can: bcm: fix data race on rx_stamp/rx_ifindex in bcm_rx_handler()

For an rx op subscribed on all interfaces (ifindex == 0), the same op
is registered once in the shared per-netns wildcard filter list, so
bcm_rx_handler() can run concurrently on different CPUs for frames
arriving on different net devices.

op->rx_stamp and op->rx_ifindex were written before bcm_rx_update_lock was
taken, allowing concurrent writers to race each other - including a torn
store of the 64-bit rx_stamp on 32-bit platforms.

Beyond a torn store bcm_send_to_user() must report the timestamp/ifindex
of the very same frame whose content it is delivering. So the assignment
is placed in the same unbroken bcm_rx_update_lock section as the content
comparison.

As a side effect, the RTR-request frame feature (which never reach
bcm_send_to_user()) no longer updates rx_stamp/rx_ifindex, since only
the notification path needs them.

Fixes: ffd980f976e7 ("[CAN]: Add broadcast manager (bcm) protocol")
Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/linux-can/20260707145135.5BC831F00A3A@smtp.kernel.org/
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260714-bcm_fixes-v15-10-562f7e3e42da@hartkopp.net
Cc: stable@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

diff --git a/net/can/bcm.c b/net/can/bcm.c
index a53dba6ab8b8901259b4d72e95b9880501d8be94..f213a0b3779158abc632a180233ea26f5763684f 100644 (file)
--- a/net/can/bcm.c
+++ b/net/can/bcm.c
@@ -800,11 +800,6 @@ static void bcm_rx_handler(struct sk_buff *skb, void *data)
        /* disable timeout */
        hrtimer_cancel(&op->timer);
 
-       /* save rx timestamp */
-       op->rx_stamp = skb->tstamp;
-       /* save originator for recvfrom() */
-       op->rx_ifindex = skb->dev->ifindex;
-
        /* op->flags/op->frames may be updated concurrently by bcm_rx_setup() */
        spin_lock_bh(&op->bcm_rx_update_lock);
 
@@ -836,6 +831,14 @@ static void bcm_rx_handler(struct sk_buff *skb, void *data)
                        traffic_flags |= RX_OWN;
        }
 
+       /* save rx timestamp and originator for recvfrom() under lock.
+        * For an op subscribed on all interfaces (ifindex == 0)
+        * bcm_rx_handler() can run concurrently on different CPUs so
+        * the CAN content and the meta data must be bundled correctly.
+        */
+       op->rx_stamp = skb->tstamp;
+       op->rx_ifindex = skb->dev->ifindex;
+
        if (op->flags & RX_FILTER_ID) {
                /* the easiest case */
                bcm_rx_update_and_send(op, op->last_frames, rxframe,