Subject: Re: [PATCH net v2] net: vlan: fix a UAF in vlan_dev_real_dev()
To:     Jakub Kicinski <kuba@kernel.org>
CC:     Petr Machata <petrm@nvidia.com>, <davem@davemloft.net>,
        <jgg@nvidia.com>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20211102021218.955277-1-william.xuanziyang@huawei.com>
 <87k0h9bb9x.fsf@nvidia.com>
 <20211115094940.138d86dc@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
 <87a6i3t2zg.fsf@nvidia.com> <7f7cbbec-8c4e-a2dc-787b-570d1049a6b4@huawei.com>
 <20211118061735.5357f739@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
From:   "Ziyang Xuan (William)" <william.xuanziyang@huawei.com>
Message-ID: <c240e0e0-256c-698b-4a98-47490869faa3@huawei.com>
Date:   Fri, 19 Nov 2021 11:29:00 +0800
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> On Thu, 18 Nov 2021 09:46:24 +0800 Ziyang Xuan (William) wrote:
>>>> I think we should move the dev_hold() to ndo_init(), otherwise 
>>>> it's hard to reason if destructor was invoked or not if
>>>> register_netdevice() errors out.  
>>>
>>> Ziyang Xuan, do you intend to take care of this?
>>> .  
>>
>> I am reading the related processes according to the problem scenario.
>> And I will give a more clear sequence and root cause as soon as possible
>> by some necessary tests.
> 
> Okay, I still don't see the fix. 
> 
> Petr would you mind submitting since you have a repro handy?
> .
The sequence for the problem maybe as following:

=============================================================
# create vlan device
vlan_newlink [assume real_dev refcnt == 2 just referenced by itself]
	register_vlan_dev
		register_netdevice(vlan_dev) [success]
		netdev_upper_dev_link [failed]
		unregister_netdevice(vlan_dev) [failure process]
		...
		netdev_run_todo [vlan_dev]
			vlan_dev_free(vlan_dev) [priv_destructor cb]
				dev_put(real_dev) [real_dev refcnt == 1]

# delete real device
unregister_netdevice(real_dev) [real_dev refcnt == 1]
	unregister_netdevice_many
		dev_put(real_dev) [real_dev refcnt == 0]
		net_set_todo(real_dev)
...
netdev_run_todo [real_dev]
	netdev_wait_allrefs(real_dev) [real_dev refcnt == 0]
		pr_emerg("unregister_netdevice: ...", ...)

I am thinking about how to fix the problem. priv_destructor() of the
net_device referenced not only by net_set_todo() but also failure process
in register_netdevice().

I need some time to test my some ideas. And anyone has good ideas, please
do not be stingy.

Thank you!