[PATCH 4.14 264/496] ip_tunnel: Clamp MTU to bounds on new link

[Greg Kroah-Hartman]

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

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

From: Stefano Brivio <sbrivio@xxxxxxxxxx>

[ Upstream commit 24fc79798b8ddfd46f2dd363a8d29072c083b977 ]

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@xxxxxxxxxx>
Acked-by: Sabrina Dubroca <sd@xxxxxxxxxxxxxxx>
Signed-off-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv4/ip_tunnel.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -1103,8 +1103,14 @@ int ip_tunnel_newlink(struct net_device
 		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: