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

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Date:   Thu, 9 Feb 2023 12:33:34 +0530
From:   Prashanth K <quic_prashk@...cinc.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Alan Stern <stern@...land.harvard.edu>,
        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 12:31 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.
will do it and share next patch

Thanks for the suggestions
Prashanth K