netdev - Re: [PATCH v5 bpf-next 5/9] bpf: cpumap: add the possibility to attach an eBPF program to cpumap

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Message-ID: <4dcc1530-ffdf-30a5-9806-0b06cd0f12d6@iogearbox.net>
Date:   Sat, 4 Jul 2020 00:24:16 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        ast@...nel.org, brouer@...hat.com, toke@...hat.com,
        lorenzo.bianconi@...hat.com, dsahern@...nel.org,
        andrii.nakryiko@...il.com
Subject: Re: [PATCH v5 bpf-next 5/9] bpf: cpumap: add the possibility to
 attach an eBPF program to cpumap

On 7/3/20 10:48 PM, Lorenzo Bianconi wrote:
>> On 6/30/20 2:49 PM, Lorenzo Bianconi wrote:
>> [...]
> 
> [...]
> 
>>>    	old_rcpu = xchg(&cmap->cpu_map[key_cpu], rcpu);
>>>    	if (old_rcpu) {
>>> +		if (old_rcpu->prog)
>>> +			bpf_prog_put(old_rcpu->prog);
>>>    		call_rcu(&old_rcpu->rcu, __cpu_map_entry_free);
>>>    		INIT_WORK(&old_rcpu->kthread_stop_wq, cpu_map_kthread_stop);
>>>    		schedule_work(&old_rcpu->kthread_stop_wq);
>>
>> Hm, not quite sure I follow the logic here. Why is the bpf_prog_put() not placed inside
>> __cpu_map_entry_free(), for example? Wouldn't this at least leave a potential small race
>> window of UAF given the rest is still live? If we already piggy-back from RCU side on
>> rcpu entry, why not having it in __cpu_map_entry_free()?
> 
> ack right, thanks for spotting this issue. I guess we can even move
> "bpf_prog_put(rcpu->prog)" in put_cpu_map_entry() so the last consumer
> of bpf_cpu_map_entry will free the attached program. Agree?

Yes, that sounds reasonable to me.

Thanks,
Daniel