[PATCH v3] usbip: vudc: Fix use after free bug in vudc_remove due to race condition

[Zheng Wang]

In vudc_probe, it calls init_vudc_hw, which bound &udc->timer with v_timer.

When it calls usbip_sockfd_store, it will call v_start_timer to start the
timer work.

When we call vudc_remove to remove the driver, theremay be a sequence as
follows:

Fix it by shutdown the timer work before cleanup in vudc_remove.

Note that removing a driver is a root-only operation, and should never
happen. But the attacker can directly unplug the usb to trigger the remove
function.

CPU0                  CPU1

                     |v_timer
vudc_remove          |
kfree(udc);          |
//free shost         |
                     |udc->gadget
                     |//use

The udc might be removed before v_timer finished, and UAF happens.

This bug was found by Codeql static analysis and might by false positive.

Fixes: b6a0ca111867 ("usbip: vudc: Add UDC specific ops")
Signed-off-by: Zheng Wang <zyytlz.wz@xxxxxxx>
---
v3:
- fix the issue by adding del_timer_sync in v_stop_timer and
invoke it in vudc_remove

v2:
- add more details about how the bug was found suggested by Shuah
---
 drivers/usb/usbip/vudc_dev.c      | 1 +
 drivers/usb/usbip/vudc_transfer.c | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/drivers/usb/usbip/vudc_dev.c b/drivers/usb/usbip/vudc_dev.c
index 2bc428f2e261..dcbfed30806d 100644
--- a/drivers/usb/usbip/vudc_dev.c
+++ b/drivers/usb/usbip/vudc_dev.c
@@ -633,6 +633,7 @@ int vudc_remove(struct platform_device *pdev)
 {
 	struct vudc *udc = platform_get_drvdata(pdev);
 
+	v_stop_timer(udc);
 	usb_del_gadget_udc(&udc->gadget);
 	cleanup_vudc_hw(udc);
 	kfree(udc);
diff --git a/drivers/usb/usbip/vudc_transfer.c b/drivers/usb/usbip/vudc_transfer.c
index 7e801fee33bf..562ea7b6ea2e 100644
--- a/drivers/usb/usbip/vudc_transfer.c
+++ b/drivers/usb/usbip/vudc_transfer.c
@@ -492,5 +492,7 @@ void v_stop_timer(struct vudc *udc)
 
 	/* timer itself will take care of stopping */
 	dev_dbg(&udc->pdev->dev, "timer stop");
+	
+	del_timer_sync(&t->timer);
 	t->state = VUDC_TR_STOPPED;
 }
-- 
2.25.1