Patch "usb: ohci: Proper handling of ed_rm_list to handle race condition between usb_kill_urb() and finish_unlinks()" has been added to the 4.9-stable tree

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

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

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
usb-ohci-proper-handling-of-ed_rm_list-to-handle-race-condition-between-usb_kill_urb-and-finish_unlinks.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: AMAN DEEP <[email protected]>
Date: Thu, 8 Feb 2018 11:55:01 +0800
Subject: usb: ohci: Proper handling of ed_rm_list to handle race condition between usb_kill_urb() and finish_unlinks()

From: AMAN DEEP <[email protected]>

commit 46408ea558df13b110e0866b99624384a33bdeba upstream.

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().

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

@@ -1018,6 +1018,8 @@ skip_ed:
 		 * 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

@@ -1076,21 +1078,22 @@ rescan_this:
 			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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/usb-ohci-proper-handling-of-ed_rm_list-to-handle-race-condition-between-usb_kill_urb-and-finish_unlinks.patch