netdev - Re: [net-next 1/2] dummy: add device MTU validation check

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Date:   Thu, 21 Sep 2017 08:02:18 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Zhang Shengju <zhangshengju@...s.chinamobile.com>
Cc:     davem@...emloft.net, willemb@...gle.com,
        stephen@...workplumber.org, netdev@...r.kernel.org
Subject: Re: [net-next 1/2] dummy: add device MTU validation check

On Thu, 2017-09-21 at 21:32 +0800, Zhang Shengju wrote:
> Currently, any mtu value can be assigned when adding a new dummy device:
> [~]# ip link add name dummy1 mtu 100000 type dummy
> [~]# ip link show dummy1
> 15: dummy1: <BROADCAST,NOARP> mtu 100000 qdisc noop state DOWN mode DEFAULT group default qlen 1000
>     link/ether 0a:61:6b:16:14:ce brd ff:ff:ff:ff:ff:ff
> 
> This patch adds device MTU validation check.

What is wrong with big MTU on dummy ?

If this is a generic rule, this check should belong in core network
stack.

> 
> Signed-off-by: Zhang Shengju <zhangshengju@...s.chinamobile.com>
> ---
>  drivers/net/dummy.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/net/dummy.c b/drivers/net/dummy.c
> index e31ab3b..0276b2b 100644
> --- a/drivers/net/dummy.c
> +++ b/drivers/net/dummy.c
> @@ -365,6 +365,14 @@ static int dummy_validate(struct nlattr *tb[], struct nlattr *data[],
>  		if (!is_valid_ether_addr(nla_data(tb[IFLA_ADDRESS])))
>  			return -EADDRNOTAVAIL;
>  	}
> +
> +	if (tb[IFLA_MTU]) {
> +		u32 mtu = nla_get_u32(tb[IFLA_MTU]);

You do not verify/validate nla_len(tb[IFLA_MTU]).

Do not ever trust user space.

> +
> +		if (mtu > ETH_MAX_MTU)
> +			return -EINVAL;
> +	}
> +
>  	return 0;
>  }
>