Subject: Re: [PATCH net v2] net: vlan: fix a UAF in vlan_dev_real_dev()
To:     Petr Machata <petrm@nvidia.com>
CC:     Jakub Kicinski <kuba@kernel.org>, <davem@davemloft.net>,
        <jgg@nvidia.com>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20211102021218.955277-1-william.xuanziyang@huawei.com>
 <87k0h9bb9x.fsf@nvidia.com>
 <20211115094940.138d86dc@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
 <87a6i3t2zg.fsf@nvidia.com> <7f7cbbec-8c4e-a2dc-787b-570d1049a6b4@huawei.com>
 <20211118061735.5357f739@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
 <c240e0e0-256c-698b-4a98-47490869faa3@huawei.com> <8735nstq62.fsf@nvidia.com>
From:   "Ziyang Xuan (William)" <william.xuanziyang@huawei.com>
Message-ID: <daae2fe3-997c-a390-afae-15ff33ba3d1c@huawei.com>
Date:   Tue, 23 Nov 2021 17:01:02 +0800
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> 
> Ziyang Xuan (William) <william.xuanziyang@huawei.com> writes:
> 
>> I need some time to test my some ideas. And anyone has good ideas, please
>> do not be stingy.
> 
> Jakub Kicinski <kuba@kernel.org> 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!