Re: [PATCH net 3/9] netfilter: nf_nat: undo erroneous tcp edemux lookup

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On 6.3.2021 16.49, Mika Penttilä wrote:

On 6.3.2021 14.12, Pablo Neira Ayuso wrote:
From: Florian Westphal <fw@xxxxxxxxx>

Under extremely rare conditions TCP early demux will retrieve the wrong

1. local machine establishes a connection to a remote server, S, on port

    This gives:
    laddr:lport -> S:p
    ... both in tcp and conntrack.

2. local machine establishes a connection to host H, on port p2.
    2a. TCP stack choses same laddr:lport, so we have
    laddr:lport -> H:p2 from TCP point of view.
    2b). There is a destination NAT rewrite in place, translating
         H:p2 to S:p.  This results in following conntrack entries:

    I)  laddr:lport -> S:p  (origin)  S:p -> laddr:lport (reply)
    II) laddr:lport -> H:p2 (origin)  S:p -> laddr:lport2 (reply)

    NAT engine has rewritten laddr:lport to laddr:lport2 to map
    the reply packet to the correct origin.
Could you eloborate where and how linux nat engine is doing the

laddr:lport to laddr:lport2

rewrite? There's only DST nat and there should be conflict (for reply) in tuple establishment afaik....

Ah I see it is the nat null binding for src to make it unique

    When server sends SYN/ACK to laddr:lport2, the PREROUTING hook
    will undo-the SNAT transformation, rewriting IP header to
    S:p -> laddr:lport

    This causes TCP early demux to associate the skb with the TCP socket
    of the first connection.

    The INPUT hook will then reverse the DNAT transformation, rewriting
    the IP header to H:p2 -> laddr:lport.

Because packet ends up with the wrong socket, the new connection
never completes: originator stays in SYN_SENT and conntrack entry
remains in SYN_RECV until timeout, and responder retransmits SYN/ACK
until it gives up.

To resolve this, orphan the skb after the input rewrite:
Because the source IP address changed, the socket must be incorrect.
We can't move the DNAT undo to prerouting due to backwards
compatibility, doing so will make iptables/nftables rules to no longer
match the way they did.

After orphan, the packet will be handed to the next protocol layer
(tcp, udp, ...) and that will repeat the socket lookup just like as if
early demux was disabled.

Fixes: 41063e9dd1195 ("ipv4: Early TCP socket demux.")
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
  net/netfilter/nf_nat_proto.c | 25 +++++++++++++++++++++----
  1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/net/netfilter/nf_nat_proto.c b/net/netfilter/nf_nat_proto.c
index e87b6bd6b3cd..4731d21fc3ad 100644
--- a/net/netfilter/nf_nat_proto.c
+++ b/net/netfilter/nf_nat_proto.c
@@ -646,8 +646,8 @@ nf_nat_ipv4_fn(void *priv, struct sk_buff *skb,
    static unsigned int
-nf_nat_ipv4_in(void *priv, struct sk_buff *skb,
-           const struct nf_hook_state *state)
+nf_nat_ipv4_pre_routing(void *priv, struct sk_buff *skb,
+            const struct nf_hook_state *state)
      unsigned int ret;
      __be32 daddr = ip_hdr(skb)->daddr;
@@ -659,6 +659,23 @@ nf_nat_ipv4_in(void *priv, struct sk_buff *skb,
      return ret;
  +static unsigned int
+nf_nat_ipv4_local_in(void *priv, struct sk_buff *skb,
+             const struct nf_hook_state *state)
+    __be32 saddr = ip_hdr(skb)->saddr;
+    struct sock *sk = skb->sk;
+    unsigned int ret;
+    ret = nf_nat_ipv4_fn(priv, skb, state);
+    if (ret == NF_ACCEPT && sk && saddr != ip_hdr(skb)->saddr &&
+        !inet_sk_transparent(sk))
+        skb_orphan(skb); /* TCP edemux obtained wrong socket */
+    return ret;
  static unsigned int
  nf_nat_ipv4_out(void *priv, struct sk_buff *skb,
          const struct nf_hook_state *state)
@@ -736,7 +753,7 @@ nf_nat_ipv4_local_fn(void *priv, struct sk_buff *skb,
  static const struct nf_hook_ops nf_nat_ipv4_ops[] = {
      /* Before packet filtering, change destination */
-        .hook        = nf_nat_ipv4_in,
+        .hook        = nf_nat_ipv4_pre_routing,
          .pf        = NFPROTO_IPV4,
          .hooknum    = NF_INET_PRE_ROUTING,
          .priority    = NF_IP_PRI_NAT_DST,
@@ -757,7 +774,7 @@ static const struct nf_hook_ops nf_nat_ipv4_ops[] = {
      /* After packet filtering, change source */
-        .hook        = nf_nat_ipv4_fn,
+        .hook        = nf_nat_ipv4_local_in,
          .pf        = NFPROTO_IPV4,
          .hooknum    = NF_INET_LOCAL_IN,
          .priority    = NF_IP_PRI_NAT_SRC,

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