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

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Ziyang Xuan (William) [off-list ref] writes:

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I need some time to test my some ideas. And anyone has good ideas, please
do not be stingy.

Jakub Kicinski [off-list ref] writes:

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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!