[PATCH 3.18 18/32] ipv4: igmp: guard against silly MTU values

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>


[ 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>
---
 include/net/ip.h     |    2 ++
 net/ipv4/devinet.c   |    2 +-
 net/ipv4/igmp.c      |   24 +++++++++++++++---------
 net/ipv4/ip_tunnel.c |    4 ++--
 4 files changed, 20 insertions(+), 12 deletions(-)

--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -33,6 +33,8 @@
 #include <net/flow.h>
 #include <net/flow_keys.h>
 
+#define IPV4_MIN_MTU		68			/* RFC 791 */
+
 struct sock;
 
 struct inet_skb_parm {
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -1328,7 +1328,7 @@ 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,
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -402,16 +402,17 @@ static int grec_size(struct ip_mc_list *
 }
 
 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;
@@ -433,10 +434,15 @@ static struct sk_buff *add_grec(struct s
 	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;
 
+	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 ||
@@ -457,7 +463,7 @@ static struct sk_buff *add_grec(struct s
 		    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;
@@ -484,12 +490,12 @@ static struct sk_buff *add_grec(struct s
 				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)
@@ -523,7 +529,7 @@ 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)
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -395,8 +395,8 @@ static int ip_tunnel_bind_dev(struct net
 	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;
 }