ip_tunnel: Clamp MTU to bounds on new link

Otherwise, it's possible to specify invalid MTU values directly
on creation of a link (via 'ip link add'). This is already
prevented on subsequent MTU changes by commit b96f9afee4eb
("ipv4/6: use core net MTU range checking").

Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 6d21068f9b55..7c76dd17b6b9 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -1108,8 +1108,14 @@ int ip_tunnel_newlink(struct net_device *dev, struct nlattr *tb[],
                 eth_hw_addr_random(dev);
 
         mtu = ip_tunnel_bind_dev(dev);
-        if (!tb[IFLA_MTU])
+        if (tb[IFLA_MTU]) {
+                unsigned int max = 0xfff8 - dev->hard_header_len - nt->hlen;
+
+                dev->mtu = clamp(dev->mtu, (unsigned int)ETH_MIN_MTU,
+                                 (unsigned int)(max - sizeof(struct iphdr)));
+        } else {
                 dev->mtu = mtu;
+        }
 
         ip_tunnel_add(itn, nt);
 out: