netdev - Re: [PATCH net-next] net, bonding: Disallow vlan+srcmac with XDP

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Date:   Thu, 12 Aug 2021 18:21:22 +0300
From:   Nikolay Aleksandrov <nikolay@...dia.com>
To:     Jussi Maki <joamaki@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Jonathan Toppins <jtoppins@...hat.com>
Subject: Re: [PATCH net-next] net, bonding: Disallow vlan+srcmac with XDP

On 12/08/2021 18:12, Jussi Maki wrote:
> On Thu, Aug 12, 2021 at 5:01 PM Nikolay Aleksandrov <nikolay@...dia.com> wrote:
>> Hi Jussi,
>> Could you please share the null ptr deref trace?
>> I'm curious how we can get a null skb at that point.
> 
> Hi Nik, this was reported by Jonathan here:
> https://lore.kernel.org/bpf/20210728234350.28796-1-joamaki@gmail.com/T/#m07a73b1886a9213feb7112ce2a0d6dfde84fd27a.
> I didn't reproduce the null ptr deref as it was fairly obvious how it
> can happen, e.g. by having a bond with xmit_policy=vlan+srcmac. The
> hashing functions were refactored to be used for both xdp_buff and
> skbuff uses and the skb pointer became optional (was meant to be used
> when packet was non-linear), but I missed fixing the vlan hashing
> function. Partially the reason leading to this was that the
> xmit_policy is very new and the bpf vmtest infra still uses an older
> iproute2 version which didn't support it, so this was untested. What
> is not tested is broken as usual.
> 
>> Also how are the xdp and null ptr deref changes related ?
> 
> They're related in that looking into the null ptr deref here I
> realized that vlan+srcmac didn't make sense with XDP since we have no
> guarantee that the vlan id is in the ethernet header. So this patch
> both fixes the deref by checking the skb pointer for NULL and it
> disallows the whole xmit policy for XDP for the aforementioned reason.
> 
> Hope this makes sense.
> 

Oh, I had totally missed the bond xdp patch-set, all makes sense now.

Thanks,
 Nik