From: Eric Dumazet <edumazet@google.com>
Date: Mon, 11 Dec 2017 07:17:39 -0800
Subject: ipv4: igmp: guard against silly MTU values
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Patch-mainline: v4.15-rc4
Git-commit: b5476022bbada3764609368f03329ca287528dc8
References: bsc#1082869

IPv4 stack reacts to changes to small MTU, by disabling itself under
RTNL.

But there is a window where threads not using RTNL can see a wrong
device mtu. This can lead to surprises, in igmp code where it is
assumed the mtu is suitable.

Fix this by reading device mtu once and checking IPv4 minimal MTU.

This patch adds missing IPV4_MIN_MTU define, to not abuse
ETH_MIN_MTU anymore.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Michal Kubecek <mkubecek@suse.cz>

---
 include/net/ip.h     |  1 +
 net/ipv4/devinet.c   |  2 +-
 net/ipv4/igmp.c      | 24 +++++++++++++++---------
 net/ipv4/ip_tunnel.c |  4 ++--
 4 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 44beac1debb2..fca53d24524f 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -34,6 +34,7 @@
 #include <net/flow_dissector.h>
 
 #define IPV4_MAX_PMTU		65535U		/* RFC 2675, Section 5.1 */
+#define IPV4_MIN_MTU		68			/* RFC 791 */
 
 struct sock;
 
diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index df14815a3b8c..4d13d7a97b2d 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -1387,7 +1387,7 @@ static void inetdev_changename(struct net_device *dev, struct in_device *in_dev)
 
 static bool inetdev_valid_mtu(unsigned int mtu)
 {
-	return mtu >= 68;
+	return mtu >= IPV4_MIN_MTU;
 }
 
 static void inetdev_send_gratuitous_arp(struct net_device *dev,
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 4e97f7ab6676..e86554e9ed70 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -404,16 +404,17 @@ static int grec_size(struct ip_mc_list *pmc, int type, int gdel, int sdel)
 }
 
 static struct sk_buff *add_grhead(struct sk_buff *skb, struct ip_mc_list *pmc,
-	int type, struct igmpv3_grec **ppgr)
+	int type, struct igmpv3_grec **ppgr, unsigned int mtu)
 {
 	struct net_device *dev = pmc->interface->dev;
 	struct igmpv3_report *pih;
 	struct igmpv3_grec *pgr;
 
-	if (!skb)
-		skb = igmpv3_newpack(dev, dev->mtu);
-	if (!skb)
-		return NULL;
+	if (!skb) {
+		skb = igmpv3_newpack(dev, mtu);
+		if (!skb)
+			return NULL;
+	}
 	pgr = skb_put(skb, sizeof(struct igmpv3_grec));
 	pgr->grec_type = type;
 	pgr->grec_auxwords = 0;
@@ -436,12 +437,17 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ip_mc_list *pmc,
 	struct igmpv3_grec *pgr = NULL;
 	struct ip_sf_list *psf, *psf_next, *psf_prev, **psf_list;
 	int scount, stotal, first, isquery, truncate;
+	unsigned int mtu;
 
 	if (pmc->multiaddr == IGMP_ALL_HOSTS)
 		return skb;
 	if (ipv4_is_local_multicast(pmc->multiaddr) && !net->ipv4.sysctl_igmp_llm_reports)
 		return skb;
 
+	mtu = READ_ONCE(dev->mtu);
+	if (mtu < IPV4_MIN_MTU)
+		return skb;
+
 	isquery = type == IGMPV3_MODE_IS_INCLUDE ||
 		  type == IGMPV3_MODE_IS_EXCLUDE;
 	truncate = type == IGMPV3_MODE_IS_EXCLUDE ||
@@ -462,7 +468,7 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ip_mc_list *pmc,
 		    AVAILABLE(skb) < grec_size(pmc, type, gdeleted, sdeleted)) {
 			if (skb)
 				igmpv3_sendpack(skb);
-			skb = igmpv3_newpack(dev, dev->mtu);
+			skb = igmpv3_newpack(dev, mtu);
 		}
 	}
 	first = 1;
@@ -498,12 +504,12 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ip_mc_list *pmc,
 				pgr->grec_nsrcs = htons(scount);
 			if (skb)
 				igmpv3_sendpack(skb);
-			skb = igmpv3_newpack(dev, dev->mtu);
+			skb = igmpv3_newpack(dev, mtu);
 			first = 1;
 			scount = 0;
 		}
 		if (first) {
-			skb = add_grhead(skb, pmc, type, &pgr);
+			skb = add_grhead(skb, pmc, type, &pgr, mtu);
 			first = 0;
 		}
 		if (!skb)
@@ -538,7 +544,7 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ip_mc_list *pmc,
 				igmpv3_sendpack(skb);
 				skb = NULL; /* add_grhead will get a new one */
 			}
-			skb = add_grhead(skb, pmc, type, &pgr);
+			skb = add_grhead(skb, pmc, type, &pgr, mtu);
 		}
 	}
 	if (pgr)
diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index e9805ad664ac..4e90082b23a6 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -349,8 +349,8 @@ static int ip_tunnel_bind_dev(struct net_device *dev)
 	dev->needed_headroom = t_hlen + hlen;
 	mtu -= (dev->hard_header_len + t_hlen);
 
-	if (mtu < 68)
-		mtu = 68;
+	if (mtu < IPV4_MIN_MTU)
+		mtu = IPV4_MIN_MTU;
 
 	return mtu;
 }
-- 
2.16.3