netdev - Re: [PATCH net 2/5] vxlan: Don't double-free default FDB entry in __vxlan_dev

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Message-ID: <wihpnu09n86.fsf@dev-r-vrt-156.mtr.labs.mlnx>
Date:   Mon, 17 Dec 2018 15:39:34 +0000
From:   Petr Machata <petrm@...lanox.com>
To:     Roopa Prabhu <roopa@...ulusnetworks.com>
CC:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Ido Schimmel <idosch@...lanox.com>
Subject: Re: [PATCH net 2/5] vxlan: Don't double-free default FDB entry in
 __vxlan_dev_create()

Roopa Prabhu <roopa@...ulusnetworks.com> writes:

> On Mon, Dec 17, 2018 at 6:58 AM Petr Machata <petrm@...lanox.com> wrote:
>>
>> When a failure occurs in rtnl_configure_link(), the current code
>> calls unregister_netdevice() to roll back the earlier call to
>> register_netdevice(), and jumps to errout, which calls
>> vxlan_fdb_destroy().
>>
>> However unregister_netdevice() calls transitively ndo_uninit(),
>> which is vxlan_uninit(), and that already takes care of deleting
>> the default FDB entry by calling vxlan_fdb_delete_default().
>> Since the entry added earlier in __vxlan_dev_create() is exactly
>> the default entry, the cleanup code in the errout block always
>> leads to double free and thus a panic.
>>
>> Fix by skipping the rollback code and just returning.
>
> I trust your test case that pointed to a potential issue..., but can
> we add a comment there on
> why there is an fdb add but no destroy in the roll back code ?. Also,

This is at a place in the code where the clean-up would normally be:

>> +       /* the all_zeros_mac entry was deleted at vxlan_uninit */

Is that what you meant?

> the destroy code from uninit path does
> look for an entry being present before attempting a destroy, its
> unclear to me yet why that ends up doing a double free ?

That code is called first, finds the FDB entry, destroys it. After it
returns, another destroy call is invoked from __vxlan_dev_create().

>> @@ -3298,8 +3299,8 @@ static int __vxlan_dev_create(struct net *net, struct net_device *dev,
>>         list_add(&vxlan->next, &vn->vxlan_list);
>>         return 0;
>>  errout:
>> -       if (f)
>> -               vxlan_fdb_destroy(vxlan, f, false);
>> +       unregister_netdevice(dev);
>> +       /* the all_zeros_mac entry was deleted at vxlan_uninit */
>>         return err;
>>  }
>>
>> --
>> 2.4.11
>>