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Date:   Thu, 8 Aug 2019 10:33:23 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Oliver Neukum <oliver@neukum.org>
cc:     andreyknvl@google.com, <syzkaller-bugs@googlegroups.com>,
        <gregkh@linuxfoundation.org>,
        syzbot <syzbot+7bbcbe9c9ff0cd49592a@syzkaller.appspotmail.com>,
        Kernel development list <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>
Subject: Re: possible deadlock in open_rio
In-Reply-To: <1565187142.15973.3.camel@neukum.org>
Message-ID: <Pine.LNX.4.44L0.1908081027560.1652-100000@iolanthe.rowland.org>
MIME-Version: 1.0
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Sender: linux-kernel-owner@vger.kernel.org
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On Wed, 7 Aug 2019, Oliver Neukum wrote:

> Am Mittwoch, den 07.08.2019, 10:07 -0400 schrieb Alan Stern:
> > On Wed, 7 Aug 2019, Oliver Neukum wrote:

> > > technically yes. However in practical terms the straight revert I sent
> > > out yesterday should fix it.
> > 
> > I didn't see the revert, and it doesn't appear to have reached the 
> > mailing list archive.  Can you post it again?
> 
> As soon as our VPN server is back up again.

The revert may not be necessay; a little fix should get rid of the
locking violation.  The key is to avoid calling the registration or
deregistration routines while holding the rio500_mutex, and to
recognize that the probe and disconnect routines are both protected by
the device mutex.

How does this patch look?

Alan Stern


#syz test: https://github.com/google/kasan.git 7f7867ff

Index: usb-devel/drivers/usb/misc/rio500.c
===================================================================
--- usb-devel.orig/drivers/usb/misc/rio500.c
+++ usb-devel/drivers/usb/misc/rio500.c
@@ -454,52 +454,54 @@ static int probe_rio(struct usb_interfac
 {
 	struct usb_device *dev = interface_to_usbdev(intf);
 	struct rio_usb_data *rio = &rio_instance;
-	int retval = 0;
+	int retval;
+	char *ibuf, *obuf;
 
-	mutex_lock(&rio500_mutex);
 	if (rio->present) {
 		dev_info(&intf->dev, "Second USB Rio at address %d refused\n", dev->devnum);
-		retval = -EBUSY;
-		goto bail_out;
-	} else {
-		dev_info(&intf->dev, "USB Rio found at address %d\n", dev->devnum);
+		return -EBUSY;
 	}
+	dev_info(&intf->dev, "USB Rio found at address %d\n", dev->devnum);
 
 	retval = usb_register_dev(intf, &usb_rio_class);
 	if (retval) {
 		dev_err(&dev->dev,
 			"Not able to get a minor for this device.\n");
-		retval = -ENOMEM;
-		goto bail_out;
+		goto err_register;
 	}
 
-	rio->rio_dev = dev;
-
-	if (!(rio->obuf = kmalloc(OBUF_SIZE, GFP_KERNEL))) {
+	obuf = kmalloc(OBUF_SIZE, GFP_KERNEL);
+	if (!obuf) {
 		dev_err(&dev->dev,
 			"probe_rio: Not enough memory for the output buffer\n");
-		usb_deregister_dev(intf, &usb_rio_class);
-		retval = -ENOMEM;
-		goto bail_out;
+		goto err_obuf;
 	}
-	dev_dbg(&intf->dev, "obuf address:%p\n", rio->obuf);
+	dev_dbg(&intf->dev, "obuf address: %p\n", obuf);
 
-	if (!(rio->ibuf = kmalloc(IBUF_SIZE, GFP_KERNEL))) {
+	ibuf = kmalloc(IBUF_SIZE, GFP_KERNEL);
+	if (!ibuf) {
 		dev_err(&dev->dev,
 			"probe_rio: Not enough memory for the input buffer\n");
-		usb_deregister_dev(intf, &usb_rio_class);
-		kfree(rio->obuf);
-		retval = -ENOMEM;
-		goto bail_out;
+		goto err_ibuf;
 	}
-	dev_dbg(&intf->dev, "ibuf address:%p\n", rio->ibuf);
+	dev_dbg(&intf->dev, "ibuf address: %p\n", ibuf);
 
+	mutex_lock(&rio500_mutex);
+	rio->rio_dev = dev;
+	rio->ibuf = ibuf;
+	rio->obuf = obuf;
 	usb_set_intfdata (intf, rio);
 	rio->present = 1;
-bail_out:
 	mutex_unlock(&rio500_mutex);
 
 	return retval;
+
+ err_ibuf:
+	kfree(obuf);
+ err_obuf:
+	usb_deregister_dev(intf, &usb_rio_class);
+ err_register:
+	return -ENOMEM;
 }
 
 static void disconnect_rio(struct usb_interface *intf)
@@ -507,10 +509,10 @@ static void disconnect_rio(struct usb_in
 	struct rio_usb_data *rio = usb_get_intfdata (intf);
 
 	usb_set_intfdata (intf, NULL);
-	mutex_lock(&rio500_mutex);
 	if (rio) {
 		usb_deregister_dev(intf, &usb_rio_class);
 
+		mutex_lock(&rio500_mutex);
 		if (rio->isopen) {
 			rio->isopen = 0;
 			/* better let it finish - the release will do whats needed */
@@ -524,8 +526,8 @@ static void disconnect_rio(struct usb_in
 		dev_info(&intf->dev, "USB Rio disconnected.\n");
 
 		rio->present = 0;
+		mutex_unlock(&rio500_mutex);
 	}
-	mutex_unlock(&rio500_mutex);
 }
 
 static const struct usb_device_id rio_table[] = {