syzbot is reporting a crash at vhci_shutdown_connection() [1]. It turned
out that it was not a NULL pointer dereference but an ERR_PTR(-EINTR)
pointer dereference, for kthread_create() became killable due to
commit 786235eeba0e1e85 ("kthread: make kthread_create() killable").

When SIGKILLed while attach_store() is calling kthread_get_run(),
ERR_PTR(-EINTR) is stored into vdev->ud.tcp_{rx,tx}, and then
kthread_stop_put() is called on vdev->ud.tcp_{rx,tx} from
vhci_shutdown_connection() because vdev->ud.tcp_{rx,tx} != NULL.

Prior to commit 9720b4bc76a83807 ("staging/usbip: convert to kthread"),
"current" pointer is assigned to vdev->ud.tcp_{rx,tx} by usbip_thread()
kernel thread, and hence vdev->ud.tcp_{rx,tx} != NULL meant a valid task
pointer. Therefore, we should make kthread_get_run() return NULL when
kthread_create() failed.

[1] https://syzkaller.appspot.com/bug?id=c21c07f3d51769405e8efc027bdb927515dcc7d6

Reported-by: syzbot <redacted>
Reported-by: syzbot <redacted>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 9720b4bc76a83807 ("staging/usbip: convert to kthread")
Cc: Arnd Bergmann <arnd@arndb.de>
---
  drivers/usb/usbip/usbip_common.h | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/drivers/usb/usbip/usbip_common.h b/drivers/usb/usbip/usbip_common.h
index 8be857a4fa13..a7c68097aa1d 100644
--- a/drivers/usb/usbip/usbip_common.h
+++ b/drivers/usb/usbip/usbip_common.h

@@ -286,6 +286,8 @@ struct usbip_device {
  	if (!IS_ERR(__k)) {						   \
  		get_task_struct(__k);					   \
  		wake_up_process(__k);					   \
+	} else {							   \
+		__k = NULL;						   \
  	}								   \
  	__k;								   \
  })