Subject: Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in ipv6_rpl_srh_rcv()

From: Jakub Kicinski <[email-masked]>
Date: Mon, 22 May 2023 13:00:50 -0700

On Sun, 21 May 2023 20:22:16 +0200 Eric Dumazet wrote:
> On Thu, May 18, 2023 at 7:05 PM Simon Horman <[email-masked]> 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.