ipv4: igmp: guard against silly MTU values

[ Upstream commit b5476022bbada3764609368f03329ca287528dc8 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 20 insertions, 12 deletions

diff --git a/include/net/ip.h b/include/net/ip.h
index 7476bb10ff37..639398af273b 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -33,6 +33,8 @@
 #include <net/flow.h>
 #include <net/flow_dissector.h>
 
+#define IPV4_MIN_MTU		68			/* RFC 791 */
+
 struct sock;
 
 struct inet_skb_parm {
diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index 0212591b0077..63f99e9a821b 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -1358,7 +1358,7 @@ skip:
 
 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 3809d523d012..6d3497bb3885 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -410,16 +410,17 @@ 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 = (struct igmpv3_grec *)skb_put(skb, sizeof(struct igmpv3_grec));
 	pgr->grec_type = type;
 	pgr->grec_auxwords = 0;
@@ -441,12 +442,17 @@ 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) && !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 ||
@@ -467,7 +473,7 @@ 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;
@@ -494,12 +500,12 @@ 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)
@@ -533,7 +539,7 @@ empty_source:
 				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 3310ac75e3f3..c18245e05d26 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -400,8 +400,8 @@ 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;
 }