linux-kernel - Re: [PATCH 1/1] usb: xhci: fix possible wild pointer

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Date:   Fri, 2 Dec 2016 12:18:42 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] usb: xhci: fix possible wild pointer

On 2 December 2016 at 10:29, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> 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@...r.kernel.org> # v3.16+
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>

Nice catch. I was also curious where set xhci->current_cmd to be NULL
when current command is freed.
Tested-by: Baolin Wang <baolin.wang@...aro.org>

> ---
>  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
>
> --
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-- 
Baolin.wang
Best Regards