netdev - [PATCH] L2CAP: Fix null-ptr-deref in l2cap_sock_set_shutdown

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Date:   Wed, 18 Jan 2023 20:34:05 -0500
From:   Sungwoo Kim <iam@...g-woo.kim>
To:     unlisted-recipients:; (no To-header on input)
Cc:     daveti@...due.edu, wuruoyu@...com, benquike@...il.com,
        Sungwoo Kim <iam@...g-woo.kim>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-bluetooth@...r.kernel.org (open list:BLUETOOTH SUBSYSTEM),
        netdev@...r.kernel.org (open list:NETWORKING [GENERAL]),
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] L2CAP: Fix null-ptr-deref in l2cap_sock_set_shutdown_cb

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@...com) and Hui Peng(benquike@...il.com) has helped
the FuzzBT project.

Signed-off-by: Sungwoo Kim <iam@...g-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