[PATCH 1/1] can: bcm: prevent thrtimer UAF in rx path by checking RX_NO_AUTOTIMER

[Lee Jones <lee@kernel.org>]

Commit f1b4e32aca08 ("can: bcm: use call_rcu() instead of costly
synchronize_rcu()") removed the synchronize_rcu() call from
bcm_delete_rx_op() and introduced the RX_NO_AUTOTIMER flag to prevent
timers from being rearmed during deletion.  However, it only applied
this check to op->timer via bcm_rx_starttimer().

It missed the fact that op->thrtimer can also be rearmed by an
in-flight bcm_rx_handler() (which runs as an RCU reader) via
bcm_rx_update_and_send().  This allows op->thrtimer to be queued after
bcm_remove_op() has already cancelled it, leading to a use-after-free
when the timer fires on the deferred-freed struct bcm_op.

Address the omission by checking the RX_NO_AUTOTIMER flag
in bcm_rx_update_and_send() before starting op->thrtimer, effectively
preventing it from being rearmed concurrently with teardown.

Signed-off-by: Lee Jones <lee@kernel.org>
---
 net/can/bcm.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/can/bcm.c b/net/can/bcm.c
index a4bef2c48a55..67e5b3149a8f 100644
--- a/net/can/bcm.c
+++ b/net/can/bcm.c
@@ -539,6 +539,12 @@ static void bcm_rx_update_and_send(struct bcm_op *op,
 	if (hrtimer_active(&op->thrtimer))
 		return;
 
+	/* bcm_remove_op() may have cancelled thrtimer concurrently with this
+	 * RCU-protected handler; do not rearm it. Mirrors bcm_rx_starttimer().
+	 */
+	if (op->flags & RX_NO_AUTOTIMER)
+		return;
+
 	/* first reception with enabled throttling mode */
 	if (!op->kt_lastmsg)
 		goto rx_changed_settime;
-- 
2.54.0.rc1.555.g9c883467ad-goog