From 1426bd2c9f7e3126e2678e7469dca9fd9fc6dd3e Mon Sep 17 00:00:00 2001
From: Oliver Neukum <oneukum@suse.com>
Date: Tue, 27 Aug 2019 12:34:36 +0200
Subject: [PATCH] USB: cdc-wdm: fix race between write and disconnect due to flag abuse
Git-commit: 1426bd2c9f7e3126e2678e7469dca9fd9fc6dd3e
Patch-mainline: v5.3-rc7
References: bsc#1051510

In case of a disconnect an ongoing flush() has to be made fail.
Nevertheless we cannot be sure that any pending URB has already
finished, so although they will never succeed, they still must
not be touched.
The clean solution for this is to check for WDM_IN_USE
and WDM_DISCONNECTED in flush(). There is no point in ever
clearing WDM_IN_USE, as no further writes make sense.

The issue is as old as the driver.

Fixes: afba937e540c9 ("USB: CDC WDM driver")
Reported-by: syzbot+d232cca6ec42c2edb3fc@syzkaller.appspotmail.com
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20190827103436.21143-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/usb/class/cdc-wdm.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index a7824a51f86d..70afb2ca1eab 100644
--- a/drivers/usb/class/cdc-wdm.c
+++ b/drivers/usb/class/cdc-wdm.c
@@ -587,10 +587,20 @@ static int wdm_flush(struct file *file, fl_owner_t id)
 {
 	struct wdm_device *desc = file->private_data;
 
-	wait_event(desc->wait, !test_bit(WDM_IN_USE, &desc->flags));
+	wait_event(desc->wait,
+			/*
+			 * needs both flags. We cannot do with one
+			 * because resetting it would cause a race
+			 * with write() yet we need to signal
+			 * a disconnect
+			 */
+			!test_bit(WDM_IN_USE, &desc->flags) ||
+			test_bit(WDM_DISCONNECTING, &desc->flags));
 
 	/* cannot dereference desc->intf if WDM_DISCONNECTING */
-	if (desc->werr < 0 && !test_bit(WDM_DISCONNECTING, &desc->flags))
+	if (test_bit(WDM_DISCONNECTING, &desc->flags))
+		return -ENODEV;
+	if (desc->werr < 0)
 		dev_err(&desc->intf->dev, "Error in flush path: %d\n",
 			desc->werr);
 
@@ -974,8 +984,6 @@ static void wdm_disconnect(struct usb_interface *intf)
 	spin_lock_irqsave(&desc->iuspin, flags);
 	set_bit(WDM_DISCONNECTING, &desc->flags);
 	set_bit(WDM_READ, &desc->flags);
-	/* to terminate pending flushes */
-	clear_bit(WDM_IN_USE, &desc->flags);
 	spin_unlock_irqrestore(&desc->iuspin, flags);
 	wake_up_all(&desc->wait);
 	mutex_lock(&desc->rlock);
-- 
2.16.4