From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Ahern <dsahern@gmail.com>
Subject: Re: [PATCH net-next 15/20] net/neighbor: Update neightbl_dump_info
 for strict data checking
Date: Sun, 7 Oct 2018 19:34:53 -0600
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 <20181004213355.14899-16-dsahern@kernel.org>
 <20181007104846.cjy4zsmz5lsosq3o@brauner.io>
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Cc: netdev@vger.kernel.org, davem@davemloft.net, jbenc@redhat.com,
        stephen@networkplumber.org
To: Christian Brauner <christian@brauner.io>,
        David Ahern <dsahern@kernel.org>
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On 10/7/18 4:48 AM, Christian Brauner wrote:
>> +
>>  static int neightbl_dump_info(struct sk_buff *skb, struct netlink_callback *cb)
>>  {
>> +	const struct nlmsghdr *nlh = cb->nlh;
>>  	struct net *net = sock_net(skb->sk);
>>  	int family, tidx, nidx = 0;
>>  	int tbl_skip = cb->args[0];
>>  	int neigh_skip = cb->args[1];
>>  	struct neigh_table *tbl;
>>  
>> -	family = ((struct rtgenmsg *) nlmsg_data(cb->nlh))->rtgen_family;
>> +	if (cb->strict_check) {
>> +		int err = neightbl_valid_dump_info(nlh, cb->extack);
>> +
>> +		if (err)
>> +			return err;
>> +	}
>> +
>> +	family = ((struct rtgenmsg *)nlmsg_data(nlh))->rtgen_family;
> 
> So this already was a problem prior to your patch: what happens when you
> pass in the wrong struct? Then this case is not safe to do and might
> contain all kinds of crap.

'This case' meaning the above dereference? family is *always* the first
element in all of the header structs. It is core to the rtnetlink
processing.