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ipv4: igmp: guard against silly MTU values

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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>
This commit is contained in:
Eric Dumazet 2017-12-11 07:17:39 -08:00 committed by David S. Miller
parent b9b312a7a4
commit b5476022bb
4 changed files with 19 additions and 12 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

1
include/net/ip.h
View file

@ -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;

2
net/ipv4/devinet.c
View file

@ -1428,7 +1428,7 @@ 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,

24
net/ipv4/igmp.c
View file

@ -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 @@ 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)

4
net/ipv4/ip_tunnel.c
View file

@ -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;
}