Re: [syzbot] general protection fault in gadget_setup

[Anirudh Rayabharam]

On Tue, Apr 13, 2021 at 12:13:11PM -0400, Alan Stern wrote:
> On Tue, Apr 13, 2021 at 10:12:05AM +0200, Dmitry Vyukov wrote:
> > On Tue, Apr 13, 2021 at 10:08 AM syzbot
> > <syzbot+eb4674092e6cc8d9e0bd@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    0f4498ce Merge tag 'for-5.12/dm-fixes-2' of git://git.kern..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=124adbf6d00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=daeff30c2474a60f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=eb4674092e6cc8d9e0bd
> > > userspace arch: i386
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+eb4674092e6cc8d9e0bd@xxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > I suspect that the raw gadget_unbind() can be called while the timer
> > is still active. gadget_unbind() sets gadget data to NULL.
> > But I am not sure which unbind call this is:
> > usb_gadget_remove_driver() or right in udc_bind_to_driver() due to a
> > start error.
> 
> This certainly looks like a race between gadget_unbind and gadget_setup 
> in raw_gadget.
> 
> In theory, this race shouldn't matter.  The gadget core is supposed to 
> guarantee that there won't be any more callbacks to the gadget driver 
> once the driver's unbind routine is called.  That guarantee is enforced 
> in usb_gadget_remove_driver as follows:
> 
> 	usb_gadget_disconnect(udc->gadget);
> 	if (udc->gadget->irq)
> 		synchronize_irq(udc->gadget->irq);
> 	udc->driver->unbind(udc->gadget);
> 	usb_gadget_udc_stop(udc);
> 
> usb_gadget_disconnect turns off the pullup resistor, telling the host 
> that the gadget is no longer connected and preventing the transmission 
> of any more USB packets.  Any packets that have already been received 
> are sure to processed by the UDC driver's interrupt handler by the time 
> synchronize_irq returns.
> 
> But this doesn't work with dummy_hcd, because dummy_hcd doesn't use 
> interrupts; it uses a timer instead.  It does have code to emulate the 
> effect of synchronize_irq, but that code doesn't get invoked at the 
> right time -- it currently runs in usb_gadget_udc_stop, after the unbind 
> callback instead of before.  Indeed, there's no way for 
> usb_gadget_remove_driver to invoke this code before the unbind 
> callback,.
> 
> I thought the synchronize_irq emulation problem had been completely 
> solved, but evidently it hasn't.  It looks like the best solution is to 
> add a call of the synchronize_irq emulation code in dummy_pullup.
> 
> 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.

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);
+	}
 	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);
 
@@ -1900,6 +1899,7 @@ static void dummy_timer(struct timer_list *t)
 			if (value > 0) {
 				++dum->callback_usage;
 				spin_unlock(&dum->lock);
+				mdelay(5);
 				value = dum->driver->setup(&dum->gadget,
 						&setup);
 				spin_lock(&dum->lock);