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Date: Tue, 3 Sep 2019 09:45:35 +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 2019/9/2 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; } ... } > > Thanks > > >> Thanks, >> Yang >> >>> >>> Thanks >>> >>> . >>> >> >> > > . >
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