Re: possible deadlock in display_open

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

Since usb_register_dev() from imon_init_display() from imon_probe() holds
minor_rwsem while display_open() which holds driver_lock and ictx->lock is
called with minor_rwsem held from usb_open(), holding driver_lock or
ictx->lock when calling usb_register_dev() causes circular locking
dependency problem.

Since usb_deregister_dev() from imon_disconnect() holds minor_rwsem while
display_open() which holds driver_lock is called with minor_rwsem held,
holding driver_lock when calling usb_deregister_dev() also causes circular
locking dependency problem.

But actually do we need to hold these locks?
Could you explain possible race scenario if we do below?

 drivers/media/rc/imon.c | 21 ---------------------
 1 file changed, 21 deletions(-)

diff --git a/drivers/media/rc/imon.c b/drivers/media/rc/imon.c
index 54da6f60079b..e0e893d96cf3 100644
--- a/drivers/media/rc/imon.c
+++ b/drivers/media/rc/imon.c
@@ -439,9 +439,6 @@ static struct usb_driver imon_driver = {
 	.id_table	= imon_usb_id_table,
 };
 
-/* to prevent races between open() and disconnect(), probing, etc */
-static DEFINE_MUTEX(driver_lock);
-
 /* Module bookkeeping bits */
 MODULE_AUTHOR(MOD_AUTHOR);
 MODULE_DESCRIPTION(MOD_DESC);
@@ -499,9 +496,6 @@ static int display_open(struct inode *inode, struct file *file)
 	int subminor;
 	int retval = 0;
 
-	/* prevent races with disconnect */
-	mutex_lock(&driver_lock);
-
 	subminor = iminor(inode);
 	interface = usb_find_interface(&imon_driver, subminor);
 	if (!interface) {
@@ -534,7 +528,6 @@ static int display_open(struct inode *inode, struct file *file)
 	mutex_unlock(&ictx->lock);
 
 exit:
-	mutex_unlock(&driver_lock);
 	return retval;
 }
 
@@ -2416,9 +2409,6 @@ static int imon_probe(struct usb_interface *interface,
 	dev_dbg(dev, "%s: found iMON device (%04x:%04x, intf%d)\n",
 		__func__, vendor, product, ifnum);
 
-	/* prevent races probing devices w/multiple interfaces */
-	mutex_lock(&driver_lock);
-
 	first_if = usb_ifnum_to_if(usbdev, 0);
 	if (!first_if) {
 		ret = -ENODEV;
@@ -2456,8 +2446,6 @@ static int imon_probe(struct usb_interface *interface,
 	usb_set_intfdata(interface, ictx);
 
 	if (ifnum == 0) {
-		mutex_lock(&ictx->lock);
-
 		if (product == 0xffdc && ictx->rf_device) {
 			sysfs_err = sysfs_create_group(&interface->dev.kobj,
 						       &imon_rf_attr_group);
@@ -2468,21 +2456,17 @@ static int imon_probe(struct usb_interface *interface,
 
 		if (ictx->display_supported)
 			imon_init_display(ictx, interface);
-
-		mutex_unlock(&ictx->lock);
 	}
 
 	dev_info(dev, "iMON device (%04x:%04x, intf%d) on usb<%d:%d> initialized\n",
 		 vendor, product, ifnum,
 		 usbdev->bus->busnum, usbdev->devnum);
 
-	mutex_unlock(&driver_lock);
 	usb_put_dev(usbdev);
 
 	return 0;
 
 fail:
-	mutex_unlock(&driver_lock);
 	usb_put_dev(usbdev);
 	dev_err(dev, "unable to register, err %d\n", ret);
 
@@ -2498,9 +2482,6 @@ static void imon_disconnect(struct usb_interface *interface)
 	struct device *dev;
 	int ifnum;
 
-	/* prevent races with multi-interface device probing and display_open */
-	mutex_lock(&driver_lock);
-
 	ictx = usb_get_intfdata(interface);
 	dev = ictx->dev;
 	ifnum = interface->cur_altsetting->desc.bInterfaceNumber;
@@ -2545,8 +2526,6 @@ static void imon_disconnect(struct usb_interface *interface)
 	if (!ictx->dev_present_intf0 && !ictx->dev_present_intf1)
 		free_imon_context(ictx);
 
-	mutex_unlock(&driver_lock);
-
 	dev_dbg(dev, "%s: iMON device (intf%d) disconnected\n",
 		__func__, ifnum);
 }
-- 
2.34.1

On 2019/08/09 22:18, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    e96407b4 usb-fuzzer: main usb gadget fuzzer driver
> git tree:       https://github.com/google/kasan.git usb-fuzzer
> console output: https://syzkaller.appspot.com/x/log.txt?x=13b29b26600000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cfa2c18fb6a8068e
> dashboard link: https://syzkaller.appspot.com/bug?extid=c558267ad910fc494497
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15427002600000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=111cb61c600000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+c558267ad910fc494497@syzkaller.appspotmail.com