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

From: Petr Machata <[email-masked]>
Date: Tue, 23 Nov 2021 13:35:35 +0100

Ziyang Xuan (William) <[email-masked]> writes:

>> 
>> Ziyang Xuan (William) <[email-masked]> writes:
>> 
>>> I need some time to test my some ideas. And anyone has good ideas, please
>>> do not be stingy.
>> 
>> Jakub Kicinski <[email-masked]> writes:
>> 
>>> 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.
>> 
>> That makes sense to me. We always put real_dev in the destructor, so we
>> should always hold it in the constructor...
>
> Inject error before dev_hold(real_dev) in register_vlan_dev(), and execute
> the following testcase:
>
> ip link add dev dummy1 type dummy
> ip link add name dummy1.100 link dummy1 type vlan id 100 // failed for error injection
> ip link del dev dummy1
>
> Make the problem repro. The problem is solved using the following fix
> according to the Jakub's suggestion:
>
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index a3a0a5e994f5..abaa5d96ded2 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -184,9 +184,6 @@ int register_vlan_dev(struct net_device *dev, struct netlink_ext_ack *extack)
>         if (err)
>                 goto out_unregister_netdev;
>
> -       /* Account for reference in struct vlan_dev_priv */
> -       dev_hold(real_dev);
> -
>         vlan_stacked_transfer_operstate(real_dev, dev, vlan);
>         linkwatch_fire_event(dev); /* _MUST_ call rfc2863_policy() */
>
> diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
> index ab6dee28536d..a54535cbcf4c 100644
> --- a/net/8021q/vlan_dev.c
> +++ b/net/8021q/vlan_dev.c
> @@ -615,6 +615,9 @@ static int vlan_dev_init(struct net_device *dev)
>         if (!vlan->vlan_pcpu_stats)
>                 return -ENOMEM;
>
> +       /* Get vlan's reference to real_dev */
> +       dev_hold(real_dev);
>
>
> If there is not any other idea and objection, I will send the fix patch later.
>
> Thank you!

This fixes the issue in my repro, and does not seems to break anything.
We'll take it to full regression overnight, I'll report back tomorrow
about the result.