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Message-ID: <5D6E17A7.1020102@huawei.com>
Date: Tue, 3 Sep 2019 15:35:03 +0800
From: Yang Yingliang <yangyingliang@...wei.com>
To: Jason Wang <jasowang@...hat.com>
CC: David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
eric dumazet <eric.dumazet@...il.com>,
xiyou wangcong <xiyou.wangcong@...il.com>,
<weiyongjun1@...wei.com>
Subject: Re: [PATCH v3] tun: fix use-after-free when register netdev failed
On 2019/9/3 14:06, Jason Wang wrote:
>
> On 2019/9/3 下午1:42, Yang Yingliang wrote:
>>
>>
>> On 2019/9/3 11:03, Jason Wang wrote:
>>>
>>> On 2019/9/3 上午9:45, Yang Yingliang wrote:
>>>>
>>>>
>>>> On 2019/9/2 13:32, Jason Wang wrote:
>>>>>
>>>>> On 2019/8/23 下午5:36, Yang Yingliang wrote:
>>>>>>
>>>>>>
>>>>>> On 2019/8/23 11:05, Jason Wang wrote:
>>>>>>> ----- Original Message -----
>>>>>>>>
>>>>>>>> On 2019/8/22 14:07, Yang Yingliang wrote:
>>>>>>>>>
>>>>>>>>> On 2019/8/22 10:13, Jason Wang wrote:
>>>>>>>>>> On 2019/8/20 上午10:28, Jason Wang wrote:
>>>>>>>>>>> On 2019/8/20 上午9:25, David Miller wrote:
>>>>>>>>>>>> From: Yang Yingliang <yangyingliang@...wei.com>
>>>>>>>>>>>> Date: Mon, 19 Aug 2019 21:31:19 +0800
>>>>>>>>>>>>
>>>>>>>>>>>>> Call tun_attach() after register_netdevice() to make sure
>>>>>>>>>>>>> tfile->tun
>>>>>>>>>>>>> is not published until the netdevice is registered. So the
>>>>>>>>>>>>> read/write
>>>>>>>>>>>>> thread can not use the tun pointer that may freed by
>>>>>>>>>>>>> free_netdev().
>>>>>>>>>>>>> (The tun and dev pointer are allocated by
>>>>>>>>>>>>> alloc_netdev_mqs(), they
>>>>>>>>>>>>> can
>>>>>>>>>>>>> be freed by netdev_freemem().)
>>>>>>>>>>>> register_netdevice() must always be the last operation in
>>>>>>>>>>>> the order of
>>>>>>>>>>>> network device setup.
>>>>>>>>>>>>
>>>>>>>>>>>> At the point register_netdevice() is called, the device is
>>>>>>>>>>>> visible
>>>>>>>>>>>> globally
>>>>>>>>>>>> and therefore all of it's software state must be fully
>>>>>>>>>>>> initialized and
>>>>>>>>>>>> ready for us.
>>>>>>>>>>>>
>>>>>>>>>>>> You're going to have to find another solution to these
>>>>>>>>>>>> problems.
>>>>>>>>>>>
>>>>>>>>>>> The device is loosely coupled with sockets/queues. Each side is
>>>>>>>>>>> allowed to be go away without caring the other side. So in this
>>>>>>>>>>> case, there's a small window that network stack think the
>>>>>>>>>>> device has
>>>>>>>>>>> one queue but actually not, the code can then safely drop them.
>>>>>>>>>>> Maybe it's ok here with some comments?
>>>>>>>>>>>
>>>>>>>>>>> Or if not, we can try to hold the device before tun_attach
>>>>>>>>>>> and drop
>>>>>>>>>>> it after register_netdevice().
>>>>>>>>>>
>>>>>>>>>> Hi Yang:
>>>>>>>>>>
>>>>>>>>>> I think maybe we can try to hold refcnt instead of playing
>>>>>>>>>> real num
>>>>>>>>>> queues here. Do you want to post a V4?
>>>>>>>>> I think the refcnt can prevent freeing the memory in this case.
>>>>>>>>> When register_netdevice() failed, free_netdev() will be called
>>>>>>>>> directly,
>>>>>>>>> dev->pcpu_refcnt and dev are freed without checking refcnt of
>>>>>>>>> dev.
>>>>>>>> How about using patch-v1 that using a flag to check whether the
>>>>>>>> device
>>>>>>>> registered successfully.
