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SubjectRe: [PATCH] usb: dwc3: fix race of usb_gadget_driver operation
On Sun, Jun 27, 2021 at 10:48:56AM +0800, linyyuan@codeaurora.org wrote:
> On 2021-06-26 23:03, Alan Stern wrote:
> > On Sat, Jun 26, 2021 at 09:16:25AM +0800, linyyuan@codeaurora.org wrote:
> > > On 2021-06-26 00:37, Alan Stern wrote:

> > > > Here and in the other places, you should test dwc->async_callbacks
> > > > _before_ dropping the spinlock. Otherwise there is a race (the flag
> > > > could be written at about the same time it is checked).
> > > thanks for your comments,
> > >
> > > if you think there is race here, how to make sure gadget_driver
> > > pointer is
> > > safe,
> > > this is closest place where we can confirm it is non-NULL by checking
> > > async_callbacks ?
> >
> > I explained this twice already: We know that gadget_driver is not
> > NULL because usb_gadget_remove_driver calls synchronize_irq before
> > doing usb_gadget_udc_stop.
> >
> > Look at this timing diagram:
> >
> > CPU0 CPU1
> > ---- ----
> > IRQ happens for setup packet
> > Handler sees async_callbacks
> > is enabled
> > Handler unlocks dwc->lock
> > usb_gadget_remove_driver runs
> > Disables async callbacks
> > Calls synchronize_irq
> > Handler calls dwc-> . waits for IRQ handler to
> > gadget_driver->setup . return
> > Handler locks dwc-lock .
> > ... .
> > Handler returns .
> > . synchronize_irq returns
> > Calls usb_gadget_udc_stop
> > dwc->gadget_driver is
> > set to NULL
> >
> > As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
> > even though async_callbacks gets cleared during the time when the
> > lock is released.
> thanks for your patient explanation,
> but from this part, seem it is synchronize_irq() help to avoid NULL pointer
> crash.

That's right.

> can you also explain how async_callbacks flag help here ?

It doesn't help in the situation shown above, but it does help in other
situations. Consider this timing diagram:

CPU0 CPU1
---- ----
usb_gadget_remove_driver runs
Disables async callbacks
Calls synchronize_irq
synchronize_irq returns
Calls udc_driver_unbind
IRQ happens for disconnect
Handler sees async_callbacks
is disabled
Handler returns
Calls usb_gadget_udc_stop
dwc->gadget_driver is
set to NULL

With the async_callbacks check, everything works okay. But now look at
what would happen without the async_callbacks mechanism:

CPU0 CPU1
---- ----
usb_gadget_remove_driver runs
Calls synchronize_irq
synchronize_irq returns
Calls udc_driver_unbind
IRQ happens for disconnect
Handler unlocks dwc->lock
Calls dwc->gadget_driver->disconnect
Gadget driver has already been unbound
and is not prepared to handle a
callback, so it crashes
Calls usb_gadget_udc_stop
dwc->gadget_driver is
set to NULL

Without the async_callbacks mechanism, the gadget driver can get a
callback at the wrong time (after it has been unbound), which might
cause it to crash.

Alan Stern

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