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

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Date:   Fri, 2 Dec 2016 12:40:37 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Baolin Wang <baolin.wang@...aro.org>
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

Hi,

On 12/02/2016 12:18 PM, Baolin Wang wrote:
> 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.

Below code does:

    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@...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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>
>