net: fib_rules: Fix iif / oif matching on L3 master device

Before commit 40867d74c374 ("net: Add l3mdev index to flow struct and
avoid oif reset for port devices") it was possible to use FIB rules to
match on a L3 domain. This was done by having a FIB rule match on iif /
oif being a L3 master device. It worked because prior to the FIB rule
lookup the iif / oif fields in the flow structure were reset to the
index of the L3 master device to which the input / output device was
enslaved to.

The above scheme made it impossible to match on the original input /
output device. Therefore, cited commit stopped overwriting the iif / oif
fields in the flow structure and instead stored the index of the
enslaving L3 master device in a new field ('flowi_l3mdev') in the flow
structure.

While the change enabled new use cases, it broke the original use case
of matching on a L3 domain. Fix this by interpreting the iif / oif
matching on a L3 master device as a match against the L3 domain. In
other words, if the iif / oif in the FIB rule points to a L3 master
device, compare the provided index against 'flowi_l3mdev' rather than
'flowi_{i,o}if'.

Before cited commit, a FIB rule that matched on 'iif vrf1' would only
match incoming traffic from devices enslaved to 'vrf1'. With the
proposed change (i.e., comparing against 'flowi_l3mdev'), the rule would
also match traffic originating from a socket bound to 'vrf1'. Avoid that
by adding a new flow flag ('FLOWI_FLAG_L3MDEV_OIF') that indicates if
the L3 domain was derived from the output interface or the input
interface (when not set) and take this flag into account when evaluating
the FIB rule against the flow structure.

Avoid unnecessary checks in the data path by detecting that a rule
matches on a L3 master device when the rule is installed and marking it
as such.

Tested using the following script [1].

Output before 40867d74c374 (v5.4.291):

default dev dummy1 table 100 scope link
default dev dummy1 table 200 scope link

Output after 40867d74c374:

default dev dummy1 table 300 scope link
default dev dummy1 table 300 scope link

Output with this patch:

default dev dummy1 table 100 scope link
default dev dummy1 table 200 scope link

[1]
 #!/bin/bash

 ip link add name vrf1 up type vrf table 10
 ip link add name dummy1 up master vrf1 type dummy

 sysctl -wq net.ipv4.conf.all.forwarding=1
 sysctl -wq net.ipv4.conf.all.rp_filter=0

 ip route add table 100 default dev dummy1
 ip route add table 200 default dev dummy1
 ip route add table 300 default dev dummy1

 ip rule add prio 0 oif vrf1 table 100
 ip rule add prio 1 iif vrf1 table 200
 ip rule add prio 2 table 300

 ip route get 192.0.2.1 oif dummy1 fibmatch
 ip route get 192.0.2.1 iif dummy1 from 198.51.100.1 fibmatch

Fixes: 40867d74c374 ("net: Add l3mdev index to flow struct and avoid oif reset for port devices")
Reported-by: hanhuihui <hanhuihui5@huawei.com>
Closes: https://lore.kernel.org/netdev/ec671c4f821a4d63904d0da15d604b75@huawei.com/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250414172022.242991-2-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 43 insertions, 9 deletions

diff --git a/net/core/fib_rules.c b/net/core/fib_rules.c
index 4bc64d912a1c..7af302080a66 100644
--- a/net/core/fib_rules.c
+++ b/net/core/fib_rules.c
@@ -257,6 +257,24 @@ static int nla_put_port_range(struct sk_buff *skb, int attrtype,
 	return nla_put(skb, attrtype, sizeof(*range), range);
 }
 
+static bool fib_rule_iif_match(const struct fib_rule *rule, int iifindex,
+			       const struct flowi *fl)
+{
+	u8 iif_is_l3_master = READ_ONCE(rule->iif_is_l3_master);
+
+	return iif_is_l3_master ? l3mdev_fib_rule_iif_match(fl, iifindex) :
+				  fl->flowi_iif == iifindex;
+}
+
+static bool fib_rule_oif_match(const struct fib_rule *rule, int oifindex,
+			       const struct flowi *fl)
+{
+	u8 oif_is_l3_master = READ_ONCE(rule->oif_is_l3_master);
+
+	return oif_is_l3_master ? l3mdev_fib_rule_oif_match(fl, oifindex) :
+				  fl->flowi_oif == oifindex;
+}
+
 static int fib_rule_match(struct fib_rule *rule, struct fib_rules_ops *ops,
 			  struct flowi *fl, int flags,
 			  struct fib_lookup_arg *arg)
@@ -264,11 +282,11 @@ static int fib_rule_match(struct fib_rule *rule, struct fib_rules_ops *ops,
 	int iifindex, oifindex, ret = 0;
 
