can: bcm: add missing rcu read protection for procfs content

When the procfs content is generated for a bcm_op which is in the process
to be removed the procfs output might show unreliable data (UAF).

As the removal of bcm_op's is already implemented with rcu handling this
patch adds the missing rcu_read_lock() and makes sure the list entries
are properly removed under rcu protection.

Fixes: f1b4e32 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()")
Reported-by: Anderson Nascimento <anderson@allelesecurity.com>
Suggested-by: Anderson Nascimento <anderson@allelesecurity.com>
Tested-by: Anderson Nascimento <anderson@allelesecurity.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20250519125027.11900-2-socketcan@hartkopp.net
Cc: stable@vger.kernel.org # >= 5.4
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: hartkopp

net/can/bcm.c

@@ -219,7 +219,9 @@ static int bcm_proc_show(struct seq_file *m, void *v)
 	seq_printf(m, " / bound %s", bcm_proc_getifname(net, ifname, bo->ifindex));
 	seq_printf(m, " <<<\n");
 
-	list_for_each_entry(op, &bo->rx_ops, list) {
+	rcu_read_lock();
+
+	list_for_each_entry_rcu(op, &bo->rx_ops, list) {
 
 		unsigned long reduction;
 
@@ -275,6 +277,9 @@ static int bcm_proc_show(struct seq_file *m, void *v)
 		seq_printf(m, "# sent %ld\n", op->frames_abs);
 	}
 	seq_putc(m, '\n');
+
+	rcu_read_unlock();
+
 	return 0;
 }
 #endif /* CONFIG_PROC_FS */
@@ -858,7 +863,7 @@ static int bcm_delete_rx_op(struct list_head *ops, struct bcm_msg_head *mh,
 						  REGMASK(op->can_id),
 						  bcm_rx_handler, op);
 
-			list_del(&op->list);
+			list_del_rcu(&op->list);
 			bcm_remove_op(op);
 			return 1; /* done */
 		}
@@ -878,7 +883,7 @@ static int bcm_delete_tx_op(struct list_head *ops, struct bcm_msg_head *mh,
 	list_for_each_entry_safe(op, n, ops, list) {
 		if ((op->can_id == mh->can_id) && (op->ifindex == ifindex) &&
 		    (op->flags & CAN_FD_FRAME) == (mh->flags & CAN_FD_FRAME)) {
-			list_del(&op->list);
+			list_del_rcu(&op->list);
 			bcm_remove_op(op);
 			return 1; /* done */
 		}
@@ -1296,7 +1301,7 @@ static int bcm_rx_setup(struct bcm_msg_head *msg_head, struct msghdr *msg,
 					      bcm_rx_handler, op, "bcm", sk);
 		if (err) {
 			/* this bcm rx op is broken -> remove it */
-			list_del(&op->list);
+			list_del_rcu(&op->list);
 			bcm_remove_op(op);
 			return err;
 		}