Patch "usb: xhci: fix possible wild pointer" has been added to the 4.4-stable tree

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

usb: xhci: fix possible wild pointer

to the 4.4-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-xhci-fix-possible-wild-pointer.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Lu Baolu <[email protected]>
Date: Tue, 3 Jan 2017 18:28:46 +0200
Subject: usb: xhci: fix possible wild pointer

From: Lu Baolu <[email protected]>

commit 2b985467371a58ae44d76c7ba12b0951fee6ed98 upstream.

handle_cmd_completion() frees a command structure which might be still referenced by xhci->current_cmd.
This might cause problem when xhci->current_cmd is accessed after that.

A real-life case could be like this. The host takes a very long time to respond to a command, and the command timer is fired at the same time when the command completion event arrives. The command completion handler frees xhci->current_cmd before the timer function can grab xhci->lock. Afterward, timer function grabs the lock and go ahead with checking and setting members of xhci->current_cmd.

---
 drivers/usb/host/xhci-ring.c |   16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

@@ -1268,14 +1268,18 @@ void xhci_handle_command_timeout(unsigne
 	bool second_timeout = false;
 	xhci = (struct xhci_hcd *) data;
 
-	/* mark this command to be cancelled */
 	spin_lock_irqsave(&xhci->lock, flags);
-	if (xhci->current_cmd) {
-		if (xhci->current_cmd->status == COMP_CMD_ABORT)
-			second_timeout = true;
-		xhci->current_cmd->status = COMP_CMD_ABORT;
+
+	if (!xhci->current_cmd) {
+		spin_unlock_irqrestore(&xhci->lock, flags);
+		return;
 	}
 
+	/* mark this command to be cancelled */
+	if (xhci->current_cmd->status == COMP_CMD_ABORT)
+		second_timeout = true;
+	xhci->current_cmd->status = COMP_CMD_ABORT;
+
 	/* Make sure command ring is running before aborting it */
 	hw_ring_state = xhci_read_64(xhci, &xhci->op_regs->cmd_ring);
 	if ((xhci->cmd_ring_state & CMD_RING_STATE_RUNNING) &&

@@ -1424,6 +1428,8 @@ static void handle_cmd_completion(struct
 		xhci->current_cmd = list_entry(cmd->cmd_list.next,
 					       struct xhci_command, cmd_list);
 		mod_timer(&xhci->cmd_timer, jiffies + XHCI_CMD_DEFAULT_TIMEOUT);
+	} else if (xhci->current_cmd == cmd) {
+		xhci->current_cmd = NULL;
 	}
 
 event_handled:

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

queue-4.4/usb-xhci-fix-possible-wild-pointer.patch