Re: [PATCH] bpf: Don't redirect packets with pkt_len 0

["zhudi (E)" <zhudi2@huawei.com>]

> On Fri, Jun 17, 2022 at 9:19 AM Di Zhu <zhudi2@huawei.com> wrote:
> >
> > Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any
> > skbs, that is, the flow->head is null.
> > The root cause, as the [2] says, is because that bpf_prog_test_run_skb()
> > run a bpf prog which redirects empty skbs.
> > So we should determine whether the length of the packet modified by bpf
> > prog or others like bpf_prog_test is 0 before forwarding it directly.
> >
> > LINK: [1]
> https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16e
> c96c5
> > LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
> >
> > Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com
> > Signed-off-by: Di Zhu <zhudi2@huawei.com>
> > ---
> >  net/core/filter.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 5af58eb48587..c7fbfa90898a 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -2156,6 +2156,9 @@ static int __bpf_redirect_common(struct sk_buff
> *skb, struct net_device *dev,
> >  static int __bpf_redirect(struct sk_buff *skb, struct net_device *dev,
> >                           u32 flags)
> >  {
> > +       if (unlikely(skb->len == 0))
> > +               return -EINVAL;
> > +
> 
> You focus again on fq_codel, but we have a more generic issue at hand.
> 
> I said that most drivers will assume packets are Ethernet ones, having
> at least an Ethernet header in them.

I only focused on this particular test case...
the submission information should be modified to describe a general problem.
 
> Also returning -EINVAL will leak the skb :/

Yes... 

> 
> I think a better fix would be to make sure the skb carries an expected
> packet length,
> and this probably differs in __bpf_redirect_common() and
> __bpf_redirect_no_mac() ?

Does the no mac device need to check the length of the package?

> Current test in __bpf_redirect_common() seems not good enough.
> 
> +       /* Verify that a link layer header is carried */
> +       if (unlikely(skb->mac_header >= skb->network_header)) {
> +               kfree_skb(skb);
> +               return -ERANGE;
> +       }
> +
> 
> It should check that the link layer header size is >= dev->min_header_len

It is only valid for device with mac header. such as ip tunnel device, dev->min_header_len
has not set.

> 
> 
> >         if (dev_is_mac_header_xmit(dev))
> >                 return __bpf_redirect_common(skb, dev, flags);
> >         else
> > --
> > 2.27.0
> >