Re: Unchecked sock pointer causes panic in RAW_TP

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Yan Zhai <yan@cloudflare.com>
Date: Fri, 31 Jan 2025 12:32:57 -0800
> Hello,
> 
> We encountered a panic when tracing kfree_skb with RAW_TP. The problematic
> argument was introduced in commit ba8de796baf4 ("net: introduce
> sk_skb_reason_drop function"). It turns out that the verifier still accepted
> the program despite it didn't test sk == NULL. And this caused kernel panic. I
> attached a small reproducer and panic trace at the end. It's stably
> reproducible when packets are dropped without a receiver (e.g. run iperf2 UDP
> test toward localhost), in both 6.12.11 release and a recent bpf-next master
> snapshot (I was using commit c03320a6768c).
> 
> As a contrast, for another tracepoint like tcp_send_reset, if sk is not checked
> before dereferencing, the verifier will complain and reject the program as
> expected. So this feels like some annotation is missing? Appreciate if someone
> could help me figure out.

Maybe __nullable is missing given we annotated skb for tcp_send_reset ?
https://lore.kernel.org/netdev/20240911033719.91468-4-lulie@linux.alibaba.com/

completely untested:

---8<---
diff --git a/include/trace/events/skb.h b/include/trace/events/skb.h
index b877133cd93a..34accc5929d6 100644
--- a/include/trace/events/skb.h
+++ b/include/trace/events/skb.h
@@ -24,14 +24,14 @@ DEFINE_DROP_REASON(FN, FN)
 TRACE_EVENT(kfree_skb,
 
 	TP_PROTO(struct sk_buff *skb, void *location,
-		 enum skb_drop_reason reason, struct sock *rx_sk),
+		 enum skb_drop_reason reason, struct sock *rx_sk__nullable),
 
-	TP_ARGS(skb, location, reason, rx_sk),
+	TP_ARGS(skb, location, reason, rx_sk__nullable),
 
 	TP_STRUCT__entry(
 		__field(void *,		skbaddr)
 		__field(void *,		location)
-		__field(void *,		rx_sk)
+		__field(void *,		rx_sk__nullable)
 		__field(unsigned short,	protocol)
 		__field(enum skb_drop_reason,	reason)
 	),
@@ -39,7 +39,7 @@ TRACE_EVENT(kfree_skb,
 	TP_fast_assign(
 		__entry->skbaddr = skb;
 		__entry->location = location;
-		__entry->rx_sk = rx_sk;
+		__entry->rx_sk = rx_sk__nullable;
 		__entry->protocol = ntohs(skb->protocol);
 		__entry->reason = reason;
 	),
---8<---