[PATCH] usb: gadget: dummy_hcd: execute hrtimer callback in softirq context

[andrey . konovalov]

From: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Commit a7f3813e589f ("usb: gadget: dummy_hcd: Switch to hrtimer transfer
scheduler") switched dummy_hcd to use hrtimer and made the timer's
callback be executed in the hardirq context.

With that change, __usb_hcd_giveback_urb now gets executed in the hardirq
context, which causes problems for KCOV and KMSAN.

One problem is that KCOV now is unable to collect coverage from
the USB code that gets executed from the dummy_hcd's timer callback,
as KCOV cannot collect coverage in the hardirq context.

Another problem is that the dummy_hcd hrtimer might get triggered in the
middle of a softirq with KCOV remote coverage collection enabled, and that
causes a WARNING in KCOV, as reported by syzbot. (I sent a separate patch
to shut down this WARNING, but that doesn't fix the other two issues.)

Finally, KMSAN appears to ignore tracking memory copying operations
that happen in the hardirq context, which causes false positive
kernel-infoleaks, as reported by syzbot.

Change the hrtimer in dummy_hcd to execute the callback in the softirq
context.

Reported-by: syzbot+2388cdaeb6b10f0c13ac@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=2388cdaeb6b10f0c13ac
Reported-by: syzbot+17ca2339e34a1d863aad@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=17ca2339e34a1d863aad
Fixes: a7f3813e589f ("usb: gadget: dummy_hcd: Switch to hrtimer transfer scheduler")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

---

Marcello, would this change be acceptable for your use case?

If we wanted to keep the hardirq hrtimer, we would need teach KCOV to
collect coverage in the hardirq context (or disable it, which would be
unfortunate) and also fix whatever is wrong with KMSAN, but all that
requires some work.
---
 drivers/usb/gadget/udc/dummy_hcd.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
index f37b0d8386c1a..ff7bee78bcc49 100644
--- a/drivers/usb/gadget/udc/dummy_hcd.c
+++ b/drivers/usb/gadget/udc/dummy_hcd.c
@@ -1304,7 +1304,8 @@ static int dummy_urb_enqueue(
 
 	/* kick the scheduler, it'll do the rest */
 	if (!hrtimer_active(&dum_hcd->timer))
-		hrtimer_start(&dum_hcd->timer, ns_to_ktime(DUMMY_TIMER_INT_NSECS), HRTIMER_MODE_REL);
+		hrtimer_start(&dum_hcd->timer, ns_to_ktime(DUMMY_TIMER_INT_NSECS),
+				HRTIMER_MODE_REL_SOFT);
 
  done:
 	spin_unlock_irqrestore(&dum_hcd->dum->lock, flags);
@@ -1325,7 +1326,7 @@ static int dummy_urb_dequeue(struct usb_hcd *hcd, struct urb *urb, int status)
 	rc = usb_hcd_check_unlink_urb(hcd, urb, status);
 	if (!rc && dum_hcd->rh_state != DUMMY_RH_RUNNING &&
 			!list_empty(&dum_hcd->urbp_list))
-		hrtimer_start(&dum_hcd->timer, ns_to_ktime(0), HRTIMER_MODE_REL);
+		hrtimer_start(&dum_hcd->timer, ns_to_ktime(0), HRTIMER_MODE_REL_SOFT);
 
 	spin_unlock_irqrestore(&dum_hcd->dum->lock, flags);
 	return rc;
@@ -1995,7 +1996,8 @@ static enum hrtimer_restart dummy_timer(struct hrtimer *t)
 		dum_hcd->udev = NULL;
 	} else if (dum_hcd->rh_state == DUMMY_RH_RUNNING) {
 		/* want a 1 msec delay here */
-		hrtimer_start(&dum_hcd->timer, ns_to_ktime(DUMMY_TIMER_INT_NSECS), HRTIMER_MODE_REL);
+		hrtimer_start(&dum_hcd->timer, ns_to_ktime(DUMMY_TIMER_INT_NSECS),
+				HRTIMER_MODE_REL_SOFT);
 	}
 
 	spin_unlock_irqrestore(&dum->lock, flags);
@@ -2389,7 +2391,7 @@ static int dummy_bus_resume(struct usb_hcd *hcd)
 		dum_hcd->rh_state = DUMMY_RH_RUNNING;
 		set_link_state(dum_hcd);
 		if (!list_empty(&dum_hcd->urbp_list))
-			hrtimer_start(&dum_hcd->timer, ns_to_ktime(0), HRTIMER_MODE_REL);
+			hrtimer_start(&dum_hcd->timer, ns_to_ktime(0), HRTIMER_MODE_REL_SOFT);
 		hcd->state = HC_STATE_RUNNING;
 	}
 	spin_unlock_irq(&dum_hcd->dum->lock);
@@ -2467,7 +2469,7 @@ static DEVICE_ATTR_RO(urbs);
 
 static int dummy_start_ss(struct dummy_hcd *dum_hcd)
 {
-	hrtimer_init(&dum_hcd->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&dum_hcd->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
 	dum_hcd->timer.function = dummy_timer;
 	dum_hcd->rh_state = DUMMY_RH_RUNNING;
 	dum_hcd->stream_en_ep = 0;
@@ -2497,7 +2499,7 @@ static int dummy_start(struct usb_hcd *hcd)
 		return dummy_start_ss(dum_hcd);
 
 	spin_lock_init(&dum_hcd->dum->lock);
-	hrtimer_init(&dum_hcd->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&dum_hcd->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
 	dum_hcd->timer.function = dummy_timer;
 	dum_hcd->rh_state = DUMMY_RH_RUNNING;
 
-- 
2.25.1