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

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Date:   Thu, 19 Jan 2023 11:28:06 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, brouer@...hat.com,
        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 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.

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