pf: fix handling unreassembled fragments

If we handle a fragment and are configured not to reassemble it the
pd->proto field will show the layer 4 protocol (i.e. UDP,TCP,SCTP,...) but
pd->virtual_proto will show we're a fragment.
In that case we also don't have the layer 4 checksum pointer. Have code that
cares about L4 (e.g. NAT) check virtual_proto so it doesn't try to dereference a
NULL pcksum field.

PR:		288549
Reported by:	Danilo Egea Gondolfo <danilo@FreeBSD.org>
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D51722

sys/netpfil/pf/pf.c

@@ -621,7 +621,7 @@ static void
 pf_packet_rework_nat(struct pf_pdesc *pd, int off, struct pf_state_key *nk)
 {
 
-	switch (pd->proto) {
+	switch (pd->virtual_proto) {
 	case IPPROTO_TCP: {
 		struct tcphdr *th = &pd->hdr.tcp;
 
@@ -6391,7 +6391,7 @@ pf_translate_compat(struct pf_test_ctx *ctx)
 	KASSERT(ctx->sk != NULL, ("%s: null sk", __func__));
 	KASSERT(ctx->nk != NULL, ("%s: null nk", __func__));
 
-	switch (pd->proto) {
+	switch (pd->virtual_proto) {
 	case IPPROTO_TCP:
 		if (PF_ANEQ(&pd->nsaddr, &nk->addr[pd->sidx], pd->af) ||
 		    nk->port[pd->sidx] != pd->nsport) {

tests/sys/netpfil/pf/Makefile

@@ -55,6 +55,7 @@ ATF_TESTS_SH+=	altq \
 		tcp \
 		tos
 
+ATF_TESTS_PYTEST+=	frag4.py
 ATF_TESTS_PYTEST+=	frag6.py
 ATF_TESTS_PYTEST+=	header.py
 ATF_TESTS_PYTEST+=	icmp.py

tests/sys/netpfil/pf/frag4.py

@@ -0,0 +1,72 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2025 Rubicon Communications, LLC (Netgate)
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+import pytest
+from utils import DelayedSend
+from atf_python.sys.net.tools import ToolsHelper
+from atf_python.sys.net.vnet import VnetTestTemplate
+
+class TestFrag4_NoReassemble(VnetTestTemplate):
+    REQUIRED_MODULES = [ "pf" ]
+    TOPOLOGY = {
+        "vnet1": {"ifaces": ["if1"]},
+        "vnet2": {"ifaces": ["if1", "if2"]},
+        "vnet3": {"ifaces": ["if2"]},
+        "if1": {"prefixes4": [("192.0.2.1/24", "192.0.2.2/24")]},
+        "if2": {"prefixes4": [("198.51.100.1/24", "198.51.100.2/24")]},
+    }
+
+    def vnet2_handler(self, vnet):
+        outifname = vnet.iface_alias_map["if2"].name
+
+        ToolsHelper.print_output("/sbin/pfctl -e")
+        ToolsHelper.pf_rules([
+            "set reassemble no",
+            "nat on %s from 192.0.2.0/24 to any -> (%s)" % (outifname, outifname),
+            "pass out"
+        ])
+
+        ToolsHelper.print_output("/sbin/sysctl net.inet.ip.forwarding=1")
+
+    def vnet3_handler(self, vnet):
+        # We deliberately don't set the default gateway here, so if we get a
+        # reply from this we know we did NAT in vnet2
+        pass
+
+    @pytest.mark.require_user("root")
+    @pytest.mark.require_progs(["scapy"])
+    def test_udp_frag(self):
+        ToolsHelper.print_output("/sbin/route add default 192.0.2.2")
+        ToolsHelper.print_output("/sbin/ping -c 3 198.51.100.2")
+
+        # Import in the correct vnet, so at to not confuse Scapy
+        import scapy.all as sp
+
+        pkt = sp.IP(dst="198.51.100.2", frag=123) \
+            / sp.UDP(dport=12345, sport=54321)
+        reply = sp.sr1(pkt, timeout=3)
+        # We don't expect a reply
+        assert not reply