nat: fix accidental o2i deletion/reuse

Nat session is allocated before the port allocation. During port allocation
candidate address+port are set to o2i 6-tuple and tested against the flow hash.
If insertion fails, the port is busy and rejected. When all N attempts are
unsuccessful, "out-of-ports" error is recorded and the session is to be
deleted.

During session deletion o2i and i2o tuples are deleted from the flow hash.
In case of "out-of-ports" i2o tuple is not valid, however o2i is and it refers
to **some other** session that's known to be allocated.

By backing match tuple up session should be invalidated well enough not to
collide with any valid one.

Type: fix
Signed-off-by: Dmitry Valter <d-valter@yandex-team.ru>
Change-Id: Id30be6f26ecce7a5a63135fb971bb65ce318af82

2 files changed, 79 insertions, 2 deletions

diff --git a/src/plugins/nat/nat44-ed/nat44_ed_in2out.c b/src/plugins/nat/nat44-ed/nat44_ed_in2out.c
index f41fcac5153..deec0099933 100644
--- a/src/plugins/nat/nat44-ed/nat44_ed_in2out.c
+++ b/src/plugins/nat/nat44-ed/nat44_ed_in2out.c
@@ -105,6 +105,9 @@ nat_ed_alloc_addr_and_port_with_snat_address (
   const u16 port_thread_offset =
     (port_per_thread * snat_thread_index) + ED_USER_PORT_OFFSET;
 
+  /* Backup original match in case of failure */
+  const nat_6t_t match = s->o2i.match;
+
   s->o2i.match.daddr = a->addr;
   /* first try port suggested by caller */
   u16 port = clib_net_to_host_u16 (*outside_port);
@@ -136,6 +139,9 @@ nat_ed_alloc_addr_and_port_with_snat_address (
       --attempts;
     }
   while (attempts > 0);
+
+  /* Revert match */
+  s->o2i.match = match;
   return 1;
 }
 
diff --git a/test/test_nat44_ed.py b/test/test_nat44_ed.py
index 8a8c96870f4..7279b6a7d56 100644
--- a/test/test_nat44_ed.py
+++ b/test/test_nat44_ed.py
@@ -4,6 +4,7 @@ import unittest
 from io import BytesIO
 from random import randint, choice
 
+import re
 import scapy.compat
 from framework import tag_fixme_ubuntu2204, is_distro_ubuntu2204
 from framework import VppTestCase, VppTestRunner, VppLoInterface
@@ -48,8 +49,9 @@ class TestNAT44ED(VppTestCase):
         if not self.vpp_dead:
             self.plugin_disable()
 
-    def plugin_enable(self):
-        self.vapi.nat44_ed_plugin_enable_disable(sessions=self.max_sessions, enable=1)
+    def plugin_enable(self, max_sessions=None):
+        max_sessions = max_sessions or self.max_sessions
+        self.vapi.nat44_ed_plugin_enable_disable(sessions=max_sessions, enable=1)
 
     def plugin_disable(self):
         self.vapi.nat44_ed_plugin_enable_disable(enable=0)
@@ -616,6 +618,16 @@ class TestNAT44ED(VppTestCase):
 
         return pkts
 
+    def create_udp_stream(self, in_if, out_if, count, base_port=6303):
+        return [
+            (
+                Ether(dst=in_if.local_mac, src=in_if.remote_mac)
+                / IP(src=in_if.remote_ip4, dst=out_if.remote_ip4, ttl=64)
+                / UDP(sport=base_port + i, dport=20)
+            )
+            for i in range(count)
+        ]
+
     def create_stream_frag(
         self, src_if, dst, sport, dport, data, proto=IP_PROTOS.tcp, echo_reply=False
     ):
@@ -4821,6 +4833,65 @@ class TestNAT44EDMW(TestNAT44ED):
         )
         self.send_and_expect(self.pg0, p, self.pg1)
 
+    def test_dynamic_ports_exhausted(self):
+        """NAT44ED dynamic translation test: address ports exhaused"""
+
+        sessions_per_batch = 128
+        n_available_ports = 65536 - 1024
+        n_sessions = n_available_ports + 2 * sessions_per_batch
+
+        # set high enough session limit for ports to be exhausted
+        self.plugin_disable()
+        self.plugin_enable(max_sessions=n_sessions)
+
+        self.nat_add_inside_interface(self.pg0)
+        self.nat_add_outside_interface(self.pg1)
+
+        # set timeouts to high for sessions to reallistically expire
+        config = self.vapi.nat44_show_running_config()
+        old_timeouts = config.timeouts
+        self.vapi.nat_set_timeouts(
+            udp=21600,
+            tcp_established=old_timeouts.tcp_established,
+            tcp_transitory=old_timeouts.tcp_transitory,
+            icmp=old_timeouts.icmp,
+        )
+
+        # in2out after NAT addresses added
+        self.nat_add_address(self.nat_addr)
+
+        for i in range(n_sessions // sessions_per_batch):
+            pkts = self.create_udp_stream(
+                self.pg0,
+                self.pg1,
+                sessions_per_batch,
+                base_port=i * sessions_per_batch + 100,
+            )
+
+            self.pg0.add_stream(pkts)
+            self.pg_start()
+
+            err = self.statistics.get_err_counter(
+                "/err/nat44-ed-in2out-slowpath/out of ports"
+            )
+            if err > sessions_per_batch:
+                break
+
+        # Check for ports to be used no more than once
+        ports = set()
+        sessions = self.vapi.cli("show nat44 sessions")
+        rx = re.compile(
+            f" *o2i flow: match: saddr {self.pg1.remote_ip4} sport [0-9]+ daddr {self.nat_addr} dport ([0-9]+) proto UDP.*"
+        )
+        for line in sessions.splitlines():
+            m = rx.match(line)
+            if m:
+                port = int(m.groups()[0])
+                self.assertNotIn(port, ports)
+                ports.add(port)
+
+        self.assertGreaterEqual(err, sessions_per_batch)
+
 
 if __name__ == "__main__":
     unittest.main(testRunner=VppTestRunner)