[PATCH 1/1] usb: xhci: fix possible wild pointer

[Lu Baolu <baolu.lu@linux.intel.com>]

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.

Cc: <stable@vger.kernel.org> # v3.16+
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
---
 drivers/usb/host/xhci-ring.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index bdf6b13..13e05f6 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -1267,14 +1267,16 @@ 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;
 	}
 
+	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 +1424,10 @@ 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;
+	} else {
+		xhci_warn(xhci, "WARN current_cmd doesn't match command\n");
 	}
 
 event_handled:
-- 
2.1.4