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

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 <redacted>
---
 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