Re: [syzbot] general protection fault in gadget_setup

[Alan Stern]

On Fri, Apr 16, 2021 at 11:10:35AM +0530, Anirudh Rayabharam wrote:
> On Tue, Apr 13, 2021 at 12:13:11PM -0400, Alan Stern wrote:
> > Maybe we can test this reasoning by putting a delay just before the call 
> > to dum->driver->setup.  That runs in the timer handler, so it's not a 
> > good place to delay, but it may be okay just for testing purposes.
> > 
> > Hopefully this patch will make the race a lot more likely to occur.  Is 
> 
> Hi Alan, 
> 
> Indeed, I was able to reproduce this bug easily on my machine with your
> delay patch applied and using this syzkaller program:
> 
> syz_usb_connect$cdc_ncm(0x1, 0x6e, &(0x7f0000000040)={{0x12, 0x1, 0x0, 0x2, 0x0, 0x0, 0x8, 0x525, 0xa4a1, 0x40, 0x1, 0x2, 0x3, 0x1, [{{0x9, 0x2, 0x5c, 0x2, 0x1, 0x0, 0x0, 0x0, {{0x9, 0x4, 0x0, 0x0, 0x1, 0x2, 0xd, 0x0, 0x0, {{0x5}, {0x5}, {0xd}, {0x6}}, {{0x9, 0x5, 0x81, 0x3, 0x200}}}}}}]}}, &(0x7f0000000480)={0x0, 0x0, 0x0, 0x0, 0x3, [{0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}]})
> 
> I also tested doing the synchronize_irq emulation in dummy_pullup and it
> fixed the issue. The patch is below.

That's great!  Thanks for testing.

> Thanks!
> 
> 	- Anirudh.
> 
> diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
> index ce24d4f28f2a..931d4612d859 100644
> --- a/drivers/usb/gadget/udc/dummy_hcd.c
> +++ b/drivers/usb/gadget/udc/dummy_hcd.c
> @@ -903,6 +903,12 @@ static int dummy_pullup(struct usb_gadget *_gadget, int value)
>  	spin_lock_irqsave(&dum->lock, flags);
>  	dum->pullup = (value != 0);
>  	set_link_state(dum_hcd);
> +	/* emulate synchronize_irq(): wait for callbacks to finish */
> +	while (dum->callback_usage > 0) {
> +		spin_unlock_irqrestore(&dum->lock, flags);
> +		usleep_range(1000, 2000);
> +		spin_lock_irqsave(&dum->lock, flags);
> +	}

We should do this only if value == 0.  No synchronization is needed when 
the pullup is turned on.

Also, there should be a comment explaining that this is necessary 
because there's no other place to emulate the call made to 
synchronize_irq() in core.c:usb_gadget_remove_driver().

>  	spin_unlock_irqrestore(&dum->lock, flags);
>  
>  	usb_hcd_poll_rh_status(dummy_hcd_to_hcd(dum_hcd));
> @@ -1005,13 +1011,6 @@ static int dummy_udc_stop(struct usb_gadget *g)
>  	dum->ints_enabled = 0;
>  	stop_activity(dum);
>  
> -	/* emulate synchronize_irq(): wait for callbacks to finish */
> -	while (dum->callback_usage > 0) {
> -		spin_unlock_irq(&dum->lock);
> -		usleep_range(1000, 2000);
> -		spin_lock_irq(&dum->lock);
> -	}
> -
>  	dum->driver = NULL;
>  	spin_unlock_irq(&dum->lock);

Actually, I wanted to move this emulation code into a new subroutine and 
then call that subroutine from _both_ places.  Would you like to write 
and submit a patch that does this?

Alan Stern