netdev - Re: [PATCH net-next 3/8] net: bridge: vlan: add rtm definitions and dump support

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Date:   Tue, 14 Jan 2020 09:49:11 -0700
From:   David Ahern <dsahern@...il.com>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, roopa@...ulusnetworks.com,
        davem@...emloft.net, bridge@...ts.linux-foundation.org
Subject: Re: [PATCH net-next 3/8] net: bridge: vlan: add rtm definitions and
 dump support

On 1/14/20 9:45 AM, Nikolay Aleksandrov wrote:
> On 14/01/2020 18:36, Nikolay Aleksandrov wrote:
>> On 14/01/2020 17:34, David Ahern wrote:
>>> On 1/14/20 6:55 AM, Jakub Kicinski wrote:
>>>> On Mon, 13 Jan 2020 17:52:28 +0200, Nikolay Aleksandrov wrote:
>>>>> +static int br_vlan_rtm_dump(struct sk_buff *skb, struct netlink_callback *cb)
>>>>> +{
>>>>> +	int idx = 0, err = 0, s_idx = cb->args[0];
>>>>> +	struct net *net = sock_net(skb->sk);
>>>>> +	struct br_vlan_msg *bvm;
>>>>> +	struct net_device *dev;
>>>>> +
>>>>> +	if (cb->nlh->nlmsg_len < nlmsg_msg_size(sizeof(*bvm))) {
>>>>
>>>> I wonder if it'd be useful to make this a strict != check? At least
>>>> when strict validation is on? Perhaps we'll one day want to extend 
>>>> the request?
>>>>
>>>
>>> +1. All new code should be using the strict checks.
>>>
>>
>> IIRC, I did it to be able to add filter attributes later, but it should just use nlmsg_parse()
>> instead and all will be taken care of.
>> I'll respin v2 with that change.
>>
>> Thanks,
>>  Nik
>>
> 
> Actually nlmsg_parse() uses the same "<" check for the size before parsing. :)
> If I change to it and with no attributes to parse would be essentially equal to the
> current situation, but if I make it strict "!=" then we won't be able to add
> filter attributes later as we won't be backwards compatible. I'll continue looking
> into it, but IMO we should leave it as it is in order to be able to add the filtering later.
> 
> Thoughts ?
> 
> 
> 
> 

If the header is > sizeof(*bvm) I expect this part of
__nla_validate_parse() to kick in:

        if (unlikely(rem > 0)) {
                pr_warn_ratelimited("netlink: %d bytes leftover after
parsing attributes in process `%s'.\n",
                                    rem, current->comm);
                NL_SET_ERR_MSG(extack, "bytes leftover after parsing
attributes");
                if (validate & NL_VALIDATE_TRAILING)
                        return -EINVAL;
        }