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From: Hillf Danton <hdanton@sina.com>
To: syzbot <syzbot+75ec36af46e2098f253c@syzkaller.appspotmail.com>
Cc: linux-kernel@vger.kernel.org,
	syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [can?] KASAN: slab-use-after-free Read in j1939_xtp_rx_dat_one
Date: Sun, 14 Apr 2024 17:30:11 +0800
Message-Id: <20240414093011.2358-1-hdanton@sina.com>
In-Reply-To: <000000000000a87d17061608997e@google.com>
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On Sat, 13 Apr 2024 23:27:33 -0700
> syzbot found the following issue on:
> 
> HEAD commit:    2c71fdf02a95 Merge tag 'drm-fixes-2024-04-09' of https://g..
> git tree:       upstream
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11ae00cb180000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  2c71fdf02a95

--- x/net/can/j1939/transport.c
+++ y/net/can/j1939/transport.c
@@ -2122,10 +2122,12 @@ static void j1939_tp_cmd_recv(struct j19
 int j1939_tp_recv(struct j1939_priv *priv, struct sk_buff *skb)
 {
 	struct j1939_sk_buff_cb *skcb = j1939_skb_to_cb(skb);
+	int ret = 1;
 
 	if (!j1939_tp_im_involved_anydir(skcb) && !j1939_cb_is_broadcast(skcb))
 		return 0;
 
+	skb_get(skb);
 	switch (skcb->addr.pgn) {
 	case J1939_ETP_PGN_DAT:
 		skcb->addr.type = J1939_ETP;
@@ -2138,14 +2140,18 @@ int j1939_tp_recv(struct j1939_priv *pri
 		skcb->addr.type = J1939_ETP;
 		fallthrough;
 	case J1939_TP_PGN_CTL:
-		if (skb->len < 8)
-			return 0; /* Don't care. Nothing to extract here */
+		if (skb->len < 8) {
+			ret = 0; /* Don't care. Nothing to extract here */
+			break;
+		}
 
 		j1939_tp_cmd_recv(priv, skb);
 		break;
 	default:
-		return 0; /* no problem */
+		ret = 0; /* no problem */
+		break;
 	}
+	kfree_skb(skb);
 	return 1; /* "I processed the message" */
 }
 
--