[PATCH 3.16 365/366] ip_tunnel: restore binding to ifaces with a large mtu

Ben Hutchings
2018-10-14T15:25:41Z

From: Nicolas Dichtel <nicolas.dichtel@6wind.com>

commit 82612de1c98e610d194e34178bde3cca7dedce41 upstream.

After commit f6cc9c054e77, the following conf is broken (note that the
default loopback mtu is 65536, ie IP_MAX_MTU + 1):

$ ip tunnel add gre1 mode gre local 10.125.0.1 remote 10.125.0.2 dev lo
add tunnel "gre0" failed: Invalid argument
$ ip l a type dummy
$ ip l s dummy1 up
$ ip l s dummy1 mtu 65535
$ ip tunnel add gre1 mode gre local 10.125.0.1 remote 10.125.0.2 dev dummy1
add tunnel "gre0" failed: Invalid argument

dev_set_mtu() doesn't allow to set a mtu which is too large.
First, let's cap the mtu returned by ip_tunnel_bind_dev(). Second, remove
the magic value 0xFFF8 and use IP_MAX_MTU instead.
0xFFF8 seems to be there for ages, I don't know why this value was used.

With a recent kernel, it's also possible to set a mtu > IP_MAX_MTU:
$ ip l s dummy1 mtu 66000
After that patch, it's also possible to bind an ip tunnel on that kind of
interface.

CC: Petr Machata <petrm@mellanox.com>
CC: Ido Schimmel <idosch@mellanox.com>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/davem/netdev-vger-cvs.git/commit/?id=e5afd356a411a
Fixes: f6cc9c054e77 ("ip_tunnel: Emit events for post-register MTU changes")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16:
- Drop change in ip_tunnel_create()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -387,7 +387,7 @@ static int ip_tunnel_bind_dev(struct net

if (tdev) {
hlen = tdev->hard_header_len + tdev->needed_headroom;
- mtu = tdev->mtu;
+ mtu = min(tdev->mtu, IP_MAX_MTU);
}
dev->iflink = tunnel->parms.link;

@@ -851,7 +851,7 @@ int ip_tunnel_change_mtu(struct net_devi
int t_hlen = tunnel->hlen + sizeof(struct iphdr);

if (new_mtu < 68 ||
- new_mtu > 0xFFF8 - dev->hard_header_len - t_hlen)
+ new_mtu > IP_MAX_MTU - dev->hard_header_len - t_hlen)
return -EINVAL;
dev->mtu = new_mtu;
return 0;
@@ -979,7 +979,7 @@ int ip_tunnel_newlink(struct net_device

mtu = ip_tunnel_bind_dev(dev);
if (tb[IFLA_MTU]) {
- unsigned int max = 0xfff8 - dev->hard_header_len - nt->hlen;
+ unsigned int max = IP_MAX_MTU - dev->hard_header_len - nt->hlen;

mtu = clamp(dev->mtu, (unsigned int)ETH_MIN_MTU,
(unsigned int)(max - sizeof(struct iphdr)));