netfilter: nat: fix icmp id randomization

Sven Auhagen reported that a 2nd ping request will fail if 'fully-random'
mode is used.

Reason is that if no proto information is given, min/max are both 0,
so we set the icmp id to 0 instead of chosing a random value between
0 and 65535.

Update test case as well to catch this, without fix this yields:
[..]
ERROR: cannot ping ns1 from ns2 with ip masquerade fully-random (attempt 2)
ERROR: cannot ping ns1 from ns2 with ipv6 masquerade fully-random (attempt 2)

... becaus 2nd ping clashes with existing 'id 0' icmp conntrack and gets
dropped.

Fixes: 203f2e78 ("netfilter: nat: remove l4proto->unique_tuple")
Reported-by: Sven Auhagen <sven.auhagen@voleatech.de>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_core.c

@@ -415,9 +415,14 @@ static void nf_nat_l4proto_unique_tuple(struct nf_conntrack_tuple *tuple,
 	case IPPROTO_ICMPV6:
 		/* id is same for either direction... */
 		keyptr = &tuple->src.u.icmp.id;
-		min = range->min_proto.icmp.id;
+		if (!(range->flags & NF_NAT_RANGE_PROTO_SPECIFIED)) {
+			min = 0;
+			range_size = 65536;
+		} else {
+			min = ntohs(range->min_proto.icmp.id);
 			range_size = ntohs(range->max_proto.icmp.id) -
 				     ntohs(range->min_proto.icmp.id) + 1;
+		}
 		goto find_free_id;
 #if IS_ENABLED(CONFIG_NF_CT_PROTO_GRE)
 	case IPPROTO_GRE:

tools/testing/selftests/netfilter/nft_nat.sh

@@ -321,6 +321,7 @@ EOF
 
 test_masquerade6()
 {
+	local natflags=$1
 	local lret=0
 
 	ip netns exec ns0 sysctl net.ipv6.conf.all.forwarding=1 > /dev/null
@@ -354,13 +355,13 @@ ip netns exec ns0 nft -f - <<EOF
 table ip6 nat {
 	chain postrouting {
 		type nat hook postrouting priority 0; policy accept;
-		meta oif veth0 masquerade
+		meta oif veth0 masquerade $natflags
 	}
 }
 EOF
 	ip netns exec ns2 ping -q -c 1 dead:1::99 > /dev/null # ping ns2->ns1
 	if [ $? -ne 0 ] ; then
-		echo "ERROR: cannot ping ns1 from ns2 with active ipv6 masquerading"
+		echo "ERROR: cannot ping ns1 from ns2 with active ipv6 masquerade $natflags"
 		lret=1
 	fi
 
@@ -397,19 +398,26 @@ EOF
 		fi
 	done
 
+	ip netns exec ns2 ping -q -c 1 dead:1::99 > /dev/null # ping ns2->ns1
+	if [ $? -ne 0 ] ; then
+		echo "ERROR: cannot ping ns1 from ns2 with active ipv6 masquerade $natflags (attempt 2)"
+		lret=1
+	fi
+
 	ip netns exec ns0 nft flush chain ip6 nat postrouting
 	if [ $? -ne 0 ]; then
 		echo "ERROR: Could not flush ip6 nat postrouting" 1>&2
 		lret=1
 	fi
 
-	test $lret -eq 0 && echo "PASS: IPv6 masquerade for ns2"
+	test $lret -eq 0 && echo "PASS: IPv6 masquerade $natflags for ns2"
 
 	return $lret
 }
 
 test_masquerade()
 {
+	local natflags=$1
 	local lret=0
 
 	ip netns exec ns0 sysctl net.ipv4.conf.veth0.forwarding=1 > /dev/null
@@ -417,7 +425,7 @@ test_masquerade()
 
 	ip netns exec ns2 ping -q -c 1 10.0.1.99 > /dev/null # ping ns2->ns1
 	if [ $? -ne 0 ] ; then
-		echo "ERROR: canot ping ns1 from ns2"
+		echo "ERROR: cannot ping ns1 from ns2 $natflags"
 		lret=1
 	fi
 
@@ -443,13 +451,13 @@ ip netns exec ns0 nft -f - <<EOF
 table ip nat {
 	chain postrouting {
 		type nat hook postrouting priority 0; policy accept;
-		meta oif veth0 masquerade
+		meta oif veth0 masquerade $natflags
 	}
 }
 EOF
 	ip netns exec ns2 ping -q -c 1 10.0.1.99 > /dev/null # ping ns2->ns1
 	if [ $? -ne 0 ] ; then
-		echo "ERROR: cannot ping ns1 from ns2 with active ip masquerading"
+		echo "ERROR: cannot ping ns1 from ns2 with active ip masquere $natflags"
 		lret=1
 	fi
 
@@ -485,13 +493,19 @@ EOF
 		fi
 	done
 
+	ip netns exec ns2 ping -q -c 1 10.0.1.99 > /dev/null # ping ns2->ns1
+	if [ $? -ne 0 ] ; then
+		echo "ERROR: cannot ping ns1 from ns2 with active ip masquerade $natflags (attempt 2)"
+		lret=1
+	fi
+
 	ip netns exec ns0 nft flush chain ip nat postrouting
 	if [ $? -ne 0 ]; then
 		echo "ERROR: Could not flush nat postrouting" 1>&2
 		lret=1
 	fi
 
-	test $lret -eq 0 && echo "PASS: IP masquerade for ns2"
+	test $lret -eq 0 && echo "PASS: IP masquerade $natflags for ns2"
 
 	return $lret
 }
@@ -750,8 +764,12 @@ test_local_dnat
 test_local_dnat6
 
 reset_counters
-test_masquerade
-test_masquerade6
+test_masquerade ""
+test_masquerade6 ""
+
+reset_counters
+test_masquerade "fully-random"
+test_masquerade6 "fully-random"
 
 reset_counters
 test_redirect