[PATCH AUTOSEL 4.20 59/81] netfilter: nf_nat: skip nat clash resolution for same-origin entries

From: Sasha Levin
Date: Thu Feb 28 2019 - 10:10:26 EST

From: Martynas Pumputis <martynas@xxxxxxxxxxx>

[ Upstream commit 4e35c1cb9460240e983a01745b5f29fe3a4d8e39 ]

It is possible that two concurrent packets originating from the same
socket of a connection-less protocol (e.g. UDP) can end up having
different IP_CT_DIR_REPLY tuples which results in one of the packets
being dropped.

To illustrate this, consider the following simplified scenario:

1. Packet A and B are sent at the same time from two different threads
by same UDP socket. No matching conntrack entry exists yet.
Both packets cause allocation of a new conntrack entry.
2. get_unique_tuple gets called for A. No clashing entry found.
conntrack entry for A is added to main conntrack table.
3. get_unique_tuple is called for B and will find that the reply
tuple of B is already taken by A.
It will allocate a new UDP source port for B to resolve the clash.
4. conntrack entry for B cannot be added to main conntrack table
because its ORIGINAL direction is clashing with A and the REPLY
directions of A and B are not the same anymore due to UDP source
port reallocation done in step 3.

This patch modifies nf_conntrack_tuple_taken so it doesn't consider
colliding reply tuples if the IP_CT_DIR_ORIGINAL tuples are equal.

[ Florian: simplify patch to not use .allow_clash setting
and always ignore identical flows ]

Signed-off-by: Martynas Pumputis <martynas@xxxxxxxxxxx>
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
net/netfilter/nf_conntrack_core.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index e92e749aff53e..830b1328fe975 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -1007,6 +1007,22 @@ nf_conntrack_tuple_taken(const struct nf_conntrack_tuple *tuple,

if (nf_ct_key_equal(h, tuple, zone, net)) {
+ /* Tuple is taken already, so caller will need to find
+ * a new source port to use.
+ *
+ * Only exception:
+ * If the *original tuples* are identical, then both
+ * conntracks refer to the same flow.
+ * This is a rare situation, it can occur e.g. when
+ * more than one UDP packet is sent from same socket
+ * in different threads.
+ *
+ * Let nf_ct_resolve_clash() deal with this later.
+ */
+ if (nf_ct_tuple_equal(&ignored_conntrack->tuplehash[IP_CT_DIR_ORIGINAL].tuple,
+ &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple))
+ continue;
NF_CT_STAT_INC_ATOMIC(net, found);
return 1;