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>

diff --git a/include/net/ip.h b/include/net/ip.h
index 7f0cd17668aec570178efa0d912e9824ee21b695..55da8ad905828d71b5174bd493c84d49d96cf56d 100644 (file)
--- 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 {

diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index 464293c34b7ad5df55c478bb459a85b867514395..618d55f4aa77badc8b989a926af159509128285e 100644 (file)
--- 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,

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 1ea62ccfa884b356f2f0e87e73c7a56b930479e0..66b0381d6c18eb139552bfbda8c8614164966014 100644 (file)
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -402,16 +402,17 @@ 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;
@@ -433,10 +434,15 @@ 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;
 
+       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 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;
@@ -484,12 +490,12 @@ 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)
@@ -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)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 6822572429718b0a783e24e4e53c98cc5d54ea5f..7e01368f51ed44fc63350c73f995728e8d330573 100644 (file)
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -395,8 +395,8 @@ 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;
 }