Re: [PATCH bpf-next] bpf: Don't redirect packets with invalid pkt_len

[Daniel Borkmann]

On 7/12/22 6:58 PM, sdf@xxxxxxxxxx wrote:
> On 07/12, Zhengchao Shao 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 valid before forwarding it directly.
>
> > LINK: [1] https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5
> > LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
>
> > Reported-by: syzbot+7a12909485b94426aceb@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx>
> > ---
> >   net/core/filter.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
>
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 4ef77ec5255e..27801b314960 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -2122,6 +2122,11 @@ static int __bpf_redirect_no_mac(struct sk_buff *skb, struct net_device *dev,
> >   {
> >       unsigned int mlen = skb_network_offset(skb);
>
> > +    if (unlikely(skb->len == 0)) {
> > +        kfree_skb(skb);
> > +        return -EINVAL;
> > +    }
> > +
> >       if (mlen) {
> >           __skb_pull(skb, mlen);
>
> > @@ -2143,7 +2148,9 @@ static int __bpf_redirect_common(struct sk_buff *skb, struct net_device *dev,
> >                    u32 flags)
> >   {
> >       /* Verify that a link layer header is carried */
> > -    if (unlikely(skb->mac_header >= skb->network_header)) {
> > +    if (unlikely(skb->mac_header >= skb->network_header) ||
> > +        (min_t(u32, skb_mac_header_len(skb), skb->len) <
> > +         (u32)dev->min_header_len)) {
>
> Why check skb->len != 0 above but skb->len < dev->min_header_len here?
> I guess it doesn't make sense in __bpf_redirect_no_mac because we know
> that mac is empty, but why do we care in __bpf_redirect_common?
> Why not put this check in the common __bpf_redirect?
>
> Also, it's still not clear to me whether we should bake it into the core
> stack vs having some special checks from test_prog_run only. I'm
> assuming the issue is that we can construct illegal skbs with that
> test_prog_run interface, so maybe start by fixing that?

Agree, ideally we can prevent it right at the source rather than adding
more tests into the fast-path.

> Did you have a chance to look at the reproducer more closely? What
> exactly is it doing?
>
> >           kfree_skb(skb);
> >           return -ERANGE;
> >       }
> > --
> > 2.17.1