[PATCH] usbip: vudc: Prevent transfer timer rearm during teardown

Commit d96209626a29 ("usbip: vudc: Fix use after free bug in
vudc_remove due to race condition") made v_stop_timer() delete the
transfer timer synchronously before vudc_remove() frees its containing
struct vudc.

That does not close the race with the receive thread, which is stopped
later through usb_del_gadget_udc() and vudc_shutdown().  The thread
calls v_kick_timer() for CMD_SUBMIT and CMD_UNLINK packets, and
v_kick_timer() deliberately calls mod_timer() when the transfer state is
VUDC_TR_STOPPED.  A packet received after v_stop_timer() returns can
therefore enqueue the timer again before the vudc is freed.

timer_delete_sync() only drains the current timer instance and cannot
prevent such a rearm.  Use timer_shutdown_sync(), which makes subsequent
attempts to arm the timer no-ops.  The transfer state no longer needs to
be updated, as it is freed along with the timer.

On a KASAN and DEBUG_OBJECTS enabled x86_64 QEMU guest, repeatedly
binding the device, submitting a request and unbinding it produced a
"free active timer_list" warning followed by a slab-use-after-free in
v_timer().  With this change the same harness completed 4000 iterations
cleanly.

This issue was identified during our ongoing static-analysis research
while reviewing kernel code.

Fixes: d96209626a29 ("usbip: vudc: Fix use after free bug in vudc_remove due to race condition")
Link: https://lore.kernel.org/all/20230316180940.1601515-1-zyytlz.wz@163.com/ (local)
Cc: stable@vger.kernel.org
Assisted-by: LLM
Co-developed-by: Ijae Kim <redacted>
Signed-off-by: Ijae Kim <redacted>
Signed-off-by: Myeonghun Pak <redacted>
---
 drivers/usb/usbip/vudc_transfer.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/usbip/vudc_transfer.c b/drivers/usb/usbip/vudc_transfer.c
index d4ce85c4c6a2..4146b746a320 100644
--- a/drivers/usb/usbip/vudc_transfer.c
+++ b/drivers/usb/usbip/vudc_transfer.c

@@ -441,7 +441,7 @@ static void v_timer(struct timer_list *t)
 	spin_unlock_irqrestore(&udc->lock, flags);
 }
 
-/* All timer functions are run with udc->lock held */
+/* v_start_timer() and v_kick_timer() are called with udc->lock held. */
 
 void v_init_timer(struct vudc *udc)
 {

@@ -490,8 +490,6 @@ void v_stop_timer(struct vudc *udc)
 {
 	struct transfer_timer *t = &udc->tr_timer;
 
-	/* Delete the timer synchronously before teardown frees udc. */
 	dev_dbg(&udc->pdev->dev, "timer stop");
-	timer_delete_sync(&t->timer);
-	t->state = VUDC_TR_STOPPED;
+	timer_shutdown_sync(&t->timer);
 }

base-commit: 238650ef6c7c7cca08e032527329424c9fbd70e5
-- 
2.53.0