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Date:   Tue, 24 Nov 2020 00:47:25 +0100
Message-Id: <d7035335fdfe7493067fbf7d677db57807a42d5d.1606175031.git.andreyknvl@google.com>
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Subject: [PATCH v5] kcov, usb: only collect coverage from __usb_hcd_giveback_urb
 in softirq
From:   Andrey Konovalov <andreyknvl@google.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        kasan-dev@googlegroups.com, Dmitry Vyukov <dvyukov@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Nazime Hande Harputluoglu <handeharput@gmail.com>,
        Andrey Konovalov <andreyknvl@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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. To avoid calling
in_serving_softirq() directly in the driver code, add a couple of new kcov
wrappers.

As the result of this change, 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.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Marco Elver <elver@google.com>
---

Changes in v5:
- Don't call in_serving_softirq() in USB driver code directly, do that
  via kcov wrappers.

---
 drivers/usb/core/hcd.c |  9 +++++++--
 include/linux/kcov.h   | 21 +++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 2c6b9578a7d3..7bafd01e05fb 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1646,9 +1646,14 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
 
 	/* 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.
+	 */
+	kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
 	urb->complete(urb);
-	kcov_remote_stop();
+	kcov_remote_stop_softirq();
 
 	usb_anchor_resume_wakeups(anchor);
 	atomic_dec(&urb->use_count);
diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index a10e84707d82..4e3037dc1204 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -52,6 +52,25 @@ static inline void kcov_remote_start_usb(u64 id)
 	kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id));
 }
 
+/*
+ * The softirq flavor of kcov_remote_*() functions is introduced as a temporary
+ * work around for kcov's lack of nested remote coverage sections support in
+ * task context. Adding suport for nested sections is tracked in:
+ * https://bugzilla.kernel.org/show_bug.cgi?id=210337
+ */
+
+static inline void kcov_remote_start_usb_softirq(u64 id)
+{
+	if (in_serving_softirq())
+		kcov_remote_start_usb(id);
+}
+
+static inline void kcov_remote_stop_softirq(void)
+{
+	if (in_serving_softirq())
+		kcov_remote_stop();
+}
+
 #else
 
 static inline void kcov_task_init(struct task_struct *t) {}
@@ -66,6 +85,8 @@ static inline u64 kcov_common_handle(void)
 }
 static inline void kcov_remote_start_common(u64 id) {}
 static inline void kcov_remote_start_usb(u64 id) {}
+static inline void kcov_remote_start_usb_softirq(u64 id) {}
+static inline void kcov_remote_stop_softirq(void) {}
 
 #endif /* CONFIG_KCOV */
 #endif /* _LINUX_KCOV_H */
-- 
2.29.2.454.gaff20da3a2-goog