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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20230829093105.GA611013@medusa>
Date:   Tue, 29 Aug 2023 02:31:05 -0700
From:   Mohamed Khalfella <mkhalfella@...estorage.com>
To:     Eric Dumazet <edumazet@...gle.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 2023-08-29 10:07:59 +0200, Eric Dumazet wrote:
> 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.
> 

Okay, I see it now. I have not tested this patch with tcpdump capturing
packets at the same time. Also, during my testing I have not seen the
value of skb->head changnig. Now you are mentioning it it, I will make
sure to test with tcpdump running and see skb->head changing. Thank you
for pointing that out.

For frag, I guess something like frag = &skb_shinfo(list_skb)->frags[i];
should do the job. I have not tested it though. I will need to do more
testing before posting updated patch.