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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Stefano Brivio <sbrivio@redhat.com>,
        Sabrina Dubroca <sd@queasysnail.net>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.14 264/496] ip_tunnel: Clamp MTU to bounds on new link
Date:   Mon, 28 May 2018 12:00:49 +0200
Message-Id: <20180528100330.986219941@linuxfoundation.org>
In-Reply-To: <20180528100319.498712256@linuxfoundation.org>
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4.14-stable review patch.  If anyone has any objections, please let me know.

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

From: Stefano Brivio <sbrivio@redhat.com>

[ 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@redhat.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 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: