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

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Message-ID: <CANn89iJ0Sdy5o8WHdTygE3UwUgHpJkdxKfeYXMN0DZBKs_f6AA@mail.gmail.com>
Date: Tue, 23 May 2023 12:45:34 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Simon Horman <simon.horman@...igine.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "David S . Miller" <davem@...emloft.net>, 
	Paolo Abeni <pabeni@...hat.com>, Alexander Aring <alex.aring@...il.com>, 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 Tue, May 23, 2023 at 10:47 AM Simon Horman <simon.horman@...igine.com> wrote:
>
> 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.

I think ipv6_srh_rcv() is fine.

The "goto looped_back" does not need to reload hdr.

The only point where skb->head can change is at the pskb_expand_head() call,
which is properly followed by:

hdr = (struct ipv6_sr_hdr *)skb_transport_header(skb);

I will send a V2, because this first patch in the series can also make
ipv6_rpl_srh_rcv() similar.
(No need to move around the pskb_may_pull(skb, sizeof(*hdr)