linux-kernel - Re: [PATCH 0/6] usbip fixes to crashes found by syzbot

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Date:   Sat, 13 Mar 2021 09:48:52 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Shuah Khan <skhan@...uxfoundation.org>, shuah@...nel.org
Cc:     valentina.manea.m@...il.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 0/6] usbip fixes to crashes found by syzbot

On 2021/03/12 14:44, Tetsuo Handa wrote:
> And what you are missing in your [PATCH 4,5,6/6] is
> 
>   diff --git a/drivers/usb/usbip/vhci_sysfs.c b/drivers/usb/usbip/vhci_sysfs.c
>   index c4457026d5ad..3c64bd06ab53 100644
>   --- a/drivers/usb/usbip/vhci_sysfs.c
>   +++ b/drivers/usb/usbip/vhci_sysfs.c
>   @@ -423,6 +423,7 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
>           /* end the lock */
>   
>           wake_up_process(vdev->ud.tcp_rx);
>   +       schedule_timeout_uninterruptible(HZ); // Consider being preempted here.
>           wake_up_process(vdev->ud.tcp_tx);
>   
>           rh_port_connect(vdev, speed);
> 
> . wake_up_process(tcp_tx) can call vhci_shutdown_connection() before wake_up_process(tcp_tx) is called.

wake_up_process(tcp_rx) can call vhci_shutdown_connection() before wake_up_process(tcp_tx) is called.

> Since vhci_shutdown_connection() destroys tcp_tx thread and releases tcp_tx memory via kthread_stop_put(tcp_tx),
> wake_up_process(tcp_tx) will access already freed memory. Your patch converted "NULL pointer dereference caused by
> failing to call kthread_stop_put(tcp_tx)" into "use after free caused by succeeding to call kthread_stop_put(tcp_tx)".
> 

And note that

  diff --git a/drivers/usb/usbip/vhci_sysfs.c b/drivers/usb/usbip/vhci_sysfs.c
  index c4457026d5ad..0e1a81d4632c 100644
  --- a/drivers/usb/usbip/vhci_sysfs.c
  +++ b/drivers/usb/usbip/vhci_sysfs.c
  @@ -422,11 +422,11 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
          spin_unlock_irqrestore(&vhci->lock, flags);
          /* end the lock */
  
  -       wake_up_process(vdev->ud.tcp_rx);
  -       wake_up_process(vdev->ud.tcp_tx);
  -
          rh_port_connect(vdev, speed);
  
  +       wake_up_process(vdev->ud.tcp_tx);
  +       wake_up_process(vdev->ud.tcp_rx);
  +
          return count;
   }
   static DEVICE_ATTR_WO(attach);

is as well not sufficient, for you are still missing

  diff --git a/drivers/usb/usbip/vhci_sysfs.c b/drivers/usb/usbip/vhci_sysfs.c
  index c4457026d5ad..c958f89a9196 100644
  --- a/drivers/usb/usbip/vhci_sysfs.c
  +++ b/drivers/usb/usbip/vhci_sysfs.c
  @@ -422,11 +422,13 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
          spin_unlock_irqrestore(&vhci->lock, flags);
          /* end the lock */
  
  -       wake_up_process(vdev->ud.tcp_rx);
  -       wake_up_process(vdev->ud.tcp_tx);
  +       schedule_timeout_uninterruptible(HZ); // Consider being preempted here.
  
          rh_port_connect(vdev, speed);
  
  +       wake_up_process(vdev->ud.tcp_tx);
  +       wake_up_process(vdev->ud.tcp_rx);
  +
          return count;
   }
   static DEVICE_ATTR_WO(attach);

because vhci_port_disconnect() from detach_store() can call usbip_event_add(&vdev->ud, VDEV_EVENT_DOWN)
(same use after free bug regarding tcp_tx and tcp_rx) as soon as all shared states are set up and
spinlocks are released.

What you had better consider first is how to protect event_handler()/usbip_sockfd_store()/attach_store()/detach_store() functions
 from concurrent calls. Please respond to https://lkml.kernel.org/r/3dab66dc-2981-bc88-a370-4b3178dfd390@i-love.sakura.ne.jp
before you try to make further changes.