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

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Message-ID: <e7dde0aa-c1e2-dd6c-94a1-1e9049f0a5fb@quicinc.com>
Date:   Thu, 9 Feb 2023 19:37:01 +0530
From:   Prashanth K <quic_prashk@...cinc.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>
CC:     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 12:33 pm, Prashanth K wrote:
> 
> 
> On 09-02-23 12:31 pm, Greg Kroah-Hartman wrote:
>> On Thu, Feb 09, 2023 at 10:31:50AM +0530, Prashanth K wrote:
>>>
>>>
>>> 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?
>>
>> Yes, that would be the correct thing to do.
Hi Greg/Alan, One general doubt, if we make the spinlock static/global, 
wouldn't that be a problem when there are multiple instances, and also 
multiple interfaces can use u_serial at same time. Asking this because 
u_serial can be used by f_serial (gser) as well as f_acm (acm).

Thanks
Prahanth K

> will do it and share next patch
> 
> Thanks for the suggestions
> Prashanth K