linux-kernel - 答复: 答复: [PATCH] usb: gadget: u_serial: Fix the issue that gs_start

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Message-ID:
 <TYUPR06MB62179D03E009DA9E0A786C4DD2202@TYUPR06MB6217.apcprd06.prod.outlook.com>
Date: Tue, 19 Nov 2024 12:50:53 +0000
From: 胡连勤 <hulianqin@...o.com>
To: Prashanth K <quic_prashk@...cinc.com>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "quic_jjohnson@...cinc.com"
	<quic_jjohnson@...cinc.com>, "mwalle@...nel.org" <mwalle@...nel.org>
CC: "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	opensource.kernel <opensource.kernel@...o.com>
Subject:
 答复: 答复: [PATCH] usb: gadget: u_serial: Fix the issue that gs_start_io crashed due to accessing null pointer

Hello Prashanth:

> >
> >>> Considering that in some extreme cases, when u_serial driver is
> >>> accessed by multiple threads, Thread A is executing the open
> >>> operation and calling the gs_open, Thread B is executing the
> >>> disconnect operation and calling the gserial_disconnect function,The
> >>> port->port_usb pointer will be set to NULL.
> >>>
> >> [...]
> >>> ---
> >>>  drivers/usb/gadget/function/u_serial.c | 25
> >>> +++++++++++++++----------
> >>>  1 file changed, 15 insertions(+), 10 deletions(-)
> >>>
> >>> diff --git a/drivers/usb/gadget/function/u_serial.c
> >>> b/drivers/usb/gadget/function/u_serial.c
> >>> index 0a8c05b2746b..9ab2dbed60a8 100644
> >>> --- a/drivers/usb/gadget/function/u_serial.c
> >>> +++ b/drivers/usb/gadget/function/u_serial.c
> >>> @@ -124,6 +124,7 @@ struct gs_port {
> >>>  	struct kfifo		port_write_buf;
> >>>  	wait_queue_head_t	drain_wait;	/* wait while writes drain */
> >>>  	bool                    write_busy;
> >>> +	bool                    read_busy;
> >>>  	wait_queue_head_t	close_wait;
> >>>  	bool			suspended;	/* port suspended */
> >>>  	bool			start_delayed;	/* delay start when
> >> suspended */
> >>> @@ -331,9 +332,11 @@ __acquires(&port->port_lock)
> >>>  		/* drop lock while we call out; the controller driver
> >>>  		 * may need to call us back (e.g. for disconnect)
> >>>  		 */
> >>> +		port->read_busy = true;
> >>>  		spin_unlock(&port->port_lock);
> >>>  		status = usb_ep_queue(out, req, GFP_ATOMIC);
> >>>  		spin_lock(&port->port_lock);
> >>> +		port->read_busy = false;
> >>>
> >>>  		if (status) {
> >>>  			pr_debug("%s: %s %s err %d\n",
> >>> @@ -1412,19 +1415,21 @@ void gserial_disconnect(struct gserial *gser)
> >>>  	/* tell the TTY glue not to do I/O here any more */
> >>>  	spin_lock(&port->port_lock);
> >>>
> >>> -	gs_console_disconnect(port);
> >>> +	if (!port->read_busy) {
> >> start_tx/rx rely on port->port_usb for queuing the requests, and if
> >> its not null during disconnect, tx/rx would keep on queuing requests
> >> to UDC even after disconnect (which is not ideal). Here in your case,
> >> after read_busy is set, start_rx would queue something outside of
> >> spinlock, meanwhile disconnect happens but port_usb is still valid
> >> (because read_busy is set) and start_rx would break early. But
> >> start_tx could continue queuing into disconnected UDC (if 'started'
> >> is non-zero, which could happen due to timing). Can't you try
> >> something like this,
> >>
> >> --- a/drivers/usb/gadget/function/u_serial.c
> >> +++ b/drivers/usb/gadget/function/u_serial.c
> >> @@ -579,9 +579,12 @@ static int gs_start_io(struct gs_port *port)
> >>                  * we didn't in gs_start_tx() */
> >>                 tty_wakeup(port->port.tty);
> >>         } else {
> >> -               gs_free_requests(ep, head, &port->read_allocated);
> >> -               gs_free_requests(port->port_usb->in, &port->write_pool,
> >> -                       &port->write_allocated);
> >> +               /* Free reqs only if we are still connected */
> >> +               if (port->port_usb) {
> >> +                       gs_free_requests(ep, head, &port->read_allocated);
> >> +                       gs_free_requests(port->port_usb->in,
> >> &port->write_pool,
> >> +                               &port->write_allocated);
> >> +               }
> >>                 status = -EIO;
> >>         }
> >>
> >> This will skip freeing reqs (and your crash) if port_usb is null and
> >> freeing would be taken care by disconnect callback.
> >>
> >>
> > First of all, the patch you gave can solve the problem we are currently
> facing.
> >
> > When we first encountered this problem, we also thought about adding a
> > null check operation to deal with it, but we saw that the entry of
> > this function (gs_start_io) had a null check operation for
> > port->port_usb, so I gave up the idea of null check during free_req (maybe
> I made a simple problem complicated), and thought about optimizing it from
> the software logic, so that the port->port_usb pointer is always valid before
> gs_start_io is executed.
> >
> 
> If it solves the problem, i guess you can use the null pointer check as I
> suggested and send a new patchset, the current one will introduce new
> problems. Keep the issue analysis as it is in commit text since its descriptive
> enough to understand the problem.

OK, I'll do a stress test and send out a new patch later.

Thanks