[PATCH] net: igmp: Use correct source address on IGMPv3 reports

[Kevin Cernekee]

Closing a multicast socket after the final IPv4 address is deleted
from an interface can generate a membership report that uses the
source IP from a different interface.  The following test script, run
from an isolated netns, reproduces the issue:

    #!/bin/bash

    ip link add dummy0 type dummy
    ip link add dummy1 type dummy
    ip link set dummy0 up
    ip link set dummy1 up
    ip addr add 10.1.1.1/24 dev dummy0
    ip addr add 192.168.99.99/24 dev dummy1

    tcpdump -U -i dummy0 &
    socat EXEC:"sleep 2" \
        UDP4-DATAGRAM:239.101.1.68:8889,ip-add-membership=239.0.1.68:10.1.1.1 &

    sleep 1
    ip addr del 10.1.1.1/24 dev dummy0
    sleep 5
    kill %tcpdump

RFC 3376 specifies that the report must be sent with a valid IP source
address from the destination subnet, or from address 0.0.0.0.  Add an
extra check to make sure this is the case.

Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxxxxx>
---
 net/ipv4/igmp.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index d1f8f302dbf3..0672264c9d93 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -89,6 +89,7 @@
 #include <linux/rtnetlink.h>
 #include <linux/times.h>
 #include <linux/pkt_sched.h>
+#include <linux/byteorder/generic.h>
 
 #include <net/net_namespace.h>
 #include <net/arp.h>
@@ -321,6 +322,23 @@ igmp_scount(struct ip_mc_list *pmc, int type, int gdeleted, int sdeleted)
 	return scount;
 }
 
+/* source address selection per RFC 3376 section 4.2.13 */
+static __be32 igmpv3_get_srcaddr(struct net_device *dev,
+				 const struct flowi4 *fl4)
+{
+	struct in_device *in_dev = __in_dev_get_rcu(dev);
+
+	if (!in_dev)
+		return htonl(INADDR_ANY);
+
+	for_ifa(in_dev) {
+		if (inet_ifa_match(fl4->saddr, ifa))
+			return fl4->saddr;
+	} endfor_ifa(in_dev);
+
+	return htonl(INADDR_ANY);
+}
+
 static struct sk_buff *igmpv3_newpack(struct net_device *dev, unsigned int mtu)
 {
 	struct sk_buff *skb;
@@ -368,7 +386,7 @@ static struct sk_buff *igmpv3_newpack(struct net_device *dev, unsigned int mtu)
 	pip->frag_off = htons(IP_DF);
 	pip->ttl      = 1;
 	pip->daddr    = fl4.daddr;
-	pip->saddr    = fl4.saddr;
+	pip->saddr    = igmpv3_get_srcaddr(dev, &fl4);
 	pip->protocol = IPPROTO_IGMP;
 	pip->tot_len  = 0;	/* filled in later */
 	ip_select_ident(net, skb, NULL);
-- 
2.15.1.424.g9478a66081-goog