linux-kernel - Re: [PATCH] skbuff: skb_segment, Update nfrags after calling zero copy functions

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Message-ID: <CANn89iLbNF_kGG9S3R9Y8gpoEM71Wesoi1mTA3-at4Furc+0Fg@mail.gmail.com>
Date:   Tue, 29 Aug 2023 10:07:59 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mohamed Khalfella <mkhalfella@...estorage.com>
Cc:     willemjdebruijn <willemdebruijn.kernel@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Alexander Duyck <alexanderduyck@...com>,
        David Howells <dhowells@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:BPF [MISC]" <bpf@...r.kernel.org>
Subject: Re: [PATCH] skbuff: skb_segment, Update nfrags after calling zero
 copy functions

On Tue, Aug 29, 2023 at 8:50 AM Mohamed Khalfella
<mkhalfella@...estorage.com> wrote:
>
> On 2023-08-28 21:18:16 -0700, willemjdebruijn wrote:
> > Small point: nfrags is not the only state that needs to be refreshed
> > after a fags realloc, also frag.
>
> I am new to this code. Can you help me understand why frag needs to be
> updated too? My reading of this code is that frag points to frags array
> in shared info. As long as shared info pointer remain the same frag
> pointer should remain valid.
>

skb_copy_ubufs() could actually call skb_unclone() and thus skb->head
could be re-allocated.

I guess that if you run your patch (and a repro of the bug ?) with
KASAN enabled kernel, you should see a possible use-after-free ?

To force the skb_unclone() path, having a tcpdump catching all packets
would be enough I think.

> Am I missing something?
> >
> > Thanks for the report. I'm traveling likely without internet until the
> > weekend. Apologies if it takes a while for me to follow up.
> No problem. Thanks for the quick response!