[PATCH 3.16 375/410] ip_tunnel: Clamp MTU to bounds on new link

[Ben Hutchings]

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Stefano Brivio <sbrivio@xxxxxxxxxx>

commit 24fc79798b8ddfd46f2dd363a8d29072c083b977 upstream.

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>
[bwh: Backported to 3.16: Add definition of ETH_MIN_MTU]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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
@@ -968,8 +968,14 @@ 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);
 
--- a/include/uapi/linux/if_ether.h
+++ b/include/uapi/linux/if_ether.h
@@ -36,6 +36,8 @@
 #define ETH_FRAME_LEN	1514		/* Max. octets in frame sans FCS */
 #define ETH_FCS_LEN	4		/* Octets in the FCS		 */
 
+#define ETH_MIN_MTU	68		/* Min IPv4 MTU per RFC791	*/
+
 /*
  *	These are the defined Ethernet Protocol ID's.
  */