From 2a154903cec20fb64ff4d7d617ca53c16f8fd53a Mon Sep 17 00:00:00 2001
From: Hillf Danton <hdanton@sina.com>
Date: Wed, 5 Feb 2020 10:31:59 +0800
Subject: [PATCH] Bluetooth: prefetch channel before killing sock
Git-commit: 2a154903cec20fb64ff4d7d617ca53c16f8fd53a
Patch-mainline: v5.7-rc1
References: git-fixes

Prefetch channel before killing sock in order to fix UAF like

 BUG: KASAN: use-after-free in l2cap_sock_release+0x24c/0x290 net/bluetooth/l2cap_sock.c:1212
 Read of size 8 at addr ffff8880944904a0 by task syz-fuzzer/9751

Reported-by: syzbot+c3c5bdea7863886115dc@syzkaller.appspotmail.com
Fixes: 6c08fc896b60 ("Bluetooth: Fix refcount use-after-free issue")
Cc: Manish Mandlik <mmandlik@google.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 net/bluetooth/l2cap_sock.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index ab65304f3f63..390a9afab647 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1193,6 +1193,7 @@ static int l2cap_sock_release(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
 	int err;
+	struct l2cap_chan *chan;
 
 	BT_DBG("sock %p, sk %p", sock, sk);
 
@@ -1202,15 +1203,16 @@ static int l2cap_sock_release(struct socket *sock)
 	bt_sock_unlink(&l2cap_sk_list, sk);
 
 	err = l2cap_sock_shutdown(sock, 2);
+	chan = l2cap_pi(sk)->chan;
 
-	l2cap_chan_hold(l2cap_pi(sk)->chan);
-	l2cap_chan_lock(l2cap_pi(sk)->chan);
+	l2cap_chan_hold(chan);
+	l2cap_chan_lock(chan);
 
 	sock_orphan(sk);
 	l2cap_sock_kill(sk);
 
-	l2cap_chan_unlock(l2cap_pi(sk)->chan);
-	l2cap_chan_put(l2cap_pi(sk)->chan);
+	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 
 	return err;
 }
-- 
2.16.4