[PATCH 3.16 150/370] usb: xhci: fix possible wild pointer

[Ben Hutchings]

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>

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.

Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Signed-off-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -1292,13 +1292,16 @@ void xhci_handle_command_timeout(unsigne
 	struct xhci_command *cur_cmd = NULL;
 	xhci = (struct xhci_hcd *) data;
 
-	/* mark this command to be cancelled */
 	spin_lock_irqsave(&xhci->lock, flags);
-	if (xhci->current_cmd) {
-		cur_cmd = xhci->current_cmd;
-		cur_cmd->status = COMP_CMD_ABORT;
+
+	if (!xhci->current_cmd) {
+		spin_unlock_irqrestore(&xhci->lock, flags);
+		return;
 	}
 
+	/* mark this command to be cancelled */
+	cur_cmd = xhci->current_cmd;
+	cur_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);
@@ -1440,6 +1443,8 @@ 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: