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

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Date:   Wed, 8 Feb 2023 21:15:54 +0530
From:   Prashanth K <quic_prashk@...cinc.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 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

Issue Type: kernel panic issue
Issue AutoSignature:
pc : do_raw_spin_lock
lr : _raw_spin_lock_irqsave
Call trace:
do_raw_spin_lock
_raw_spin_lock_irqsave
gserial_resume
acm_resume
composite_resume
configfs_composite_resume
dwc3_process_event_entry
dwc3_process_event_buf
dwc3_thread_interrupt
irq_thread_fn
irq_thread
kthread
ret_from_fork

Thanks in advance,
Prashanth
> 
> thanks,
> 
> greg k-h