[PATCH v14 10/11] can: bcm: fix data race on rx_stamp/rx_ifindex in bcm_rx_handler()

From: Oliver Hartkopp <[email protected]>

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: [email protected]
Link: https://lore.kernel.org/linux-can/[email protected]/
Signed-off-by: Oliver Hartkopp <[email protected]>
---
 net/can/bcm.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/can/bcm.c b/net/can/bcm.c
index 55226e5fec07..9c6fe559c4e6 100644
--- a/net/can/bcm.c
+++ b/net/can/bcm.c
@@ -790,15 +790,10 @@ 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);
 
 	rtr_frame = op->flags & RX_RTR_FRAME;
 	if (rtr_frame) {
@@ -826,10 +821,18 @@ static void bcm_rx_handler(struct sk_buff *skb, void *data)
 		traffic_flags |= RX_LOCAL;
 		if (skb->sk == op->sk)
 			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,
 				       traffic_flags);
 		goto rx_starttimer;

-- 
2.53.0