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

Pablo Neira Ayuso
2012-08-17T13:36:12Z

On Thu, Aug 09, 2012 at 10:08:49PM +0200, kaber@trash.net wrote:
> From: Patrick McHardy <kaber@trash.net>
>
> 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 <kaber@trash.net>
> ---
> 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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