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

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Subject: Re: [syzbot] [can?] WARNING: refcount bug in sk_skb_reason_drop
Author: dmantipov@yandex.ru

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 7ec462100ef9142344ddbf86f2c3008b97acddbe 

From eb58509f460225df7cf181a07772d577a270091b Mon Sep 17 00:00:00 2001
From: Dmitry Antipov <dmantipov@yandex.ru>
Date: Thu, 3 Oct 2024 20:24:12 +0300
Subject: [PATCH] can: fix skb reference counting in j1939_session_new()

Since 'j1939_session_skb_queue()' do an extra 'skb_get()' for each
new skb, I assume that the same should be done for an initial one
in 'j1939_session_new()' just to avoid refcount underflow.

Reported-by: syzbot+d4e8dc385d9258220c31@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d4e8dc385d9258220c31
Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
---
 net/can/j1939/transport.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index 319f47df3330..fecee36dbb70 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1505,7 +1505,8 @@ static struct j1939_session *j1939_session_new(struct j1939_priv *priv,
 	session->state = J1939_SESSION_NEW;
 
 	skb_queue_head_init(&session->skb_queue);
-	skb_queue_tail(&session->skb_queue, skb);
+	/* Take an extra ref just like in j1939_session_skb_queue(). */
+	skb_queue_tail(&session->skb_queue, skb_get(skb));
 
 	skcb = j1939_skb_to_cb(skb);
 	memcpy(&session->skcb, skcb, sizeof(session->skcb));
-- 
2.46.2