigned int hooknum,
> @@ -242,7 +261,7 @@ static unsigned int ipv6_conntrack_local(unsigned int hooknum,
>  		net_notice_ratelimited("ipv6_conntrack_local: packet too short\n");
>  		return NF_ACCEPT;
>  	}
> -	return __ipv6_conntrack_in(dev_net(out), hooknum, skb, okfn);
> +	return __ipv6_conntrack_in(dev_net(out), hooknum, skb, in, out, okfn);
>  }
>  
>  static struct nf_hook_ops ipv6_conntrack_ops[] __read_mostly = {
> diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
> index c9c78c2..f94fb3a 100644
> --- a/net/ipv6/netfilter/nf_conntrack_reasm.c
> +++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
> @@ -190,6 +190,7 @@ static int nf_ct_frag6_queue(struct nf_ct_frag6_queue *fq, struct sk_buff *skb,
>  			     const struct frag_hdr *fhdr, int nhoff)
>  {
>  	struct sk_buff *prev, *next;
> +	unsigned int payload_len;
>  	int offset, end;
>  
>  	if (fq->q.last_in & INET_FRAG_COMPLETE) {
> @@ -197,8 +198,10 @@ static int nf_ct_frag6_queue(struct nf_ct_frag6_queue *fq, struct sk_buff *skb,
>  		goto err;
>  	}
>  
> +	payload_len = ntohs(ipv6_hdr(skb)->payload_len);
> +
>  	offset = ntohs(fhdr->frag_off) & ~0x7;
> -	end = offset + (ntohs(ipv6_hdr(skb)->payload_len) -
> +	end = offset + (payload_len -
>  			((u8 *)(fhdr + 1) - (u8 *)(ipv6_hdr(skb) + 1)));
>  
>  	if ((unsigned int)end > IPV6_MAXPLEN) {
> @@ -307,6 +310,8 @@ found:
>  	skb->dev = NULL;
>  	fq->q.stamp = skb->tstamp;
>  	fq->q.meat += skb->len;
> +	if (payload_len > fq->q.max_size)
> +		fq->q.max_size = payload_len;
>  	atomic_add(skb->truesize, &nf_init_frags.mem);
>  
>  	/* The first fragment.
> @@ -412,10 +417,12 @@ nf_ct_frag6_reasm(struct nf_ct_frag6_queue *fq, struct net_device *dev)
>  	}
>  	atomic_sub(head->truesize, &nf_init_frags.mem);
>  
> +	head->local_df = 1;
>  	head->next = NULL;
>  	head->dev = dev;
>  	head->tstamp = fq->q.stamp;
>  	ipv6_hdr(head)->payload_len = htons(payload_len);
> +	IP6CB(head)->frag_max_size = sizeof(struct ipv6hdr) + fq->q.max_size;
>  
>  	/* Yes, and fold redundant checksum back. 8) */
>  	if (head->ip_summed == CHECKSUM_COMPLETE)
> @@ -592,6 +599,7 @@ void nf_ct_frag6_output(unsigned int hooknum, struct sk_buff *skb,
>  			int (*okfn)(struct sk_buff *))
>  {
>  	struct sk_buff *s, *s2;
> +	unsigned int ret = 0;
>  
>  	for (s = NFCT_FRAG6_CB(skb)->orig; s;) {
>  		nf_conntrack_put_reasm(s->nfct_reasm);
> @@ -601,8 +609,13 @@ void nf_ct_frag6_output(unsigned int hooknum, struct sk_buff *skb,
>  		s2 = s->next;
>  		s->next = NULL;
>  
> -		NF_HOOK_THRESH(NFPROTO_IPV6, hooknum, s, in, out, okfn,
> -			       NF_IP6_PRI_CONNTRACK_DEFRAG + 1);
> +		if (ret != -ECANCELED)
> +			ret = NF_HOOK_THRESH(NFPROTO_IPV6, hooknum, s,
> +					     in, out, okfn,
> +					     NF_IP6_PRI_CONNTRACK_DEFRAG + 1);
> +		else
> +			kfree_skb(s);
> +
>  		s = s2;
>  	}
>  	nf_conntrack_put_reasm(skb);
> -- 
> 1.7.1
> 
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