Re: [PATCH net v2] net: vlan: fix a UAF in vlan_dev_real_dev()

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 15 Nov 2021 18:04:42 +0100 Petr Machata wrote:
> Ziyang Xuan <william.xuanziyang@huawei.com> writes:
> 
> > diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> > index 55275ef9a31a..a3a0a5e994f5 100644
> > --- a/net/8021q/vlan.c
> > +++ b/net/8021q/vlan.c
> > @@ -123,9 +123,6 @@ void unregister_vlan_dev(struct net_device *dev, struct list_head *head)
> >  	}
> >  
> >  	vlan_vid_del(real_dev, vlan->vlan_proto, vlan_id);
> > -
> > -	/* Get rid of the vlan's reference to real_dev */
> > -	dev_put(real_dev);
> >  }
> >  
> >  int vlan_check_real_dev(struct net_device *real_dev,
> > diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
> > index 0c21d1fec852..aeeb5f90417b 100644
> > --- a/net/8021q/vlan_dev.c
> > +++ b/net/8021q/vlan_dev.c
> > @@ -843,6 +843,9 @@ static void vlan_dev_free(struct net_device *dev)
> >  
> >  	free_percpu(vlan->vlan_pcpu_stats);
> >  	vlan->vlan_pcpu_stats = NULL;
> > +
> > +	/* Get rid of the vlan's reference to real_dev */
> > +	dev_put(vlan->real_dev);
> >  }
> >  
> >  void vlan_setup(struct net_device *dev)  
> 
> This is causing reference counting issues when vetoing is involved.
> Consider the following snippet:
> 
>     ip link add name bond1 type bond mode 802.3ad
>     ip link set dev swp1 master bond1
>     ip link add name bond1.100 link bond1 type vlan protocol 802.1ad id 100
>     # ^ vetoed, no netdevice created
>     ip link del dev bond1
> 
> The setup process goes like this: vlan_newlink() calls
> register_vlan_dev() calls netdev_upper_dev_link() calls
> __netdev_upper_dev_link(), which issues a notifier
> NETDEV_PRECHANGEUPPER, which yields a non-zero error,
> because a listener vetoed it.
> 
> So it unwinds, skipping dev_hold(real_dev), but eventually the VLAN ends
> up decreasing reference count of the real_dev. Then when when the bond
> netdevice is removed, we get an endless loop of:
> 
>     kernel:unregister_netdevice: waiting for bond1 to become free. Usage count = 0 
> 
> Moving the dev_hold(real_dev) to always happen even if the
> netdev_upper_dev_link() call makes the issue go away.
> 
> I'm not sure why this wasn't happening before. After the veto,
> register_vlan_dev() follows with a goto out_unregister_netdev, which
> calls unregister_netdevice() calls unregister_netdevice_queue(), which
> issues a notifier NETDEV_UNREGISTER, which invokes vlan_device_event(),
> which calls unregister_vlan_dev(), which used to dev_put(real_dev),
> which seems like it should have caused the same issue. Dunno.

Does the notifier trigger unregister_vlan_dev()? I thought the notifier
triggers when lower dev is unregistered.

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.