[PATCH v2 1/3] ipv4: fix inet_select_addr() when enable route_localnet

[luoshijie <luoshijie1@huawei.com>]

From: Shijie Luo <luoshijie1@huawei.com>

Suppose we have two interfaces eth0 and eth1 in two hosts, follow
the same steps in the two hosts:
 # sysctl -w net.ipv4.conf.eth1.route_localnet=1
 # sysctl -w net.ipv4.conf.eth1.arp_announce=2
 # ip route del 127.0.0.0/8 dev lo table local
and then set ip to eth1 in host1 like:
 # ifconfig eth1 127.25.3.4/24
set ip to eth2 in host2 and ping host1:
 # ifconfig eth1 127.25.3.14/24
 # ping -I eth1 127.25.3.4
Well, host2 cannot connect to host1.

When set a ip address with head 127, the scope of the address defaults
to RT_SCOPE_HOST. In this situation, host2 will use arp_solicit() to
send a arp request for the mac address of host1 with ip
address 127.25.3.14. When arp_announce=2, inet_select_addr() cannot
select a correct saddr with condition ifa->ifa_scope > scope, because
ifa_scope is RT_SCOPE_HOST and scope is RT_SCOPE_LINK. Then,
inet_select_addr() will go to no_in_dev to lookup all interfaces to find
a primary ip and finally get the primary ip of eth0.

Here I add a localnet_scope defaults to RT_SCOPE_HOST, and when
route_localnet is enabled, this value changes to RT_SCOPE_LINK to make
inet_select_addr() find a correct primary ip as saddr of arp request.

Fixes: d0daebc3d622 ("ipv4: Add interface option to enable routing of 127.0.0.0/8")

Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
---
 net/ipv4/devinet.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index c6bd0f7a020a..08c6c7c41749 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -1254,6 +1254,7 @@ static __be32 in_dev_select_addr(const struct in_device *in_dev,
 __be32 inet_select_addr(const struct net_device *dev, __be32 dst, int scope)
 {
 	__be32 addr = 0;
+	unsigned char localnet_scope = RT_SCOPE_HOST;
 	struct in_device *in_dev;
 	struct net *net = dev_net(dev);
 	int master_idx;
@@ -1263,8 +1264,11 @@ __be32 inet_select_addr(const struct net_device *dev, __be32 dst, int scope)
 	if (!in_dev)
 		goto no_in_dev;
 
+	if (unlikely(IN_DEV_ROUTE_LOCALNET(in_dev)))
+		localnet_scope = RT_SCOPE_LINK;
+
 	for_primary_ifa(in_dev) {
-		if (ifa->ifa_scope > scope)
+		if (min(ifa->ifa_scope, localnet_scope) > scope)
 			continue;
 		if (!dst || inet_ifa_match(dst, ifa)) {
 			addr = ifa->ifa_local;
-- 
2.19.1