[PATCH net] can: j1939: fix lockless local-destination check

Shuhao Fu
2026-04-19T14:17:26Z

j1939_priv.ents[].nusers is documented as protected by priv->lock, and
its updates already happen under that lock. j1939_can_recv() also reads
it under read_lock_bh(). However, j1939_session_skb_queue() and
j1939_tp_send() still read priv->ents[da].nusers without taking the
lock.

Those transport-side checks decide whether to set J1939_ECU_LOCAL_DST, so
they can race with j1939_local_ecu_get() and j1939_local_ecu_put() while
userspace is binding or releasing sockets concurrently with TP traffic.
This can misclassify TP/ETP sessions as local or remote and take the wrong
transport path.

Fix both transport paths by routing the destination-locality check through
a helper that reads ents[].nusers under read_lock_bh(&priv->lock).

Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Signed-off-by: Shuhao Fu <sfual@cse.ust.hk>
---
net/can/j1939/transport.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index df93d57907da7e..8a31cb23bc76d0 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -351,6 +351,18 @@ static void j1939_session_skb_drop_old(struct j1939_session *session)
}
}

+static bool j1939_address_is_local(struct j1939_priv *priv, u8 addr)
+{
+ bool local = false;
+
+ read_lock_bh(&priv->lock);
+ if (j1939_address_is_unicast(addr) && priv->ents[addr].nusers)
+ local = true;
+ read_unlock_bh(&priv->lock);
+
+ return local;
+}
+
void j1939_session_skb_queue(struct j1939_session *session,
struct sk_buff *skb)
{
@@ -359,8 +371,7 @@ void j1939_session_skb_queue(struct j1939_session *session,

j1939_ac_fixup(priv, skb);

- if (j1939_address_is_unicast(skcb->addr.da) &&
- priv->ents[skcb->addr.da].nusers)
+ if (j1939_address_is_local(priv, skcb->addr.da))
skcb->flags |= J1939_ECU_LOCAL_DST;

skcb->flags |= J1939_ECU_LOCAL_SRC;
@@ -2038,8 +2049,7 @@ struct j1939_session *j1939_tp_send(struct j1939_priv *priv,
return ERR_PTR(ret);

/* fix DST flags, it may be used there soon */
- if (j1939_address_is_unicast(skcb->addr.da) &&
- priv->ents[skcb->addr.da].nusers)
+ if (j1939_address_is_local(priv, skcb->addr.da))
skcb->flags |= J1939_ECU_LOCAL_DST;

/* src is always local, I'm sending ... */
--
2.25.1