Re: [PATCH 4/6] usbip: fix stub_dev usbip_sockfd_store() races leading to gpf

[Shuah Khan]

On 3/8/21 3:10 AM, Tetsuo Handa wrote:
> On 2021/03/08 16:35, Tetsuo Handa wrote:
> > 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) {
>
> Oops. This is
>
> 	if (sdev->ud.status == SDEV_ST_AVAILABLE) {
>
> of course.
>
> >          /* 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.
>
> And I think that same serialization is required between "rh_port_connect() from attach_store()" and
> "rh_port_disconnect() from vhci_shutdown_connection() via usbip_event_add(&vdev->ud, VDEV_EVENT_DOWN)
>   from vhci_port_disconnect() from detach_store()", for both vhci_rx_pdu() from vhci_rx_loop() and
> vhci_port_disconnect() from detach_store() can queue VDEV_EVENT_DOWN event which can be processed
> without waiting for attach_store() to complete.

Yes. We might need synchronization between events, threads, and shutdown
in usbip_host side and in connection polling and threads in vhci.

I am also looking at the shutdown sequences closely as well since the
local state is referenced without usbip_device lock in these paths.

I am approaching these problems as peeling the onion an expression so
we can limit the changes and take a spot fix approach. We have the
goal to address these crashes and not introduce regressions.

I don't seem to be able to reproduce these problems consistently in my
env. with the reproducer. I couldn't reproduce them in normal case at
all. Hence, the this cautious approach that reduces the chance of
regressions and if we see regressions, they can fixed easily.

https://syzkaller.appspot.com/text?tag=ReproC&x=14801034d00000

If this patch series fixes the problems you are seeing, I would like
get these fixes in and address the other two potential race conditions
in another round of patches. I also want to soak these in the next
for a few weeks.

Please let me know if these patches fix the problems you are seeing in 
your env.

thanks,
-- Shuah