[PATCH v4] RDMA/core: Reject unregistering netdevs in ib_get_eth_speed

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

ib_device_get_netdev() intentionally returns a referenced net_device even
when it is unregistering, so matching and cleanup callers can still find
the association. The reference keeps struct net_device allocated, but does
not guarantee that the device remains operational.

ib_get_eth_speed() uses the returned device operationally by invoking its
ethtool callback. Although that call is made under RTNL, the function does
not verify the registration state first. An asynchronous RDMA port query
can therefore call into a netdev after NETDEV_UNREGISTER and ndo_uninit
have completed.

Check for NETREG_REGISTERED while holding RTNL and return -ENODEV for a
device which is being unregistered. Keeping RTNL across the check and the
ethtool operation prevents unregister from starting between them.

Keep the speed fallback and warning under RTNL as well, so the warning can
safely read netdev->name. Drop the netdev reference before releasing RTNL
once all accesses to the device are complete.

Fixes: d41861942fc5 ("IB/core: Add generic function to extract IB speed from netdev")
Reported-by: syzbot+5fe14f2ff4ccbace9a26@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5fe14f2ff4ccbace9a26
Signed-off-by: Krystian Kaniewski <krystianmkaniewski@gmail.com>
---
v4:
  - Keep the speed fallback and warning under RTNL and use netdev->name
    directly, as suggested by Jason Gunthorpe.
  - Release the netdev reference before rtnl_unlock() on both paths.
  - Drop the [PATCH net] subject prefix because the patch targets RDMA.
  - Rebase onto the RDMA for-next branch.
v3: https://lore.kernel.org/all/20260810133124.44513-1-krystianmkaniewski@gmail.com/
v2: https://lore.kernel.org/all/20260803121140.261329-1-krystianmkaniewski@gmail.com/

 drivers/infiniband/core/verbs.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/infiniband/core/verbs.c b/drivers/infiniband/core/verbs.c
index 367822efff36..b31d61513335 100644
--- a/drivers/infiniband/core/verbs.c
+++ b/drivers/infiniband/core/verbs.c
@@ -2058,11 +2058,13 @@ int ib_get_eth_speed(struct ib_device *dev, u32 port_num, u16 *speed, u8 *width)
 		return -ENODEV;
 
 	rtnl_lock();
-	rc = __ethtool_get_link_ksettings(netdev, &lksettings);
-	rtnl_unlock();
-
-	dev_put(netdev);
+	if (READ_ONCE(netdev->reg_state) != NETREG_REGISTERED) {
+		dev_put(netdev);
+		rtnl_unlock();
+		return -ENODEV;
+	}
 
+	rc = __ethtool_get_link_ksettings(netdev, &lksettings);
 	if (!rc && lksettings.base.speed != (u32)SPEED_UNKNOWN) {
 		netdev_speed = lksettings.base.speed;
 	} else {
@@ -2071,6 +2073,8 @@ int ib_get_eth_speed(struct ib_device *dev, u32 port_num, u16 *speed, u8 *width)
 			pr_warn("%s speed is unknown, defaulting to %u\n",
 				netdev->name, netdev_speed);
 	}
+	dev_put(netdev);
+	rtnl_unlock();
 
 	ib_get_width_and_speed(netdev_speed, lksettings.lanes,
 			       speed, width);
-- 
2.53.0