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

Greg Kroah-Hartman
2021-02-15T16:22:02Z

From: Andrey Konovalov <andreyknvl@google.com>

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@google.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://lore.kernel.org/r/f3a7a153f0719cb53ec385b16e912798bd3e4cf9.1602856358.git.andreyknvl@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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);