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SubjectRe: [PATCH net v2] net: vlan: fix a UAF in vlan_dev_real_dev()
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Jakub Kicinski <kuba@kernel.org> writes:

> 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 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?

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