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

[Baolin Wang]

On 2 December 2016 at 12:40, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
> Hi,
>
> On 12/02/2016 12:18 PM, Baolin Wang wrote:
>> On 2 December 2016 at 10:29, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> # v3.16+
>>> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
>> Nice catch. I was also curious where set xhci->current_cmd to be NULL
>> when current command is freed.
>
> Below code does:

Yes, I just means you did that, but the original code did not. Sorry
for confusing.

>
>     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");
>         }
>
>
> Best regards,
> Lu Baolu
>
>> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxx>
>>
>>> ---
>>>  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