[PATCH net] net: phy: allow phy_detach() before netdev registration

From: Chenguang Zhao <redacted>

phylink_connect_phy() may be called from probe without RTNL while the
net_device is still NETREG_UNINITIALIZED. If PHY bring-up fails,
phy_detach() uses rtnl_dereference(dev->hwprov) and lockdep reports
suspicious RCU usage.

hwprov cannot have been assigned before register_netdevice(). Relax the
RCU check for that state only; registered devices still require RTNL.

Fixes: 35f7cad1743e ("net: Add the possibility to support a selected hwtstamp in netdevice")
Reported-by: syzbot+694b49f41098a5df4fd7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=694b49f41098a5df4fd7

Signed-off-by: Chenguang Zhao <redacted>
---
 drivers/net/phy/phy_device.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index 94b2e85e00a3..acd8d658a29e 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c

@@ -1941,7 +1941,9 @@ void phy_detach(struct phy_device *phydev)
 		/* hwprov may technically be protected by ops lock but
 		 * not for devices with a phydev, see phy_link_topo_add_phy()
 		 */
-		hwprov = rtnl_dereference(dev->hwprov);
+		hwprov = rcu_dereference_protected(dev->hwprov,
+						   lockdep_rtnl_is_held() ||
+						   dev->reg_state == NETREG_UNINITIALIZED);
 		/* Disable timestamp if it is the one selected */
 		if (hwprov && hwprov->phydev == phydev) {
 			rcu_assign_pointer(dev->hwprov, NULL);

-- 
2.25.1