Re: [PATCH v2,bpf-next] bpf: Don't redirect packets with invalid pkt_len

[Daniel Borkmann]

On 7/14/22 8:22 PM, Stanislav Fomichev wrote:
> On Wed, Jul 13, 2022 at 11:05 PM Zhengchao Shao
> <shaozhengchao@xxxxxxxxxx> 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 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>
>
> Reviewed-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
>
> Daniel, do you see any issues with this approach?

I think it's fine, maybe this could be folded into:

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 750d7d173a20..256cd18cfe22 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -957,6 +957,8 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)

        if (!__skb)
                return 0;
+       if (!skb->len)
+               return -EINVAL;

        /* make sure the fields we don't use are zeroed */
        if (!range_is_zero(__skb, 0, offsetof(struct __sk_buff, mark)))

> I wonder if we might also want to add some WARN_ON to the
> __bpf_redirect_common routine gated by #ifdef CONFIG_DEBUG_NET ?
> In case syscaller manages to hit similar issues elsewhere..

Assuming the issue is generic (and CONFIG_DEBUG_NET only enabled by things like
syzkaller), couldn't we do something like the below?

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index f6a27ab19202..c9988a785294 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2459,6 +2459,17 @@ static inline void skb_set_tail_pointer(struct sk_buff *skb, const int offset)

 #endif /* NET_SKBUFF_DATA_USES_OFFSET */

+static inline void skb_assert_len(struct sk_buff *skb)
+{
+#ifdef CONFIG_DEBUG_NET
+       if (unlikey(!skb->len)) {
+               pr_err("%s\n", __func__);
+               skb_dump(KERN_ERR, skb, false);
+               WARN_ON_ONCE(1);
+       }
+#endif /* CONFIG_DEBUG_NET */
+}
+
 /*
  *     Add data to an sk_buff
  */
diff --git a/net/core/dev.c b/net/core/dev.c
index 978ed0622d8f..53c4b9fd22c0 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4168,7 +4168,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
        bool again = false;

        skb_reset_mac_header(skb);
-
+       skb_assert_len(skb);
        if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP))
                __skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SCHED);



> > ---
> > v1: should not check len in fast path
> >
> >   net/bpf/test_run.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > index 2ca96acbc50a..750d7d173a20 100644
> > --- a/net/bpf/test_run.c
> > +++ b/net/bpf/test_run.c
> > @@ -1152,6 +1152,12 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
> >          ret = convert___skb_to_skb(skb, ctx);
> >          if (ret)
> >                  goto out;
> > +
> > +       if (skb->len == 0) {
> > +               ret = -EINVAL;
> > +               goto out;
> > +       }
> > +
> >          ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false);
> >          if (ret)
> >                  goto out;
> > --
> > 2.17.1