netfilter: ipv6: avoid nf_iterate recursion

The previous patch changed nf_ct_frag6_gather() to morph reassembled skb
with the previous one.

This means that the return value is always NULL or the skb argument.
So change it to an err value.

Instead of invoking NF_HOOK recursively with threshold to skip already-called hooks
we can now just return NF_ACCEPT to move on to the next hook except for
-EINPROGRESS (which means skb has been queued for reassembly), in which case we
return NF_STOLEN.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

2 files changed, 36 insertions, 49 deletions

diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
index 1a86a08adbe5..912bc3afc183 100644
--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -361,14 +361,15 @@ err:
 
 /*
  *      Check if this packet is complete.
- *      Returns NULL on failure by any reason, and pointer
- *      to current nexthdr field in reassembled frame.
  *
  *      It is called with locked fq, and caller must check that
  *      queue is eligible for reassembly i.e. it is not COMPLETE,
  *      the last and the first frames arrived and all the bits are here.
+ *
+ *      returns true if *prev skb has been transformed into the reassembled
+ *      skb, false otherwise.
  */
-static struct sk_buff *
+static bool
 nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev,  struct net_device *dev)
 {
         struct sk_buff *fp, *head = fq->q.fragments;
@@ -382,22 +383,21 @@ nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev,  struct net_devic
 
         ecn = ip_frag_ecn_table[fq->ecn];
         if (unlikely(ecn == 0xff))
-                goto out_fail;
+                return false;
 
         /* Unfragmented part is taken from the first segment. */
         payload_len = ((head->data - skb_network_header(head)) -
                        sizeof(struct ipv6hdr) + fq->q.len -
                        sizeof(struct frag_hdr));
         if (payload_len > IPV6_MAXPLEN) {
-                pr_debug("payload len is too large.\n");
-                goto out_oversize;
+                net_dbg_ratelimited("nf_ct_frag6_reasm: payload len = %d\n",
+                                    payload_len);
+                return false;
         }
 
         /* Head of list must not be cloned. */
-        if (skb_unclone(head, GFP_ATOMIC)) {
-                pr_debug("skb is cloned but can't expand head");
-                goto out_oom;
-        }
+        if (skb_unclone(head, GFP_ATOMIC))
+                return false;
 
         /* If the first fragment is fragmented itself, we split
          * it to two chunks: the first with data and paged part
@@ -408,7 +408,7 @@ nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev,  struct net_devic
 
                 clone = alloc_skb(0, GFP_ATOMIC);
                 if (clone == NULL)
-                        goto out_oom;
+                        return false;
 
                 clone->next = head->next;
                 head->next = clone;
@@ -438,7 +438,7 @@ nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev,  struct net_devic
 
                 fp = skb_clone(prev, GFP_ATOMIC);
                 if (!fp)
-                        goto out_oom;
+                        return false;
 
                 fp->next = prev->next;
                 skb_queue_walk(head, iter) {
@@ -494,16 +494,7 @@ nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev,  struct net_devic
         fq->q.fragments = NULL;
         fq->q.fragments_tail = NULL;
 
-        return head;
-
-out_oversize:
-        net_dbg_ratelimited("nf_ct_frag6_reasm: payload len = %d\n",
-                            payload_len);
-        goto out_fail;
-out_oom:
-        net_dbg_ratelimited("nf_ct_frag6_reasm: no memory for reassembly\n");
-out_fail:
-        return NULL;
+        return true;
 }
 
 /*
@@ -569,27 +560,26 @@ find_prev_fhdr(struct sk_buff *skb, u8 *prevhdrp, int *prevhoff, int *fhoff)
         return 0;
 }
 
-struct sk_buff *nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
+int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 {
         struct net_device *dev = skb->dev;
+        int fhoff, nhoff, ret;
         struct frag_hdr *fhdr;
         struct frag_queue *fq;
         struct ipv6hdr *hdr;
-        int fhoff, nhoff;
         u8 prevhdr;
-        struct sk_buff *ret_skb = NULL;
 
         /* Jumbo payload inhibits frag. header */
         if (ipv6_hdr(skb)->payload_len == 0) {
                 pr_debug("payload len = 0\n");
-                return skb;
+                return -EINVAL;
         }
 
         if (find_prev_fhdr(skb, &prevhdr, &nhoff, &fhoff) < 0)
-                return skb;
+                return -EINVAL;
 
         if (!pskb_may_pull(skb, fhoff + sizeof(*fhdr)))
-                return skb;
+                return -ENOMEM;
 
         skb_set_transport_header(skb, fhoff);
         hdr = ipv6_hdr(skb);
@@ -598,27 +588,28 @@ struct sk_buff *nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 use
         fq = fq_find(net, fhdr->identification, user, &hdr->saddr, &hdr->daddr,
                      ip6_frag_ecn(hdr));
         if (fq == NULL)
-                return skb;
+                return -ENOMEM;
 
         spin_lock_bh(&fq->q.lock);
 
         if (nf_ct_frag6_queue(fq, skb, fhdr, nhoff) < 0) {
-                spin_unlock_bh(&fq->q.lock);
-                pr_debug("Can't insert skb to queue\n");
-                inet_frag_put(&fq->q, &nf_frags);
-                return skb;
+                ret = -EINVAL;
+                goto out_unlock;
         }
 
+        /* after queue has assumed skb ownership, only 0 or -EINPROGRESS
+         * must be returned.
+         */
+        ret = -EINPROGRESS;
         if (fq->q.flags == (INET_FRAG_FIRST_IN | INET_FRAG_LAST_IN) &&
-            fq->q.meat == fq->q.len) {
-                ret_skb = nf_ct_frag6_reasm(fq, skb, dev);
-                if (ret_skb == NULL)
-                        pr_debug("Can't reassemble fragmented packets\n");
-        }
-        spin_unlock_bh(&fq->q.lock);
+            fq->q.meat == fq->q.len &&
+            nf_ct_frag6_reasm(fq, skb, dev))
+                ret = 0;
 
+out_unlock:
+        spin_unlock_bh(&fq->q.lock);
         inet_frag_put(&fq->q, &nf_frags);
-        return ret_skb;
+        return ret;
 }
 EXPORT_SYMBOL_GPL(nf_ct_frag6_gather);
 
diff --git a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
index fb96b1018884..f7aab5ab93a5 100644
--- a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
+++ b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
@@ -55,7 +55,7 @@ static unsigned int ipv6_defrag(void *priv,
                                 struct sk_buff *skb,
                                 const struct nf_hook_state *state)
 {
-        struct sk_buff *reasm;
+        int err;
 
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)
         /* Previously seen (loopback)?  */
@@ -63,17 +63,13 @@ static unsigned int ipv6_defrag(void *priv,
                 return NF_ACCEPT;
 #endif
 
-        reasm = nf_ct_frag6_gather(state->net, skb,
-                                   nf_ct6_defrag_user(state->hook, skb));
+        err = nf_ct_frag6_gather(state->net, skb,
+                                 nf_ct6_defrag_user(state->hook, skb));
         /* queued */
-        if (reasm == NULL)
+        if (err == -EINPROGRESS)
                 return NF_STOLEN;
 
-        NF_HOOK_THRESH(NFPROTO_IPV6, state->hook, state->net, state->sk, reasm,
-                       state->in, state->out,
-                       state->okfn, NF_IP6_PRI_CONNTRACK_DEFRAG + 1);
-
-        return NF_STOLEN;
+        return NF_ACCEPT;
 }
 
 static struct nf_hook_ops ipv6_defrag_ops[] = {