netdev - [PATCH net] macvlan: enforce a consistent minimal mtu

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Message-Id: <20221007225743.1633333-1-eric.dumazet@gmail.com>
Date:   Fri,  7 Oct 2022 15:57:43 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: [PATCH net] macvlan: enforce a consistent minimal mtu

From: Eric Dumazet <edumazet@...gle.com>

macvlan should enforce a minimal mtu of 68, even at link creation.

This patch avoids the current behavior (which could lead to crashes
in ipv6 stack if the link is brought up)

$ ip link add macvlan1 link eno1 mtu 8 type macvlan  # This should fail !
$ ip link sh dev macvlan1
5: macvlan1@...1: <BROADCAST,MULTICAST> mtu 8 qdisc noop
    state DOWN mode DEFAULT group default qlen 1000
    link/ether 02:47:6c:24:74:82 brd ff:ff:ff:ff:ff:ff
$ ip link set macvlan1 mtu 67
Error: mtu less than device minimum.
$ ip link set macvlan1 mtu 68
$ ip link set macvlan1 mtu 8
Error: mtu less than device minimum.

Fixes: 91572088e3fd ("net: use core MTU range checking in core net infra")
Reported-by: syzbot <syzkaller@...glegroups.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 drivers/net/macvlan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index 713e3354cb2eb14cbf4009e67858c261508a037b..8f8f73099de8d8308b45b7e3bf9179e8311182ee 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -1192,7 +1192,7 @@ void macvlan_common_setup(struct net_device *dev)
 {
 	ether_setup(dev);
 
-	dev->min_mtu		= 0;
+	/* ether_setup() has set dev->min_mtu to ETH_MIN_MTU. */
 	dev->max_mtu		= ETH_MAX_MTU;
 	dev->priv_flags	       &= ~IFF_TX_SKB_SHARING;
 	netif_keep_dst(dev);
-- 
2.38.0.rc1.362.ged0d419d3c-goog