can: j1939: j1939_session_deactivate(): clarify lifetime of session object

The j1939_session_deactivate() is decrementing the session ref-count and
potentially can free() the session. This would cause use-after-free
situation.

However, the code calling j1939_session_deactivate() does always hold
another reference to the session, so that it would not be free()ed in
this code path.

This patch adds a comment to make this clear and a WARN_ON, to ensure
that future changes will not violate this requirement. Further this
patch avoids dereferencing the session pointer as a precaution to avoid
use-after-free if the session is actually free()ed.

Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Link: https://lore.kernel.org/r/20210714111602.24021-1-o.rempel@pengutronix.deReported-by: Xiaochen Zou <xzou017@ucr.edu>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/j1939/transport.c

@@ -1075,11 +1075,16 @@ static bool j1939_session_deactivate_locked(struct j1939_session *session)
 
 static bool j1939_session_deactivate(struct j1939_session *session)
 {
+	struct j1939_priv *priv = session->priv;
 	bool active;
 
-	j1939_session_list_lock(session->priv);
+	j1939_session_list_lock(priv);
+	/* This function should be called with a session ref-count of at
+	 * least 2.
+	 */
+	WARN_ON_ONCE(kref_read(&session->kref) < 2);
 	active = j1939_session_deactivate_locked(session);
-	j1939_session_list_unlock(session->priv);
+	j1939_session_list_unlock(priv);
 
 	return active;
 }