netdev - Re: [PATCH net-next V2 2/2] net: kfree_skb_list use kmem_cache_free

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Message-ID: <d0ecbed5-0588-9624-7ecb-014a3bebf192@redhat.com>
Date:   Thu, 19 Jan 2023 12:22:37 +0100
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     brouer@...hat.com, Jakub Kicinski <kuba@...nel.org>,
        netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        pabeni@...hat.com
Subject: Re: [PATCH net-next V2 2/2] net: kfree_skb_list use
 kmem_cache_free_bulk



On 19/01/2023 11.28, Eric Dumazet wrote:
> On Thu, Jan 19, 2023 at 11:18 AM Jesper Dangaard Brouer
> <jbrouer@...hat.com> wrote:
>>
>>
>>
>> On 19/01/2023 03.26, Jakub Kicinski wrote:
>>> On Wed, 18 Jan 2023 22:37:47 +0100 Jesper Dangaard Brouer wrote:
>>>>> +           skb_mark_not_on_list(segs);
>>>>
>>>> The syzbot[1] bug goes way if I remove this skb_mark_not_on_list().
>>>>
>>>> I don't understand why I cannot clear skb->next here?
>>>
>>> Some of the skbs on the list are not private?
>>> IOW we should only unlink them if skb_unref().
>>
>> Yes, you are right.
>>
>> The skb_mark_not_on_list() should only be called if __kfree_skb_reason()
>> returns true, meaning the SKB is ready to be free'ed (as it calls/check
>> skb_unref()).
> 
> 
> This was the case already before your changes.
> 
> skb->next/prev can not be shared by multiple users.
> 
> One skb can be put on a single list by definition.
> 
> Whoever calls kfree_skb_list(list) owns all the skbs->next|prev found
> in the list
> 
> So you can mangle skb->next as you like, even if the unref() is
> telling that someone
> else has a reference on skb.

Then why does the bug go way if I remove the skb_mark_not_on_list() call 
then?

>>
>> I will send a proper fix patch shortly... after syzbot do a test on it.
>>

I've send a patch for syzbot that only calls skb_mark_not_on_list() when 
unref() and __kfree_skb_reason() "permits" this.
I tested it locally with reproducer and it also fixes/"removes" the bug.

--Jesper