[PATCH 5.10 104/104] kcov, usb: only collect coverage from __usb_hcd_giveback_urb in softirq

[Greg Kroah-Hartman]

From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

commit aee9ddb1d3718d3ba05b50c51622d7792ae749c9 upstream.

Currently there's a KCOV remote coverage collection section in
__usb_hcd_giveback_urb(). Initially that section was added based on the
assumption that usb_hcd_giveback_urb() can only be called in interrupt
context as indicated by a comment before it. This is what happens when
syzkaller is fuzzing the USB stack via the dummy_hcd driver.

As it turns out, it's actually valid to call usb_hcd_giveback_urb() in task
context, provided that the caller turned off the interrupts; USB/IP does
exactly that. This can lead to a nested KCOV remote coverage collection
sections both trying to collect coverage in task context. This isn't
supported by KCOV, and leads to a WARNING.

Change __usb_hcd_giveback_urb() to only call kcov_remote_*() callbacks
when it's being executed in a softirq. As the result, the coverage from
USB/IP related usb_hcd_giveback_urb() calls won't be collected, but the
WARNING is fixed.

A potential future improvement would be to support nested remote coverage
collection sections, but this patch doesn't address that.

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Link: https://lore.kernel.org/r/f3a7a153f0719cb53ec385b16e912798bd3e4cf9.1602856358.git.andreyknvl@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/usb/core/hcd.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1646,9 +1646,16 @@ static void __usb_hcd_giveback_urb(struc
 
 	/* pass ownership to the completion handler */
 	urb->status = status;
-	kcov_remote_start_usb((u64)urb->dev->bus->busnum);
+	/*
+	 * This function can be called in task context inside another remote
+	 * coverage collection section, but KCOV doesn't support that kind of
+	 * recursion yet. Only collect coverage in softirq context for now.
+	 */
+	if (in_serving_softirq())
+		kcov_remote_start_usb((u64)urb->dev->bus->busnum);
 	urb->complete(urb);
-	kcov_remote_stop();
+	if (in_serving_softirq())
+		kcov_remote_stop();
 
 	usb_anchor_resume_wakeups(anchor);
 	atomic_dec(&urb->use_count);