[PATCH v2] usb: core: hcd: flush giveback work on stop

[Igor Putko <igorpetindev@gmail.com>]

When an HCD is stopped/removed (either during module unload, driver
unbind, or an error path in probe), pending URBs are unlinked and
completed. However, for root hubs or HCDs with giveback done in BH
context, the work function usb_giveback_urb_bh() may still be executing
on another CPU after __usb_hcd_giveback_urb() drops the URB use_count
and wakes up waiters in usb_kill_urb().

Since the caller may immediately free the struct usb_hcd via usb_put_hcd()
after usb_stop_hcd() or usb_remove_hcd(), the running worker accesses
the embedded giveback_urb_bh structure in its epilogue (e.g. clearing
completing_ep or clearing the running flag) after the memory has been
freed, triggering a slab-use-after-free write detected by KASAN.

Ensure all giveback work has fully completed in usb_stop_hcd() by calling
flush_work() on both high and low priority BHs before deallocating.

Fixes: 8fea0c8fda30 ("usb: core: hcd: Convert from tasklet to BH workqueue")
Cc: stable@vger.kernel.org
Reported-by: syzbot+a5b220cacb0618384d5a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspotmail.com/bug?extid=a5b220cacb0618384d5a
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Igor Putko <igorpetindev@gmail.com>
---
v1 -> v2:
- Use flush_work() instead of cancel_work_sync() to ensure no pending
  work items are dropped, as suggested by Alan Stern.
---
 drivers/usb/core/hcd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 8e8a5f59b319..e6854d67ed8c 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -2787,6 +2787,9 @@ static void usb_stop_hcd(struct usb_hcd *hcd)
 	/* In case the HCD restarted the timer, stop it again. */
 	clear_bit(HCD_FLAG_POLL_RH, &hcd->flags);
 	timer_delete_sync(&hcd->rh_timer);
+
+	flush_work(&hcd->high_prio_bh.bh);
+	flush_work(&hcd->low_prio_bh.bh);
 }
 
 /**
-- 
2.47.3