Patchwork [05/19] netfilter: nf_conntrack_ipv6: improve fragmentation handling

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Submitter Patrick McHardy
Date Aug. 18, 2012, 12:43 p.m.
Message ID <Pine.GSO.4.63.1208181438060.17600@stinky-local.trash.net>
Download mbox | patch
Permalink /patch/178459/
State Superseded
Headers show

Comments

Patrick McHardy - Aug. 18, 2012, 12:43 p.m.
On Fri, 17 Aug 2012, Pablo Neira Ayuso wrote:

> On Thu, Aug 09, 2012 at 10:08:49PM +0200, kaber@trash.net wrote:
>> +
>> +		/* 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)

Fixed, thanks.

>
> - 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.

I see. Also fixed, thanks. New patch attached.
commit f4266409e20dc0d2d22447aa7935c997f38914a1
Author: Patrick McHardy <kaber@trash.net>
Date:   Tue Aug 14 15:27:55 2012 +0200

    netfilter: nf_conntrack_ipv6: improve fragmentation handling
    
    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>

Patch

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..9f36e38 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 +200,14 @@  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;
+	const struct nf_conn_help *help;

+	struct nf_conn *ct;

+	enum ip_conntrack_info ctinfo;

 
 	/* This packet is fragmented and has reassembled packet. */
 	if (reasm) {
@@ -213,6 +219,23 @@  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 && !nf_ct_is_untracked(ct)) {

+			help = nfct_help(ct);

+			if (help && help->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 +251,7 @@  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 +265,7 @@  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);