Re: [PATCH] media: imon: remove redundant serialization

Sean Young
2022-04-26T07:57:38Z

On Tue, Apr 26, 2022 at 09:54:18AM +0900, Tetsuo Handa wrote:
> 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.
>
> Sean Young explained that the problem is there are imon devices which have
> two usb interfaces, even though it is one device. The probe and disconnect
> function of both usb interfaces can run concurrently.
>
> Alan Stern responded that the driver and USB cores guarantee that when an
> interface is probed, both the interface and its USB device are locked.
> Ditto for when the disconnect callback gets run. So concurrent probing/
> disconnection of multiple interfaces on the same device is not possible.
>
> Therefore, simply remove redundant serialization.
>
> Link: https://syzkaller.appspot.com/bug?extid=c558267ad910fc494497
> Reported-by: syzbot <syzbot+c558267ad910fc494497@syzkaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Sean Young <sean@mess.org>
> Cc: Alan Stern <stern@rowland.harvard.edu>

Thanks, looks good!

> ---
> This patch might be OK for solving circular locking dependency problem.
> But https://syzkaller.appspot.com/text?tag=CrashReport&x=1632f89f700000 says
> free_imon_context() from imon_disconnect() can call kfree(ictx) before
> display_close() is called, resulting in use-after-free problem. I guess that
> we need to defer free_imon_context() using refcount till display_close() is
> called. Fix as a separate patch or include into this patch?

That is a separate issue, so that needs a separate patch.

Sean

>
> 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
>