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* [PATCH v1] can: j1939: j1939_session_deactivate(): clarify lifetime of session object
@ 2021-07-14 11:16 Oleksij Rempel
  2021-07-14 19:32 ` Marc Kleine-Budde
  0 siblings, 1 reply; 2+ messages in thread
From: Oleksij Rempel @ 2021-07-14 11:16 UTC (permalink / raw)
  To: dev.kurt, mkl, wg
  Cc: Oleksij Rempel, Xiaochen Zou, kernel, linux-can, netdev,
	David Jander, Zhang Changzhong

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")
Reported-by: Xiaochen Zou <>
Signed-off-by: Oleksij Rempel <>
 net/can/j1939/transport.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index cb358646e382..cff9812a5585 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1056,11 +1056,16 @@ 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;

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2021-07-14 11:16 [PATCH v1] can: j1939: j1939_session_deactivate(): clarify lifetime of session object Oleksij Rempel
2021-07-14 19:32 ` Marc Kleine-Budde

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