[PATCH v4 04/12] usb: usbip: stub: serialize usbip_sockfd_store() against event_handler()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

usbip_sockfd_store() must perform

      if (sdev->ud.status != SDEV_ST_AVAILABLE) {
        /* misc assignments for attach operation */
        sdev->ud.status = SDEV_ST_USED;
      }

under a lock, or multiple ud->tcp_{tx,rx} are created (which will later
cause a crash like [1]) and refcount on ud->tcp_socket is leaked when
usbip_sockfd_store() is concurrently called.

Since I don't want to replace "return" with "goto" in order to call
usbip_event_unlock(), wrap whole function in order to keep changes
minimal.

[1] https://syzkaller.appspot.com/bug?extid=95ce4b142579611ef0a9

Reported-by: syzbot <syzbot+95ce4b142579611ef0a9@syzkaller.appspotmail.com>
Tested-by: syzbot <syzbot+a93fba6d384346a761e3@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/usb/usbip/stub_dev.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/usbip/stub_dev.c b/drivers/usb/usbip/stub_dev.c
index 2305d425e6c9..cea5663d1994 100644
--- a/drivers/usb/usbip/stub_dev.c
+++ b/drivers/usb/usbip/stub_dev.c
@@ -39,8 +39,8 @@ static DEVICE_ATTR_RO(usbip_status);
  * is used to transfer usbip requests by kernel threads. -1 is a magic number
  * by which usbip connection is finished.
  */
-static ssize_t usbip_sockfd_store(struct device *dev, struct device_attribute *attr,
-			    const char *buf, size_t count)
+static ssize_t __usbip_sockfd_store(struct device *dev, struct device_attribute *attr,
+				    const char *buf, size_t count)
 {
 	struct stub_device *sdev = dev_get_drvdata(dev);
 	int sockfd = 0;
@@ -104,6 +104,17 @@ static ssize_t usbip_sockfd_store(struct device *dev, struct device_attribute *a
 	spin_unlock_irq(&sdev->ud.lock);
 	return -EINVAL;
 }
+static ssize_t usbip_sockfd_store(struct device *dev, struct device_attribute *attr,
+				  const char *buf, size_t count)
+{
+	ssize_t ret = usbip_event_lock_killable();
+
+	if (ret)
+		return ret;
+	ret = __usbip_sockfd_store(dev, attr, buf, count);
+	usbip_event_unlock();
+	return ret;
+}
 static DEVICE_ATTR_WO(usbip_sockfd);
 
 static struct attribute *usbip_attrs[] = {
-- 
2.18.4