Re: possible deadlock in display_open

Tetsuo Handa
2022-04-25T05:29:46Z

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