[PATCH 5.13 102/104] can: j1939: j1939_session_deactivate(): clarify lifetime of session object

From: Greg Kroah-Hartman
Date: Mon Aug 02 2021 - 10:14:40 EST

From: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>

commit 0c71437dd50dd687c15d8ca80b3b68f10bb21d63 upstream.

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

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: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Link: https://lore.kernel.org/r/20210714111602.24021-1-o.rempel@xxxxxxxxxxxxxx
Reported-by: Xiaochen Zou <xzou017@xxxxxxx>
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/can/j1939/transport.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1075,11 +1075,16 @@ static bool j1939_session_deactivate_loc

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;