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From:   Sungwoo Kim <iam@sung-woo.kim>
Cc:     daveti@purdue.edu, wuruoyu@me.com, benquike@gmail.com,
        Sungwoo Kim <iam@sung-woo.kim>,
        Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        linux-bluetooth@vger.kernel.org (open list:BLUETOOTH SUBSYSTEM),
        netdev@vger.kernel.org (open list:NETWORKING [GENERAL]),
        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH] L2CAP: Fix null-ptr-deref in l2cap_sock_set_shutdown_cb
Date:   Wed, 18 Jan 2023 20:34:05 -0500
Message-Id: <20230119013405.3870506-1-iam@sung-woo.kim>
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The L2CAP socket shutdown invokes l2cap_sock_destruct without a lock
on conn->chan_lock, assigning NULL to chan->data *just before*
the l2cap_disconnect_req thread that accesses to chan->data.
This patch prevent it by adding a null check for a workaround, instead
of fixing a lock.

This bug is found by FuzzBT, a modified Syzkaller by Sungwoo Kim(me).
Ruoyu Wu(wuruoyu@me.com) and Hui Peng(benquike@gmail.com) has helped
the FuzzBT project.

Signed-off-by: Sungwoo Kim <iam@sung-woo.kim>
---
 net/bluetooth/l2cap_sock.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index ca8f07f35..350c7afdf 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1681,9 +1681,11 @@ static void l2cap_sock_set_shutdown_cb(struct l2cap_chan *chan)
 {
 	struct sock *sk = chan->data;
 
-	lock_sock(sk);
-	sk->sk_shutdown = SHUTDOWN_MASK;
-	release_sock(sk);
+	if (!sk) {
+		lock_sock(sk);
+		sk->sk_shutdown = SHUTDOWN_MASK;
+		release_sock(sk);
+	}
 }
 
 static long l2cap_sock_get_sndtimeo_cb(struct l2cap_chan *chan)
-- 
2.25.1