netdev - Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in ipv6_rpl_srh

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Date: Tue, 23 May 2023 10:47:15 +0200
From: Simon Horman <simon.horman@...igine.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>,
	Alexander Aring <alex.aring@...il.com>,
	David Lebrun <david.lebrun@...ouvain.be>, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in
 ipv6_rpl_srh_rcv()

On Mon, May 22, 2023 at 01:00:50PM -0700, Jakub Kicinski wrote:
> On Sun, 21 May 2023 20:22:16 +0200 Eric Dumazet wrote:
> > On Thu, May 18, 2023 at 7:05 PM Simon Horman <simon.horman@...igine.com> wrote:
> > > Not far below this line there is a call to pskb_pull():
> > >
> > >                 if (hdr->nexthdr == NEXTHDR_IPV6) {
> > >                         int offset = (hdr->hdrlen + 1) << 3;
> > >
> > >                         skb_postpull_rcsum(skb, skb_network_header(skb),
> > >                                            skb_network_header_len(skb));
> > >
> > >                         if (!pskb_pull(skb, offset)) {
> > >                                 kfree_skb(skb);
> > >                                 return -1;
> > >                         }
> > >                         skb_postpull_rcsum(skb, skb_transport_header(skb),
> > >                                            offset);
> > >
> > > Should hdr be reloaded after the call to pskb_pull() too?  
> > 
> > I do not think so, because @hdr is not used between this pskb_pull()
> > and the return -1:
> > 
> >        if (hdr->nexthdr == NEXTHDR_IPV6) {
> >             int offset = (hdr->hdrlen + 1) << 3;
> > 
> >             skb_postpull_rcsum(skb, skb_network_header(skb),
> >                        skb_network_header_len(skb));
> > 
> >             if (!pskb_pull(skb, offset)) {
> >                 kfree_skb(skb);
> >                 return -1;
> >             }
> >             skb_postpull_rcsum(skb, skb_transport_header(skb),
> >                        offset);
> > 
> >             skb_reset_network_header(skb);
> >             skb_reset_transport_header(skb);
> >             skb->encapsulation = 0;
> > 
> >             __skb_tunnel_rx(skb, skb->dev, net);
> > 
> >             netif_rx(skb);
> >             return -1;
> >         }
> 
> Hum, there's very similar code in ipv6_srh_rcv() (a different function
> but with a very similar name) which calls pskb_pull() and then checks
> if hdr->nexthdr is v4. I'm guessing that's the one Simon was referring
> to.

Yes, that does seem to be the case.