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

[Eric Dumazet]

On Tue, Aug 29, 2023 at 8:50 AM Mohamed Khalfella
<mkhalfella@xxxxxxxxxxxxxxx> 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!