Re: [PATCH] usb: gadget: u_serial: Add null pointer check in gserial_resume

[Prashanth K]

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@xxxxxxxxxxx>
> > > > > > > ---
> > > > > > >     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