[PATCH v4 01/12] usb: usbip: introduce usbip_event_mutex for serialization

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting a NULL pointer dereference [1] because usbip modules
do not use serialization when attaching/detaching. As a preparation for
serializing attach/detach operations, introduce a global mutex, export
it via usbip_event_lock_killable()/usbip_event_unlock(), and use it in
event_handler().

We don't need to use per a device mutex because event_handler() is
processed by a singlethreaded workqueue and ud->tcp_{tx,rx} must not
wait for event_handler() to flush.

[1] https://syzkaller.appspot.com/bug?extid=95ce4b142579611ef0a9

Reported-by: syzbot <syzbot+95ce4b142579611ef0a9@syzkaller.appspotmail.com>
Tested-by: syzbot <syzbot+a93fba6d384346a761e3@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/usb/usbip/usbip_common.h |  2 ++
 drivers/usb/usbip/usbip_event.c  | 15 +++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/drivers/usb/usbip/usbip_common.h b/drivers/usb/usbip/usbip_common.h
index a7dd6c66aee5..f8e16c75b154 100644
--- a/drivers/usb/usbip/usbip_common.h
+++ b/drivers/usb/usbip/usbip_common.h
@@ -327,6 +327,8 @@ void usbip_stop_eh(struct usbip_device *ud);
 void usbip_event_add(struct usbip_device *ud, unsigned long event);
 int usbip_event_happened(struct usbip_device *ud);
 int usbip_in_eh(struct task_struct *task);
+int usbip_event_lock_killable(void);
+void usbip_event_unlock(void);
 
 static inline int interface_to_busnum(struct usb_interface *interface)
 {
diff --git a/drivers/usb/usbip/usbip_event.c b/drivers/usb/usbip/usbip_event.c
index 5d88917c9631..e05b858f346d 100644
--- a/drivers/usb/usbip/usbip_event.c
+++ b/drivers/usb/usbip/usbip_event.c
@@ -58,6 +58,19 @@ static struct usbip_device *get_event(void)
 }
 
 static struct task_struct *worker_context;
+static DEFINE_MUTEX(usbip_event_mutex);
+
+int usbip_event_lock_killable(void)
+{
+	return mutex_lock_killable(&usbip_event_mutex);
+}
+EXPORT_SYMBOL_GPL(usbip_event_lock_killable);
+
+void usbip_event_unlock(void)
+{
+	mutex_unlock(&usbip_event_mutex);
+}
+EXPORT_SYMBOL_GPL(usbip_event_unlock);
 
 static void event_handler(struct work_struct *work)
 {
@@ -68,6 +81,7 @@ static void event_handler(struct work_struct *work)
 	}
 
 	while ((ud = get_event()) != NULL) {
+		mutex_lock(&usbip_event_mutex);
 		usbip_dbg_eh("pending event %lx\n", ud->event);
 
 		/*
@@ -91,6 +105,7 @@ static void event_handler(struct work_struct *work)
 			unset_event(ud, USBIP_EH_UNUSABLE);
 		}
 
+		mutex_unlock(&usbip_event_mutex);
 		wake_up(&ud->eh_waitq);
 	}
 }
-- 
2.18.4