[PATCH net v2] netdevsim: take bus device refcount before registering device

[Adriano Cordova <adrianox@gmail.com>]

nsim_bus_dev_release() drops nsim_bus_devs when a device is freed, but
the matching increment was done in new_device_store() after
nsim_bus_dev_new() returned.  When device_register() failed inside
nsim_bus_dev_new(), put_device() invoked the release callback and
decremented without an increment, driving the refcount to zero; a later
creation then incremented from zero (refcount_t: addition on 0).

Increment nsim_bus_devs in nsim_bus_dev_new() before device_register(),
and drop the bus reference in nsim_bus_exit() only after every device has
been removed, so the completion cannot be signaled early.

Reported-by: syzbot+989ff6506f5c6d1ff56b@syzkaller.appspot.com
Closes: https://syzkaller.appspot.com/bug?extid=989ff6506f5c6d1ff56b
Fixes: 6aff7cbfe7bfa ("netdevsim: Block until all devices are released")
Signed-off-by: Adriano Cordova <adrianox@gmail.com>
---
v2: https://lore.kernel.org/netdev/20260831183952.1345042-1-adrianox@gmail.com/
    - also drop the bus reference in nsim_bus_exit() only after all
      devices have been removed, per Jakub Kicinski, so that
      wait_for_completion() cannot be signaled early

 drivers/net/netdevsim/bus.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c
index 5c55c308487b..b792ee10b1c1 100644
--- a/drivers/net/netdevsim/bus.c
+++ b/drivers/net/netdevsim/bus.c
@@ -194,7 +194,6 @@ new_device_store(const struct bus_type *bus, const char *buf, size_t count)
 		goto err;
 	}
 
-	refcount_inc(&nsim_bus_devs);
 	/* Allow using nsim_bus_dev */
 	smp_store_release(&nsim_bus_dev->init, true);
 
@@ -465,6 +464,7 @@ nsim_bus_dev_new(unsigned int id, unsigned int port_count, unsigned int num_queu
 	/* Disallow using nsim_bus_dev */
 	smp_store_release(&nsim_bus_dev->init, false);
 
+	refcount_inc(&nsim_bus_devs);
 	err = device_register(&nsim_bus_dev->dev);
 	if (err)
 		goto err_nsim_bus_dev_id_free;
@@ -520,8 +520,6 @@ void nsim_bus_exit(void)
 
 	/* Disallow using resources */
 	smp_store_release(&nsim_bus_enable, false);
-	if (refcount_dec_and_test(&nsim_bus_devs))
-		complete(&nsim_bus_devs_released);
 
 	mutex_lock(&nsim_bus_dev_list_lock);
 	list_for_each_entry_safe(nsim_bus_dev, tmp, &nsim_bus_dev_list, list) {
@@ -530,6 +528,9 @@ void nsim_bus_exit(void)
 	}
 	mutex_unlock(&nsim_bus_dev_list_lock);
 
+	if (refcount_dec_and_test(&nsim_bus_devs))
+		complete(&nsim_bus_devs_released);
+
 	wait_for_completion(&nsim_bus_devs_released);
 
 	driver_unregister(&nsim_driver);
-- 
2.51.0