net: ethtool: Fix suspicious rcu_dereference usage

The __ethtool_get_ts_info function can be called with or without the
rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
triggers a warning due to the lack of lock context.

Add an rcu_read_lock() to ensure the lock is acquired and to maintain
synchronization.

Reported-by: syzbot+a344326c05c98ba19682@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@google.com/
Fixes: b9e3f7d ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241220083741.175329-1-kory.maincent@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kmaincent

net/ethtool/common.c

@@ -870,7 +870,8 @@ int __ethtool_get_ts_info(struct net_device *dev,
 {
 	struct hwtstamp_provider *hwprov;
 
-	hwprov = rtnl_dereference(dev->hwprov);
+	rcu_read_lock();
+	hwprov = rcu_dereference(dev->hwprov);
 	/* No provider specified, use default behavior */
 	if (!hwprov) {
 		const struct ethtool_ops *ops = dev->ethtool_ops;
@@ -887,9 +888,11 @@ int __ethtool_get_ts_info(struct net_device *dev,
 		info->so_timestamping |= SOF_TIMESTAMPING_RX_SOFTWARE |
 					 SOF_TIMESTAMPING_SOFTWARE;
 
+		rcu_read_unlock();
 		return err;
 	}
 
+	rcu_read_unlock();
 	return ethtool_get_ts_info_by_phc(dev, info, &hwprov->desc);
 }