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@syzkaller.appspotmail.com
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
---
v2: need move checking to convert___skb_to_skb and add debug info
v1: should not check len in fast path

 include/linux/skbuff.h | 11 +++++++++++
 net/bpf/test_run.c     |  3 +++
 net/core/dev.c         |  1 +
 3 files changed, 15 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index f6a27ab19202..8ea41ad0786b 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 (unlikely(!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/bpf/test_run.c b/net/bpf/test_run.c
index 2ca96acbc50a..dc9dc0bedca0 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -955,6 +955,9 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
 {
 	struct qdisc_skb_cb *cb = (struct qdisc_skb_cb *)skb->cb;
 
+	if (!skb->len)
+		return -EINVAL;
+
 	if (!__skb)
 		return 0;
 
diff --git a/net/core/dev.c b/net/core/dev.c
index d588fd0a54ce..716df64fcfa5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4168,6 +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);
-- 
2.17.1

On Fri, 15 Jul 2022 11:22:33 +0800 Zhengchao Shao wrote:
> +#ifdef CONFIG_DEBUG_NET
> +	if (unlikely(!skb->len)) {
> +		pr_err("%s\n", __func__);
> +		skb_dump(KERN_ERR, skb, false);
> +		WARN_ON_ONCE(1);
> +	}

Is there a reason to open code WARN_ONCE() like that?

#ifdef CONFIG_DEBUG_NET
	if (WARN_ONCE(!skb->len, "%s\n", __func__))
		skb_dump(KERN_ERR, skb, false);

or

	if (IS_ENABLED(CONFIG_DEBUG_NET) &&
	    WARN_ONCE(!skb->len, "%s\n", __func__))
		skb_dump(KERN_ERR, skb, false);

On Fri, Jul 15, 2022 at 6:30 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Fri, 15 Jul 2022 11:22:33 +0800 Zhengchao Shao wrote:
> > +#ifdef CONFIG_DEBUG_NET
> > +     if (unlikely(!skb->len)) {
> > +             pr_err("%s\n", __func__);
> > +             skb_dump(KERN_ERR, skb, false);
> > +             WARN_ON_ONCE(1);
> > +     }
>
> Is there a reason to open code WARN_ONCE() like that?
>
> #ifdef CONFIG_DEBUG_NET
>         if (WARN_ONCE(!skb->len, "%s\n", __func__))
>                 skb_dump(KERN_ERR, skb, false);
>
> or
>
>         if (IS_ENABLED(CONFIG_DEBUG_NET) &&
>             WARN_ONCE(!skb->len, "%s\n", __func__))
>                 skb_dump(KERN_ERR, skb, false);


Also the skb_dump() needs to be done once.

DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);