Re: [syzbot] [can?] WARNING: refcount bug in j1939_session_put

[Edward Adam Davis]

Fixes: c9c0ee5f20c5 ("net: skbuff: Skip early return in skb_unref when debugging")

Root cause: In commit c9c0ee5f20c5, There are following rules:
In debug builds (CONFIG_DEBUG_NET set), the reference count is always  decremented, even when it's 1

This rule will cause the reference count to be 0 after calling skc_unref,
which will affect the release of skb.

The solution I have proposed is:
Before releasing the SKB during session destroy, check the CONFIG_DEBUG_NET
and skb_unref return values to avoid reference count errors caused by a 
reference count of 0 when releasing the SKB.

#syz test: net-next 743ff02152bc

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index 4be73de5033c..50d96015c125 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -278,7 +278,8 @@ static void j1939_session_destroy(struct j1939_session *session)
 
 	while ((skb = skb_dequeue(&session->skb_queue)) != NULL) {
 		/* drop ref taken in j1939_session_skb_queue() */
-		skb_unref(skb);
+		if (!skb_unref(skb) && IS_ENABLED(CONFIG_DEBUG_NET))
+			skb_get(skb);
 		kfree_skb(skb);
 	}
 	__j1939_session_drop(session);