xfrm: policy: fix netlink/pf_key policy lookups

Colin Ian King says:
 Static analysis with CoverityScan found a potential issue [..]
 It seems that pointer pol is set to NULL and then a check to see if it
 is non-null is used to set pol to tmp; howeverm this check is always
 going to be false because pol is always NULL.

Fix this and update test script to catch this.  Updated script only:
./xfrm_policy.sh ; echo $?
RTNETLINK answers: No such file or directory
FAIL: ip -net ns3 xfrm policy get src 10.0.1.0/24 dst 10.0.2.0/24 dir out
RTNETLINK answers: No such file or directory
[..]
PASS: policy before exception matches
PASS: ping to .254 bypassed ipsec tunnel
PASS: direct policy matches
PASS: policy matches
1

Fixes: 6be3b0db ("xfrm: policy: add inexact policy search tree infrastructure")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -1663,7 +1663,10 @@ struct xfrm_policy *xfrm_policy_bysel_ctx(struct net *net, u32 mark, u32 if_id,
 			tmp = __xfrm_policy_bysel_ctx(cand.res[i], mark,
 						      if_id, type, dir,
 						      sel, ctx);
-			if (tmp && pol && tmp->pos < pol->pos)
+			if (!tmp)
+				continue;
+
+			if (!pol || tmp->pos < pol->pos)
 				pol = tmp;
 		}
 	} else {

tools/testing/selftests/net/xfrm_policy.sh

@@ -21,6 +21,7 @@
 # Kselftest framework requirement - SKIP code is 4.
 ksft_skip=4
 ret=0
+policy_checks_ok=1
 
 KEY_SHA=0xdeadbeef1234567890abcdefabcdefabcdefabcd
 KEY_AES=0x0123456789abcdef0123456789012345
@@ -45,6 +46,26 @@ do_esp() {
     ip -net $ns xfrm policy add src $rnet dst $lnet dir fwd tmpl src $remote dst $me proto esp mode tunnel priority 100 action allow
 }
 
+do_esp_policy_get_check() {
+    local ns=$1
+    local lnet=$2
+    local rnet=$3
+
+    ip -net $ns xfrm policy get src $lnet dst $rnet dir out > /dev/null
+    if [ $? -ne 0 ] && [ $policy_checks_ok -eq 1 ] ;then
+        policy_checks_ok=0
+        echo "FAIL: ip -net $ns xfrm policy get src $lnet dst $rnet dir out"
+        ret=1
+    fi
+
+    ip -net $ns xfrm policy get src $rnet dst $lnet dir fwd > /dev/null
+    if [ $? -ne 0 ] && [ $policy_checks_ok -eq 1 ] ;then
+        policy_checks_ok=0
+        echo "FAIL: ip -net $ns xfrm policy get src $rnet dst $lnet dir fwd"
+        ret=1
+    fi
+}
+
 do_exception() {
     local ns=$1
     local me=$2
@@ -112,31 +133,31 @@ check_xfrm() {
 	# 1: iptables -m policy rule count != 0
 	rval=$1
 	ip=$2
-	ret=0
+	lret=0
 
 	ip netns exec ns1 ping -q -c 1 10.0.2.$ip > /dev/null
 
 	check_ipt_policy_count ns3
 	if [ $? -ne $rval ] ; then
-		ret=1
+		lret=1
 	fi
 	check_ipt_policy_count ns4
 	if [ $? -ne $rval ] ; then
-		ret=1
+		lret=1
 	fi
 
 	ip netns exec ns2 ping -q -c 1 10.0.1.$ip > /dev/null
 
 	check_ipt_policy_count ns3
 	if [ $? -ne $rval ] ; then
-		ret=1
+		lret=1
 	fi
 	check_ipt_policy_count ns4
 	if [ $? -ne $rval ] ; then
-		ret=1
+		lret=1
 	fi
 
-	return $ret
+	return $lret
 }
 
 #check for needed privileges
@@ -227,6 +248,11 @@ do_esp ns4 dead:3::10 dead:3::1 dead:2::/64 dead:1::/64 $SPI2 $SPI1
 do_dummies4 ns3
 do_dummies6 ns4
 
+do_esp_policy_get_check ns3 10.0.1.0/24 10.0.2.0/24
+do_esp_policy_get_check ns4 10.0.2.0/24 10.0.1.0/24
+do_esp_policy_get_check ns3 dead:1::/64 dead:2::/64
+do_esp_policy_get_check ns4 dead:2::/64 dead:1::/64
+
 # ping to .254 should use ipsec, exception is not installed.
 check_xfrm 1 254
 if [ $? -ne 0 ]; then