patch "usb: xhci: fix possible wild pointer" added to usb-linus

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

usb: xhci: fix possible wild pointer

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

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

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(-)

index bdf6b13d9b67..acc37311c938 100644
@@ -1267,14 +1267,18 @@ void xhci_handle_command_timeout(unsigned long data)
 	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) &&

@@ -1422,6 +1426,8 @@ static void handle_cmd_completion(struct xhci_hcd *xhci,
 		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:

2.11.0