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Date:   Wed, 8 Sep 2021 19:21:29 +0900
From:   Jung Daehwan <dh10.jung@samsung.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Felipe Balbi <balbi@kernel.org>
Cc:     "open list:USB GADGET/PERIPHERAL SUBSYSTEM" 
        <linux-usb@vger.kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: usb: gadget: u_serial: check Null pointer in EP callback
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On Fri, Aug 06, 2021 at 05:13:34PM +0200, Greg Kroah-Harman wrote:
> On Fri, Aug 06, 2021 at 04:53:26PM +0900, Daehwan Jung wrote:
> > From: taehyun cho <taehyun.cho@samsung.com>
> > 
> > Endpoint complete function in u_serial can be executed when 'gs_port' 
> > is Null. This situation happens when 'dwc3_gadget_pullup' returns 
> > ETIMEDOUT. The reason why ETIMEDOUT is returned is that whole system 
> > is out of order including interrupt regardless of USB.
> > 
> > pc : __lock_acquire+0x54/0x5ec
> > lr : lock_acquire+0xe8/0x198
> > sp : ffffffc03914b9d0
> > x29: ffffffc03914b9d0 x28: ffffff895f13b680
> > x27: 0000000000000000 x26: 0000000000000000
> > x25: 00000000000003c8 x24: 0000000000000080
> > x23: ffffffc010a8f650 x22: 0000000000000000
> > x21: 0000000000000000 x20: 0000000000000000
> > x19: ffffffc010a8f650 x18: ffffffc02d70a0b0
> > x17: 0000000000000000 x16: 00000000000229e0
> > x15: 0000000000000004 x14: 00000000000004f2
> > x13: ffffffc0120fe178 x12: 0000000000000003
> > x11: 00000000ffffffff x10: 0000000100000001
> > x9 : 0000000000000001 x8 : 00000000000003c8
> > x7 : 0000000000000000 x6 : ffffffc010a8f650
> > x5 : 0000000000000000 x4 : 0000000000000080
> > x3 : 0000000000000000 x2 : 0000000000000000
> > x1 : 0000000000000000 x0 : 00000000000003c8 Call trace:
> >  __lock_acquire+0x54/0x5ec
> >  lock_acquire+0xe8/0x198
> >  _raw_spin_lock+0x70/0x88
> >  gs_read_complete+0x48/0xac
> >  usb_gadget_giveback_request+0x48/0x80
> >  dwc3_gadget_giveback+0xcc/0xe8
> >  dwc3_remove_requests+0xa8/0x188
> >  __dwc3_gadget_ep_disable+0x98/0x110
> >  dwc3_gadget_ep_disable+0x50/0xbc
> >  usb_ep_disable+0x44/0x94
> >  gserial_disconnect+0xc0/0x250
> >  acm_free_func+0x30/0x48
> >  usb_put_function+0x34/0x68
> >  config_usb_cfg_unlink+0xdc/0xf8
> >  configfs_unlink+0x144/0x264
> >  vfs_unlink+0x134/0x218
> >  do_unlinkat+0x13c/0x2a0
> >  __arm64_sys_unlinkat+0x48/0x60
> >  el0_svc_common.llvm.10277270529376503802+0xb8/0x1b4
> >  do_el0_svc+0x24/0x8c
> >  el0_svc+0x10/0x1c
> >  el0_sync_handler+0x68/0xac
> >  el0_sync+0x18c/0x1c0
> > 
> > Signed-off-by: taehyun cho <taehyun.cho@samsung.com>
> > ---
> >  drivers/usb/gadget/function/u_serial.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> > 
> > diff --git a/drivers/usb/gadget/function/u_serial.c 
> > b/drivers/usb/gadget/function/u_serial.c
> > index 6f68cbe..af08a18 100644
> > --- a/drivers/usb/gadget/function/u_serial.c
> > +++ b/drivers/usb/gadget/function/u_serial.c
> > @@ -450,6 +450,15 @@ static void gs_read_complete(struct usb_ep *ep, 
> > struct usb_request *req)  {
> >  	struct gs_port	*port = ep->driver_data;
> >  
> > +	/*
> > +	 * Port became NULL when 'dwc3_gadget_pullup' returns ETIMEDOUT.
> > +	 * Return here to avoid panic.
> > +	 */
> > +	if (!port) {
> > +		pr_err("%s, failed to get port\n", __func__);
> > +		return;
> > +	}
> > +
> 
	spin_lock(&port->port_lock);
	...
	spin_unlock(&port->port_lock);
> What prevents port from being null right after checking this?  Where is the
> lock to prevent this?
>
It tries to get lock first in gs_read_complete/gs_write_complete like above.
That's why the panic occured during getting lock but this issue is not related
to lock. We just want to prevent doing something after port becomes null.
> >  	/* Queue all received data until the tty layer is ready for it. */
> >  	spin_lock(&port->port_lock);
> >  	list_add_tail(&req->list, &port->read_queue); @@ -461,6 +470,15 @@ 
> > static void gs_write_complete(struct usb_ep *ep, struct usb_request 
> > *req)  {
> >  	struct gs_port	*port = ep->driver_data;
> >  
> > +	/*
> > +	 * port became NULL when 'dwc3_gadget_pullup' returns ETIMEDOUT.
> > +	 * Return here to avoid panic.
> > +	 */
> > +	if (!port) {
> > +		pr_err("%s, failed to get port\n", __func__);
> > +		return;
> > +	}
> 
> Same here, where is the lock?
> 
> And why report an error, what can a user do about it?
> 

It could happen to access null pointer and occur whole system panic.

Best Regards,
Jung Daehwan

> thanks,
> 
> greg k-h
> 
> 


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