Subject: Re: [bpf-next PATCH 1/3] net: fix generic XDP to handle if eth header was mangled

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 3 Oct 2018 16:04:13 +0200

On Tue, 25 Sep 2018 22:36:39 -0700
Song Liu <[email-masked]> wrote:

> On Tue, Sep 25, 2018 at 7:26 AM Jesper Dangaard Brouer
> <[email-masked]> wrote:
> >
> > XDP can modify (and resize) the Ethernet header in the packet.
> >
> > There is a bug in generic-XDP, because skb->protocol and skb->pkt_type
> > are setup before reaching (netif_receive_)generic_xdp.
> >
> > This bug was hit when XDP were popping VLAN headers (changing
> > eth->h_proto), as skb->protocol still contains VLAN-indication
> > (ETH_P_8021Q) causing invocation of skb_vlan_untag(skb), which corrupt
> > the packet (basically popping the VLAN again).
> >
> > This patch catch if XDP changed eth header in such a way, that SKB
> > fields needs to be updated.
> >
> > Fixes: d445516966dc ("net: xdp: support xdp generic on virtual devices")
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >  net/core/dev.c |   14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index ca78dc5a79a3..db6d89f536cb 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4258,6 +4258,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >         struct netdev_rx_queue *rxqueue;
> >         void *orig_data, *orig_data_end;
> >         u32 metalen, act = XDP_DROP;
> > +       __be16 orig_eth_type;
> > +       struct ethhdr *eth;
> > +       bool orig_bcast;
> >         int hlen, off;
> >         u32 mac_len;
> >
> > @@ -4298,6 +4301,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >         xdp->data_hard_start = skb->data - skb_headroom(skb);
> >         orig_data_end = xdp->data_end;
> >         orig_data = xdp->data;
> > +       eth = (struct ethhdr *)xdp->data;
> > +       orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
> > +       orig_eth_type = eth->h_proto;
> >
> >         rxqueue = netif_get_rxqueue(skb);
> >         xdp->rxq = &rxqueue->xdp_rxq;
> > @@ -4321,6 +4327,14 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
> >
> >         }
> >
> > +       /* check if XDP changed eth hdr such SKB needs update */
> > +       eth = (struct ethhdr *)xdp->data;
> > +       if ((orig_eth_type != eth->h_proto) ||
> > +           (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {  
> 
> Is the actions below always correct for the condition above? Do we need
> to confirm the SKB is updated properly?

I cannot find the issue that you are hinting to?

If the BPF prog used bpf_xdp_adjust_head(), which the included selftest
program does, then skb->data have been appropriately adjusted just
above (with __skb_pull(skb, off) or __skb_push(skb, -off)), which makes
the call to skb_reset_mac_header(skb) inside eth_type_trans() correct.

I've double checked the code, and I cannot find anything wrong...
please let me know if I missed something!?

> > +               __skb_push(skb, mac_len);
> > +               skb->protocol = eth_type_trans(skb, skb->dev);

We could change mac_len to be ETH_HLEN, because inside eth_type_trans()
the constant ETH_HLEN is used, that way we are 100% sure the
skb_push/skb_pull are "paired".  Will that be better for you?

> > +       }
> > +
> >         switch (act) {
> >         case XDP_REDIRECT:
> >         case XDP_TX:

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer