USB: core: Fix races in character device registration and deregistraion

Commit Message

Alan Stern Aug. 12, 2019, 8:11 p.m. UTC

The syzbot fuzzer has found two (!) races in the USB character device
registration and deregistration routines.  This patch fixes the races.

The first race results from the fact that usb_deregister_dev() sets
usb_minors[intf->minor] to NULL before calling device_destroy() on the
class device.  This leaves a window during which another thread can
allocate the same minor number but will encounter a duplicate name
error when it tries to register its own class device.  A typical error
message in the system log would look like:

    sysfs: cannot create duplicate filename '/class/usbmisc/ldusb0'

The patch fixes this race by destroying the class device first.

The second race is in usb_register_dev().  When that routine runs, it
first allocates a minor number, then drops minor_rwsem, and then
creates the class device.  If the device creation fails, the minor
number is deallocated and the whole routine returns an error.  But
during the time while minor_rwsem was dropped, there is a window in
which the minor number is allocated and so another thread can
successfully open the device file.  Typically this results in
use-after-free errors or invalid accesses when the other thread closes
its open file reference, because the kernel then tries to release
resources that were already deallocated when usb_register_dev()
failed.  The patch fixes this race by keeping minor_rwsem locked
throughout the entire routine.

Reported-and-tested-by: [e-mail]
Signed-off-by: Alan Stern <[e-mail]>
CC: <[e-mail]>

---

[as1907]


 drivers/usb/core/file.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

Comments

Greg KH Aug. 12, 2019, 8:52 p.m. UTC | #1

On Mon, Aug 12, 2019 at 04:11:07PM -0400, Alan Stern wrote:
> The syzbot fuzzer has found two (!) races in the USB character device
> registration and deregistration routines.  This patch fixes the races.
> 
> The first race results from the fact that usb_deregister_dev() sets
> usb_minors[intf->minor] to NULL before calling device_destroy() on the
> class device.  This leaves a window during which another thread can
> allocate the same minor number but will encounter a duplicate name
> error when it tries to register its own class device.  A typical error
> message in the system log would look like:
> 
>     sysfs: cannot create duplicate filename '/class/usbmisc/ldusb0'
> 
> The patch fixes this race by destroying the class device first.
> 
> The second race is in usb_register_dev().  When that routine runs, it
> first allocates a minor number, then drops minor_rwsem, and then
> creates the class device.  If the device creation fails, the minor
> number is deallocated and the whole routine returns an error.  But
> during the time while minor_rwsem was dropped, there is a window in
> which the minor number is allocated and so another thread can
> successfully open the device file.  Typically this results in
> use-after-free errors or invalid accesses when the other thread closes
> its open file reference, because the kernel then tries to release
> resources that were already deallocated when usb_register_dev()
> failed.  The patch fixes this race by keeping minor_rwsem locked
> throughout the entire routine.
> 
> Reported-and-tested-by: [e-mail]
> Signed-off-by: Alan Stern <[e-mail]>
> CC: <[e-mail]>
> 
> ---
> 
> [as1907]

Thanks for this, now queued up.

greg k-h

Patch

Index: usb-devel/drivers/usb/core/file.c
===================================================================
--- usb-devel.orig/drivers/usb/core/file.c
+++ usb-devel/drivers/usb/core/file.c
@@ -193,9 +193,10 @@  int usb_register_dev(struct usb_interfac
 		intf->minor = minor;
 		break;
 	}
-	up_write(&minor_rwsem);
-	if (intf->minor < 0)
+	if (intf->minor < 0) {
+		up_write(&minor_rwsem);
 		return -EXFULL;
+	}
 
 	/* create a usb class device for this usb interface */
 	snprintf(name, sizeof(name), class_driver->name, minor - minor_base);
@@ -203,12 +204,11 @@  int usb_register_dev(struct usb_interfac
 				      MKDEV(USB_MAJOR, minor), class_driver,
 				      "%s", kbasename(name));
 	if (IS_ERR(intf->usb_dev)) {
-		down_write(&minor_rwsem);
 		usb_minors[minor] = NULL;
 		intf->minor = -1;
-		up_write(&minor_rwsem);
 		retval = PTR_ERR(intf->usb_dev);
 	}
+	up_write(&minor_rwsem);
 	return retval;
 }
 EXPORT_SYMBOL_GPL(usb_register_dev);
@@ -234,12 +234,12 @@  void usb_deregister_dev(struct usb_inter
 		return;
 
 	dev_dbg(&intf->dev, "removing %d minor\n", intf->minor);
+	device_destroy(usb_class->class, MKDEV(USB_MAJOR, intf->minor));
 
 	down_write(&minor_rwsem);
 	usb_minors[intf->minor] = NULL;
 	up_write(&minor_rwsem);
 
-	device_destroy(usb_class->class, MKDEV(USB_MAJOR, intf->minor));
 	intf->usb_dev = NULL;
 	intf->minor = -1;
 	destroy_usb_class();