RE: [syzbot] KASAN: use-after-free Read in dev_uevent

Zhang, Qiang1
2022-02-24T03:15:05Z

On Wed, Feb 23, 2022 at 05:00:12PM +0100, gregkh@linuxfoundation.org wrote:
> On Wed, Feb 23, 2022 at 09:38:20AM -0500, stern@rowland.harvard.edu wrote:
> > Which bus locks are you referring to? I'm not aware of any locks
> > that synchronize dev_uevent() with anything (in particular, with
> > driver unbinding).
>
> The locks in the driver core that handle the binding and unbinding of
> drivers to devices.
>
> > And as far as I know, usb_gadget_remove_driver() doesn't play any
> > odd tricks with pointers.
>
> Ah, I never noticed that this is doing a "fake" bus and does the
> bind/unbind itself outside of the driver core. It should just be a
> normal bus type and have the core do the work for it, but oh well.
>
> And there is a lock that should serialize all of this already, so it's
> odd that this is able to be triggered at all.

>>I guess at a minimum the UDC core should hold the device lock when it registers, unregisters, binds, or unbinds UDC and gadget devices.
>>Would that be enough to fix the problem? I really don't understand how sysfs file access gets synchronized with device removal.

>>>
>>>Agree with you, in usb_gadget_remove_driver() function, the udc->dev.driver and udc->gadget->dev.driver be set to null without any protection, so when the udevd accessed the dev->driver, this address may be invalid at this time.
>>>maybe the operation of dev->driver can be protected by device_lock().
>>>

Is it enough that we just need to protect "dev.driver" ?

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 3d6430eb0c6a..9bd2624973d7 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -2316,8 +2316,10 @@ static int dev_uevent(struct kobject *kobj, struct kobj_uevent_env *env)
if (dev->type && dev->type->name)
add_uevent_var(env, "DEVTYPE=%s", dev->type->name);

+ device_lock(dev);
if (dev->driver)
add_uevent_var(env, "DRIVER=%s", dev->driver->name);
+ device_unlock(dev);

/* Add common DT information about the device */
of_device_uevent(dev, env);
diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
index 568534a0d17c..7877142397d3 100644
--- a/drivers/usb/gadget/udc/core.c
+++ b/drivers/usb/gadget/udc/core.c
@@ -1436,8 +1436,14 @@ static void usb_gadget_remove_driver(struct usb_udc *udc)
usb_gadget_udc_stop(udc);

udc->driver = NULL;
+
+ device_lock(&udc->dev);
udc->dev.driver = NULL;
+ device_unlock(&udc->dev);
+
+ device_lock(&udc->gadget->dev);
udc->gadget->dev.driver = NULL;
+ device_unlock(&udc->gadget->dev);
}

/**
@@ -1498,8 +1504,14 @@ static int udc_bind_to_driver(struct usb_udc *udc, struct usb_gadget_driver *dri
driver->function);

udc->driver = driver;
+
+ device_lock(&udc->dev);
udc->dev.driver = &driver->driver;
+ device_unlock(&udc->dev);
+
+ device_lock(&udc->gadget->dev);
udc->gadget->dev.driver = &driver->driver;
+ device_unlock(&udc->gadget->dev);

usb_gadget_udc_set_speed(udc, driver->max_speed);

@@ -1521,8 +1533,14 @@ static int udc_bind_to_driver(struct usb_udc *udc, struct usb_gadget_driver *dri
dev_err(&udc->dev, "failed to start %s: %d\n",
udc->driver->function, ret);
udc->driver = NULL;
+
+ device_lock(&udc->dev);
udc->dev.driver = NULL;
+ device_unlock(&udc->dev);
+
+ device_lock(&udc->gadget->dev);
udc->gadget->dev.driver = NULL;
+ device_unlock(&udc->gadget->dev);
return ret;
}

Thanks,
Zqiang

>>>Thanks,
>>>Zqiang

> Unless the device is being removed at the same time it was manually
> unbound from the driver? If so, then this really should be fixed up
> to use the driver core logic instead...
>>
>>Device removal does of course trigger unbinding, but they always take place in the same thread so it isn't an issue.
>>
>>Probably part of the reason people don't want to use the driver core here is so that they can specify which UDC a gadget driver should bind to. The driver core would always bind each new gadget to the first registered gadget driver.
>>
>>When Dave Brownell originally wrote the gadget subsystem, I believe he didn't bother to integrate it with the driver core because it was a "bus" with only a single device and a single driver. The ability to have multiple UDCs in the system was added later.
>>
>>Alan Stern