can: isotp: fix tx.buf use-after-free in isotp_sendmsg()

isotp_sendmsg() uses only cmpxchg() on so->tx.state to serialize access
to so->tx.buf. isotp_release() waits for ISOTP_IDLE via
wait_event_interruptible() and then calls kfree(so->tx.buf).

If a signal interrupts the wait_event_interruptible() inside close()
while tx.state is ISOTP_SENDING, the loop exits early and release
proceeds to force ISOTP_SHUTDOWN and continues to kfree(so->tx.buf)
while sendmsg may still be reading so->tx.buf for the final CAN frame
in isotp_fill_dataframe().

The so->tx.buf can be allocated once when the standard tx.buf length needs
to be extended. Move the kfree() of this potentially extended tx.buf to
sk_destruct time when either isotp_sendmsg() and isotp_release() are done.

Fixes: 96d1c81 ("can: isotp: add module parameter for maximum pdu size")
Cc: [email protected]
Reported-by: Ali Norouzi <[email protected]>
Co-developed-by: Ali Norouzi <[email protected]>
Signed-off-by: Ali Norouzi <[email protected]>
Signed-off-by: Oliver Hartkopp <[email protected]>
Link: https://patch.msgid.link/20260319-fix-can-gw-and-can-isotp-v2-2-c45d52c6d2d8@pengutronix.de
Signed-off-by: Marc Kleine-Budde <[email protected]>

Author: hartkopp

net/can/isotp.c

@@ -1248,12 +1248,6 @@ static int isotp_release(struct socket *sock)
 	so->ifindex = 0;
 	so->bound = 0;
 
-	if (so->rx.buf != so->rx.sbuf)
-		kfree(so->rx.buf);
-
-	if (so->tx.buf != so->tx.sbuf)
-		kfree(so->tx.buf);
-
 	sock_orphan(sk);
 	sock->sk = NULL;
 
@@ -1622,6 +1616,21 @@ static int isotp_notifier(struct notifier_block *nb, unsigned long msg,
 	return NOTIFY_DONE;
 }
 
+static void isotp_sock_destruct(struct sock *sk)
+{
+	struct isotp_sock *so = isotp_sk(sk);
+
+	/* do the standard CAN sock destruct work */
+	can_sock_destruct(sk);
+
+	/* free potential extended PDU buffers */
+	if (so->rx.buf != so->rx.sbuf)
+		kfree(so->rx.buf);
+
+	if (so->tx.buf != so->tx.sbuf)
+		kfree(so->tx.buf);
+}
+
 static int isotp_init(struct sock *sk)
 {
 	struct isotp_sock *so = isotp_sk(sk);
@@ -1666,6 +1675,9 @@ static int isotp_init(struct sock *sk)
 	list_add_tail(&so->notifier, &isotp_notifier_list);
 	spin_unlock(&isotp_notifier_lock);
 
+	/* re-assign default can_sock_destruct() reference */
+	sk->sk_destruct = isotp_sock_destruct;
+
 	return 0;
 }