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

From: Petr Machata <[email-masked]>
Date: Wed, 17 Nov 2021 12:50:59 +0100

Jakub Kicinski <[email-masked]> writes:

> On Mon, 15 Nov 2021 18:04:42 +0100 Petr Machata wrote:
>> Ziyang Xuan <[email-masked]> 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 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?