[PATCH net-next v6 1/2] net: phy: refuse a second attach before touching the PHY

phy_attach_direct() tests phydev->attached_dev only after it has taken
the driver module and, for a PHY with no driver, bound the generic one,
and then leaves through the error path that calls
phy_detach_internal(). That call works on the PHY's current attachment,
so a second attach of a PHY in use tears down the first consumer: it
clears that netdev's phydev pointer and the PHY's attached_dev, removes
the sysfs links and suspends the PHY, while the first consumer goes on
using it. A PHY on the generic driver never reaches the test: the
attach runs its probe again and, when the bind fails, clears d->driver
under the first consumer.

Found in review of a change that makes phy_detach() put only the
module the attach recorded: with it, this path leaves the first
consumer's reference pinned. On a Keenetic KN-1012, a test module
attaching a second netdev to the EN8811H that the lan4 DSA port holds
got -EBUSY and left lan4 with no PHY; with this change lan4 kept it.

Test whether the PHY is attached before the driver module is taken or
the generic driver bound, and refuse from there. A PHY attached without
a netdev, as DSA does for its CPU and link ports, has no attached_dev,
so test phy_link_change, which every attach sets and every detach
clears. A second attach of such a PHY used to succeed and share it;
with that change it would also leak a module reference.

Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <redacted>
---
v6: dropped the Fixes tag.

 drivers/net/phy/phy_device.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index bfce8b893644..6bc9cdb7fa69 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c

@@ -1811,6 +1811,13 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
 
 	get_device(d);
 
+	/* Set by every attach, with or without a netdev */
+	if (phydev->phy_link_change) {
+		phydev_err(phydev, "PHY already attached\n");
+		err = -EBUSY;
+		goto error_put_device;
+	}
+
 	/* Assume that if there is no driver, that it doesn't
 	 * exist, and we should use the genphy driver.
 	 */

@@ -1838,12 +1845,6 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
 			goto error_module_put;
 	}
 
-	if (phydev->attached_dev) {
-		dev_err(&dev->dev, "PHY already attached\n");
-		err = -EBUSY;
-		goto error;
-	}
-
 	phydev->phy_link_change = phy_link_change;
 	if (dev) {
 		phydev->attached_dev = dev;

-- 
2.53.0