linux-kernel - Re: [PATCH 4/6] usbip: fix stub_dev usbip_sockfd

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Date:   Mon, 8 Mar 2021 16:35:22 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Shuah Khan <skhan@...uxfoundation.org>, shuah@...nel.org,
        valentina.manea.m@...il.com, gregkh@...uxfoundation.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] usbip: fix stub_dev usbip_sockfd_store() races
 leading to gpf

On 2021/03/08 12:53, Shuah Khan wrote:
> Fix the above problems:
> - Stop using kthread_get_run() macro to create/start threads.
> - Create threads and get task struct reference.
> - Add kthread_create() failure handling and bail out.
> - Hold usbip_device lock to update local and shared states after
>   creating rx and tx threads.
> - Update usbip_device status to SDEV_ST_USED.
> - Update usbip_device tcp_socket, sockfd, tcp_rx, and tcp_tx
> - Start threads after usbip_device (tcp_socket, sockfd, tcp_rx, tcp_tx,
>   and status) is complete.

No, the whole usbip_sockfd_store() etc. should be serialized using a mutex,
for two different threads can open same file and write the same content at
the same moment. This results in seeing SDEV_ST_AVAILABLE and creating two
threads and overwiting global variables and setting SDEV_ST_USED and starting
two threads by each of two thread, which will later fail to call kthread_stop()
on one of two thread because global variables are overwritten.

kthread_crate() (which involves GFP_KERNEL allocation) can take long time
enough to hit

  usbip_sockfd_store() must perform

      if (sdev->ud.status != SDEV_ST_AVAILABLE) {
        /* misc assignments for attach operation */
        sdev->ud.status = SDEV_ST_USED;
      }

  under a lock, or multiple ud->tcp_{tx,rx} are created (which will later
  cause a crash like [1]) and refcount on ud->tcp_socket is leaked when
  usbip_sockfd_store() is concurrently called.

problem. That's why my patch introduced usbip_event_mutex lock.