 	iifindex = READ_ONCE(rule->iifindex);
-	if (iifindex && (iifindex != fl->flowi_iif))
+	if (iifindex && !fib_rule_iif_match(rule, iifindex, fl))
 		goto out;
 
 	oifindex = READ_ONCE(rule->oifindex);
-	if (oifindex && (oifindex != fl->flowi_oif))
+	if (oifindex && !fib_rule_oif_match(rule, oifindex, fl))
 		goto out;
 
 	if ((rule->mark ^ fl->flowi_mark) & rule->mark_mask)
@@ -736,16 +754,20 @@ static int fib_nl2rule_rtnl(struct fib_rule *nlrule,
 		struct net_device *dev;
 
 		dev = __dev_get_by_name(nlrule->fr_net, nlrule->iifname);
-		if (dev)
+		if (dev) {
 			nlrule->iifindex = dev->ifindex;
+			nlrule->iif_is_l3_master = netif_is_l3_master(dev);
+		}
 	}
 
 	if (tb[FRA_OIFNAME]) {
 		struct net_device *dev;
 
 		dev = __dev_get_by_name(nlrule->fr_net, nlrule->oifname);
-		if (dev)
+		if (dev) {
 			nlrule->oifindex = dev->ifindex;
+			nlrule->oif_is_l3_master = netif_is_l3_master(dev);
+		}
 	}
 
 	return 0;
@@ -1336,11 +1358,17 @@ static void attach_rules(struct list_head *rules, struct net_device *dev)
 
 	list_for_each_entry(rule, rules, list) {
 		if (rule->iifindex == -1 &&
-		    strcmp(dev->name, rule->iifname) == 0)
+		    strcmp(dev->name, rule->iifname) == 0) {
 			WRITE_ONCE(rule->iifindex, dev->ifindex);
+			WRITE_ONCE(rule->iif_is_l3_master,
+				   netif_is_l3_master(dev));
+		}
 		if (rule->oifindex == -1 &&
-		    strcmp(dev->name, rule->oifname) == 0)
+		    strcmp(dev->name, rule->oifname) == 0) {
 			WRITE_ONCE(rule->oifindex, dev->ifindex);
+			WRITE_ONCE(rule->oif_is_l3_master,
+				   netif_is_l3_master(dev));
+		}
 	}
 }
 
@@ -1349,10 +1377,14 @@ static void detach_rules(struct list_head *rules, struct net_device *dev)
 	struct fib_rule *rule;
 
 	list_for_each_entry(rule, rules, list) {
-		if (rule->iifindex == dev->ifindex)
+		if (rule->iifindex == dev->ifindex) {
 			WRITE_ONCE(rule->iifindex, -1);
-		if (rule->oifindex == dev->ifindex)
+			WRITE_ONCE(rule->iif_is_l3_master, false);
+		}
+		if (rule->oifindex == dev->ifindex) {
 			WRITE_ONCE(rule->oifindex, -1);
+			WRITE_ONCE(rule->oif_is_l3_master, false);
+		}
 	}
 }
 
diff --git a/net/l3mdev/l3mdev.c b/net/l3mdev/l3mdev.c
index ca10916340b0..5432a5f2dfc8 100644
--- a/net/l3mdev/l3mdev.c
+++ b/net/l3mdev/l3mdev.c
@@ -277,8 +277,10 @@ void l3mdev_update_flow(struct net *net, struct flowi *fl)
 	if (fl->flowi_oif) {
 		dev = dev_get_by_index_rcu(net, fl->flowi_oif);
 		if (dev) {
-			if (!fl->flowi_l3mdev)
+			if (!fl->flowi_l3mdev) {
 				fl->flowi_l3mdev = l3mdev_master_ifindex_rcu(dev);
+				fl->flowi_flags |= FLOWI_FLAG_L3MDEV_OIF;
+			}
 
 			/* oif set to L3mdev directs lookup to its table;
 			 * reset to avoid oif match in fib_lookup