[PATCH v6] usb: ohci: Proper handling of ed_rm_list to handle race condition between usb_kill_urb() and finish_unlinks()

[Jeffy Chen]

From: AMAN DEEP <aman.deep@xxxxxxxxxxx>

There is a race condition between finish_unlinks->finish_urb() function
and usb_kill_urb() in ohci controller case. The finish_urb calls
spin_unlock(&ohci->lock) before usb_hcd_giveback_urb() function call,
then if during this time, usb_kill_urb is called for another endpoint,
then new ed will be added to ed_rm_list at beginning for unlink, and
ed_rm_list will point to newly added.

When finish_urb() is completed in finish_unlinks() and ed->td_list
becomes empty as in below code (in finish_unlinks() function):

        if (list_empty(&ed->td_list)) {
                *last = ed->ed_next;
                ed->ed_next = NULL;
        } else if (ohci->rh_state == OHCI_RH_RUNNING) {
                *last = ed->ed_next;
                ed->ed_next = NULL;
                ed_schedule(ohci, ed);
        }

The *last = ed->ed_next will make ed_rm_list to point to ed->ed_next
and previously added ed by usb_kill_urb will be left unreferenced by
ed_rm_list. This causes usb_kill_urb() hang forever waiting for
finish_unlink to remove added ed from ed_rm_list.

The main reason for hang in this race condtion is addition and removal
of ed from ed_rm_list in the beginning during usb_kill_urb and later
last* is modified in finish_unlinks().

As suggested by Alan Stern, the solution for proper handling of
ohci->ed_rm_list is to remove ed from the ed_rm_list before finishing
any URBs. Then at the end, we can add ed back to the list if necessary.

This properly handle the updated ohci->ed_rm_list in usb_kill_urb().

Fixes:977dcfdc6031("USB:OHCI:don't lose track of EDs when a controller dies")
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
CC: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Aman Deep <aman.deep@xxxxxxxxxxx>
Signed-off-by: Jeffy Chen <jeffy.chen@xxxxxxxxxxxxxx>
---

Changes in v6:
This is a resend of Aman Deep's v5 patch [0], which solved the hang we
hit [1]. (Thanks Aman :)

The v5 has some format issues, so i slightly adjust the commit message.

[0] https://www.spinics.net/lists/linux-usb/msg129010.html
[1] https://bugs.chromium.org/p/chromium/issues/detail?id=803749

 drivers/usb/host/ohci-q.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/drivers/usb/host/ohci-q.c b/drivers/usb/host/ohci-q.c
index b2ec8c399363..4ccb85a67bb3 100644
--- a/drivers/usb/host/ohci-q.c
+++ b/drivers/usb/host/ohci-q.c
@@ -1019,6 +1019,8 @@ static void finish_unlinks(struct ohci_hcd *ohci)
 		 * have modified this list.  normally it's just prepending
 		 * entries (which we'd ignore), but paranoia won't hurt.
 		 */
+		*last = ed->ed_next;
+		ed->ed_next = NULL;
 		modified = 0;
 
 		/* unlink urbs as requested, but rescan the list after
@@ -1077,21 +1079,22 @@ static void finish_unlinks(struct ohci_hcd *ohci)
 			goto rescan_this;
 
 		/*
-		 * If no TDs are queued, take ED off the ed_rm_list.
+		 * If no TDs are queued, ED is now idle.
 		 * Otherwise, if the HC is running, reschedule.
-		 * If not, leave it on the list for further dequeues.
+		 * If the HC isn't running, add ED back to the
+		 * start of the list for later processing.
 		 */
 		if (list_empty(&ed->td_list)) {
-			*last = ed->ed_next;
-			ed->ed_next = NULL;
 			ed->state = ED_IDLE;
 			list_del(&ed->in_use_list);
 		} else if (ohci->rh_state == OHCI_RH_RUNNING) {
-			*last = ed->ed_next;
-			ed->ed_next = NULL;
 			ed_schedule(ohci, ed);
 		} else {
-			last = &ed->ed_next;
+			ed->ed_next = ohci->ed_rm_list;
+			ohci->ed_rm_list = ed;
+			/* Don't loop on the same ED */
+			if (last == &ohci->ed_rm_list)
+				last = &ed->ed_next;
 		}
 
 		if (modified)
-- 
2.11.0