linux-kernel - Re: [PATCH] usb: gadget: u_serial: Add null pointer check in gserial

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Date:   Thu, 9 Feb 2023 10:31:50 +0530
From:   Prashanth K <quic_prashk@...cinc.com>
To:     Alan Stern <stern@...land.harvard.edu>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Xiu Jianfeng <xiujianfeng@...wei.com>,
        Pratham Pratap <quic_ppratap@...cinc.com>,
        Jack Pham <quic_jackp@...cinc.com>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: gadget: u_serial: Add null pointer check in
 gserial_resume



On 09-02-23 01:51 am, Alan Stern wrote:
> On Wed, Feb 08, 2023 at 09:15:54PM +0530, Prashanth K wrote:
>>
>>
>> On 08-02-23 08:24 pm, Greg Kroah-Hartman wrote:
>>> On Wed, Feb 08, 2023 at 07:24:47PM +0530, Prashanth K wrote:
>>>> Consider a case where gserial_disconnect has already cleared
>>>> gser->ioport. And if a wakeup interrupt triggers afterwards,
>>>> gserial_resume gets called, which will lead to accessing of
>>>> gserial->port and thus causing null pointer dereference.Add
>>>> a null pointer check to prevent this.
>>>>
>>>> Fixes: aba3a8d01d62 (" usb: gadget: u_serial: add suspend resume callbacks")
>>>
>>> Nit, and our tools will complain, no " " before the "usb:" string here,
>>> right?
>>>
>> Will fix it in next patch.
>>>
>>>
>>>> Signed-off-by: Prashanth K <quic_prashk@...cinc.com>
>>>> ---
>>>>    drivers/usb/gadget/function/u_serial.c | 3 +++
>>>>    1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
>>>> index 840626e..98be2b8 100644
>>>> --- a/drivers/usb/gadget/function/u_serial.c
>>>> +++ b/drivers/usb/gadget/function/u_serial.c
>>>> @@ -1428,6 +1428,9 @@ void gserial_resume(struct gserial *gser)
>>>>    	struct gs_port *port = gser->ioport;
>>>>    	unsigned long	flags;
>>>> +	if (!port)
>>>> +		return;
>>>> +
>>>
>>> What prevents port from going to NULL right after this check?
>> In our case we got a null pointer de-reference while performing USB
>> compliance tests, as the gser->port was null. Because in gserial_resume,
>> spinlock_irq_save(&port->port_lock) accesses a null-pointer as port was
>> already marked null by gserial_disconnect.
>>
>> And after gserial_resume acquires the spinlock, gserial_disconnect cant mark
>> it null until the spinlock is released. We need to check if the port->lock
>> is valid before accessing it, otherwise it can lead to the above mentioned
>> scenario
> 
> What happens if gserial_disconnect sets gser->port to NULL immediately
> after your new check occurs, but before
> spinlock_irq_save(&port->port_lock) gets called?
> 
> You may need to add a static spinlock to prevent this from happening.
> 
> Alan Stern
In that case i guess we have to make port_lock a global variable and 
take it out of gs_port structure.

+ static DEFINE_SPINLOCK(port_lock);

struct gs_port {
	struct tty_port port;
-	spinlock_t port_lock;

This will require us to change all the spinlock(port->port_lock) used in 
u_serial.c, what do you suggest?