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+eb4674092e6cc8d9e...@syzkaller.appspotmail.com> 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+eb4674092e6cc8d9e...@syzkaller.appspotmail.com
> >
> > 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);
