patch "usb: gadget: Fix use-after-free bug by not setting udc->dev.driver" added to usb-linus

This is a note to let you know that I've just added the patch titled

    usb: gadget: Fix use-after-free bug by not setting udc->dev.driver

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 16b1941eac2bd499f065a6739a40ce0011a3d740 Mon Sep 17 00:00:00 2001
From: Alan Stern <[email protected]>
Date: Sat, 5 Mar 2022 21:47:22 -0500
Subject: usb: gadget: Fix use-after-free bug by not setting udc->dev.driver

The syzbot fuzzer found a use-after-free bug:

BUG: KASAN: use-after-free in dev_uevent+0x712/0x780 drivers/base/core.c:2320
Read of size 8 at addr ffff88802b934098 by task udevd/3689

CPU: 2 PID: 3689 Comm: udevd Not tainted 5.17.0-rc4-syzkaller-00229-g4f12b742eb2b #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x303 mm/kasan/report.c:255
 __kasan_report mm/kasan/report.c:442 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:459
 dev_uevent+0x712/0x780 drivers/base/core.c:2320
 uevent_show+0x1b8/0x380 drivers/base/core.c:2391
 dev_attr_show+0x4b/0x90 drivers/base/core.c:2094

Although the bug manifested in the driver core, the real cause was a race with the gadget core. dev_uevent() does:

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

and between the test and the dereference of dev->driver, the gadget core sets dev->driver to NULL.

The race wouldn't occur if the gadget core registered its devices on a real bus, using the standard synchronization techniques of the driver core. However, it's not necessary to make such a large change in order to fix this bug; all we need to do is make sure that udc->dev.driver is always NULL.

In fact, there is no reason for udc->dev.driver ever to be set to anything, let alone to the value it currently gets: the address of the gadget's driver. After all, a gadget driver only knows how to manage a gadget, not how to manage a UDC.

This patch simply removes the statements in the gadget core that touch udc->dev.driver.

---
 drivers/usb/gadget/udc/core.c | 3 ---
 1 file changed, 3 deletions(-)

index 568534a0d17c..c109b069f511 100644
@@ -1436,7 +1436,6 @@ static void usb_gadget_remove_driver(struct usb_udc *udc)
 	usb_gadget_udc_stop(udc);
 
 	udc->driver = NULL;
-	udc->dev.driver = NULL;
 	udc->gadget->dev.driver = NULL;
 }
 

@@ -1498,7 +1497,6 @@ static int udc_bind_to_driver(struct usb_udc *udc, struct usb_gadget_driver *dri
 			driver->function);
 
 	udc->driver = driver;
-	udc->dev.driver = &driver->driver;
 	udc->gadget->dev.driver = &driver->driver;
 
 	usb_gadget_udc_set_speed(udc, driver->max_speed);

@@ -1521,7 +1519,6 @@ 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;
-	udc->dev.driver = NULL;
 	udc->gadget->dev.driver = NULL;
 	return ret;
 }

2.35.1