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

  refcount_t: addition on 0; use-after-free.

Increment nsim_bus_devs in nsim_bus_dev_new() before device_register() so
the release callback's decrement is balanced on both the success and error
paths.

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>
---
 drivers/net/netdevsim/bus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c
index 5c55c308487b..426fb438b922 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;
-- 
2.51.0

On Mon, 31 Aug 2026 14:39:52 -0400 Adriano Cordova wrote:
> nsim_bus_dev_release() drops nsim_bus_devs when a device is freed, but
> the matching increment was done in new_device_store() only after
> nsim_bus_dev_new() returned successfully.  When device_register() failed
> inside nsim_bus_dev_new(), put_device() invoked the release callback and
> decremented nsim_bus_devs without the increment, driving the refcount to
> zero.  A later successful device creation then incremented from zero and
> triggered:
> 
>   refcount_t: addition on 0; use-after-free.
> 
> Increment nsim_bus_devs in nsim_bus_dev_new() before device_register() so
> the release callback's decrement is balanced on both the success and error
> paths.

This is still racy, please squash this into v2:

@@ -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);
-- 
pw-bot: cr