>>>>>>>>
>>>>>>> As I said, it lacks sufficient locks or barriers. To be clear, I
>>>>>>> meant
>>>>>>> something like (compile-test only):
>>>>>>>
>>>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>>>>>> index db16d7a13e00..e52678f9f049 100644
>>>>>>> --- a/drivers/net/tun.c
>>>>>>> +++ b/drivers/net/tun.c
>>>>>>> @@ -2828,6 +2828,7 @@ static int tun_set_iff(struct net *net,
>>>>>>> struct file *file, struct ifreq *ifr)
>>>>>>> (ifr->ifr_flags & TUN_FEATURES);
>>>>>>> INIT_LIST_HEAD(&tun->disabled);
>>>>>>> + dev_hold(dev);
>>>>>>> err = tun_attach(tun, file, false,
>>>>>>> ifr->ifr_flags & IFF_NAPI,
>>>>>>> ifr->ifr_flags & IFF_NAPI_FRAGS);
>>>>>>> if (err < 0)
>>>>>>> @@ -2836,6 +2837,7 @@ static int tun_set_iff(struct net *net,
>>>>>>> struct file *file, struct ifreq *ifr)
>>>>>>> err = register_netdevice(tun->dev);
>>>>>>> if (err < 0)
>>>>>>> goto err_detach;
>>>>>>> + dev_put(dev);
>>>>>>> }
>>>>>>> netif_carrier_on(tun->dev);
>>>>>>> @@ -2852,11 +2854,13 @@ static int tun_set_iff(struct net *net,
>>>>>>> struct file *file, struct ifreq *ifr)
>>>>>>> return 0;
>>>>>>> err_detach:
>>>>>>> + dev_put(dev);
>>>>>>> tun_detach_all(dev);
>>>>>>> /* register_netdevice() already called
>>>>>>> tun_free_netdev() */
>>>>>>> goto err_free_dev;
>>>>>>> err_free_flow:
>>>>>>> + dev_put(dev);
>>>>>>> tun_flow_uninit(tun);
>>>>>>> security_tun_dev_free_security(tun->security);
>>>>>>> err_free_stat:
>>>>>>>
>>>>>>> What's your thought?
>>>>>>
>>>>>> The dev pointer are freed without checking the refcount in
>>>>>> free_netdev() called by err_free_dev
>>>>>>
>>>>>> path, so I don't understand how the refcount protects this pointer.
>>>>>>
>>>>>
>>>>> The refcount are guaranteed to be zero there, isn't it?
>>>> No, it's not.
>>>>
>>>> err_free_dev:
>>>> free_netdev(dev);
>>>>
>>>> void free_netdev(struct net_device *dev)
>>>> {
>>>> ...
>>>> /* pcpu_refcnt can be freed without checking refcount */
>>>> free_percpu(dev->pcpu_refcnt);
>>>> dev->pcpu_refcnt = NULL;
>>>>
>>>> /* Compatibility with error handling in drivers */
>>>> if (dev->reg_state == NETREG_UNINITIALIZED) {
>>>> /* dev can be freed without checking refcount */
>>>> netdev_freemem(dev);
>>>> return;
>>>> }
>>>> ...
>>>> }
>>>
>>>
>>> Right, but what I meant is in my patch, when code reaches
>>> free_netdev() the refcnt is zero. What did I miss?
>> Yes, but it can't fix the UAF problem.
>
>
> Well, it looks to me that the dev_put() in tun_put() won't release the
> device in this case.
The device is not released in tun_put().
This is how the UAF occurs:
CPUA CPUB
tun_set_iff()
alloc_netdev_mqs()
tun_attach()
tun_chr_read_iter()
tun_get()
tun_do_read()
tun_ring_recv()
register_netdevice() <-- inject error
goto err_detach
tun_detach_all() <-- set RCV_SHUTDOWN
free_netdev() <-- called from
err_free_dev path
netdev_freemem() <-- free the memory
without check refcount
(In this path, the refcount cannot prevent
freeing the memory of dev, and the memory
will be used by dev_put() called by
tun_chr_read_iter() on CPUB.)
(Break from tun_ring_recv(), because RCV_SHUTDOWN is set)
tun_put()
dev_put() <-- use the memory freed by netdev_freemem()
>
> Thanks
>
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