linux-kernel - Re: [PATCH] staging: greybus: Convert timers to use timer

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Message-ID: <CAGXu5j+z12uJrZQyt0gnM2rMYzoSZWOpm+ekxLWD0vKotaXb=A@mail.gmail.com>
Date:   Tue, 24 Oct 2017 06:14:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Bryan O'Donoghue" <pure.logic@...us-software.ie>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>,
        greybus-dev@...ts.linaro.org, devel@...verdev.osuosl.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: greybus: Convert timers to use timer_setup()

On Tue, Oct 24, 2017 at 5:52 AM, Bryan O'Donoghue
<pure.logic@...us-software.ie> wrote:
> On 24/10/17 13:47, Kees Cook wrote:
>>
>> On Tue, Oct 24, 2017 at 2:40 AM, Bryan O'Donoghue
>> <pure.logic@...us-software.ie> wrote:
>>>
>>> On 24/10/17 10:35, Bryan O'Donoghue wrote:
>>>>
>>>>
>>>> On 24/10/17 09:25, Kees Cook wrote:
>>>>>
>>>>>
>>>>> In preparation for unconditionally passing the struct timer_list
>>>>> pointer
>>>>> to
>>>>> all timer callbacks, switch to using the new timer_setup() and
>>>>> from_timer()
>>>>> to pass the timer pointer explicitly.
>>>>>
>>>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>>> Cc: "Bryan O'Donoghue" <pure.logic@...us-software.ie>
>>>>> Cc: Johan Hovold <johan@...nel.org>
>>>>> Cc: Alex Elder <elder@...nel.org>
>>>>> Cc: greybus-dev@...ts.linaro.org
>>>>> Cc: devel@...verdev.osuosl.org
>>>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>>>> ---
>>>>>    drivers/staging/greybus/loopback.c  | 14 ++++----------
>>>>>    drivers/staging/greybus/operation.c |  7 +++----
>>>>>    2 files changed, 7 insertions(+), 14 deletions(-)
>>>>>
>>>>> diff --git a/drivers/staging/greybus/loopback.c
>>>>> b/drivers/staging/greybus/loopback.c
>>>>> index 08e255884206..045aaf81113a 100644
>>>>> --- a/drivers/staging/greybus/loopback.c
>>>>> +++ b/drivers/staging/greybus/loopback.c
>>>>> @@ -572,16 +572,11 @@ static void
>>>>> gb_loopback_async_operation_work(struct
>>>>> work_struct *work)
>>>>>        gb_loopback_async_operation_put(op_async);
>>>>>    }
>>>>> -static void gb_loopback_async_operation_timeout(unsigned long data)
>>>>> +static void gb_loopback_async_operation_timeout(struct timer_list *t)
>>>>>    {
>>>>> -    struct gb_loopback_async_operation *op_async;
>>>>> -    u16 id = data;
>>>>> +    struct gb_loopback_async_operation *op_async =
>>>>> +        from_timer(op_async, t, timer);
>>>>> -    op_async = gb_loopback_operation_find(id);
>>>>> -    if (!op_async) {
>>>>> -        pr_err("operation %d not found - time out ?\n", id);
>>>>> -        return;
>>>>> -    }
>>>>
>>>>
>>>>
>>>> Hi Kees, you need to add
>>>>
>>>>       gb_loopback_async_operation_get(op_async); when dropping the
>>>> gb_loopback_operation_find() call here.
>>>
>>>
>>>
>>> Actually:
>>>
>>>          spin_lock_irqsave(&gb_dev.lock, flags);
>>>          gb_loopback_async_operation_get(op_async);
>>>          spin_unlock_irqrestore(&gb_dev.lock, flags);
>>
>> Shouldn't the get/put follow the lifetime of the timer running
>> instead? It shouldnt' be possible to free the op_async while the timer
>> is still pending/running.
>
> The timeout timer runs for an operation that never completed but on the
> regular "everything is good" path you end up doing
> del_timer_sync(&op_async->timer); so the timer doesn't run.

As far as I can tell, the get/put is already happening external to the timer:

gb_loopback_async_operation():
...
        op_async = kzalloc(sizeof(*op_async), GFP_KERNEL);
...
        INIT_WORK(&op_async->work, gb_loopback_async_operation_work);
        kref_init(&op_async->kref);
...
        op_async->pending = true;
...
        ret = gb_operation_request_send(operation,
                                        gb_loopback_async_operation_callback,
                                        0,
                                        GFP_KERNEL);
...
        timer_setup(&op_async->timer, gb_loopback_async_operation_timeout, 0);
        op_async->timer.expires = jiffies + gb->jiffy_timeout;
        add_timer(&op_async->timer);


gb_loopback_async_operation_callback(): (run by operation)
...
                op_async->pending = false;
                del_timer_sync(&op_async->timer);
                gb_loopback_async_operation_put(op_async);


gb_loopback_async_operation_work(): (scheduled by timer)
...
                op_async->pending = false;
                gb_loopback_async_operation_put(op_async);

(Doing a get/put in the timer callback itself shouldn't be happening:
it must already be pinned in memory for the callback to run.)

-Kees

-- 
Kees Cook
Pixel Security