Date: Mon, 8 Apr 2024 10:28:51 +0200
From: Armin Wolf <W_Armin@....de>
To: Hans de Goede <hdegoede@...hat.com>, ilpo.jarvinen@...ux.intel.com
Cc: rafael@...nel.org, linux-acpi@...r.kernel.org,
 platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] platform/x86: quickstart: Fix race condition when
 reporting input event

Am 08.04.24 um 09:49 schrieb Hans de Goede:

> Hi,
>
> On 4/8/24 8:01 AM, Armin Wolf wrote:
>> Am 07.04.24 um 17:32 schrieb Hans de Goede:
>>
>>> Hi,
>>>
>>> On 4/6/24 8:57 PM, Armin Wolf wrote:
>>>> Am 27.03.24 um 22:45 schrieb Armin Wolf:
>>>>
>>>>> Since commit e2ffcda16290 ("ACPI: OSL: Allow Notify () handlers to run
>>>>> on all CPUs"), the ACPI core allows multiple notify calls to be active
>>>>> at the same time. This means that two instances of quickstart_notify()
>>>>> running at the same time can mess which each others input sequences.
>>>>>
>>>>> Fix this by protecting the input sequence with a mutex.
>>>>>
>>>>> Compile-tested only.
>>>> Any thoughts on this?
>>> I wonder if we need this at all ?
>>>
>>> The input_event() / input_report_key() / input_sync() functions
>>> which underpin sparse_keymap_report_event() all are safe to be called
>>> from multiple threads at the same time AFAIK.
>>>
>>> The only thing which can then still go "wrong" if we have
>>> 2 sparse_keymap_report_event() functions racing for the same
>>> quickstart button and thus for the same keycode is that we may
>>> end up with:
>>>
>>> input_report_key(dev, keycode, 1);
>>> input_report_key(dev, keycode, 1); /* This is a no-op */
>>> input_sync(); /* + another input_sync() somewhere which is a no-op */
>>> input_report_key(dev, keycode, 0);
>>> input_report_key(dev, keycode, 0); /* This is a no-op */
>>> input_sync(); /* + another input_sync() somewhere which is a no-op */
>>>
>>> IOW if 2 racing notifiers hit the perfect race conditions then
>>> only 1 key press is reported, instead of 2 which seems like
>>> it is not a problem since arguably if the same event gets
>>> reported twice at the exact same time it probably really
>>> is only a single button press.
>>>
>>> Also I think it is highly unlikely we will actually see
>>> 2 notifiers for this racing in practice.
>>>
>>> So I don't think we need this at all. But if others feel strongly
>>> about adding this I can still merge it... ?
>>>
>>> Regards,
>>>
>>> Hans
>> Hi,
>>
>> the locking issue was originally brought up by Ilpo Jarvinen during the review of the lenovo-wmi-camera driver.
>> Also the race condition can cause an invalid input sequence to be emitted:
>>
>> input_report_key(dev, keycode, 1);
>> input_sync();
>> input_report_key(dev, keycode, 0);    // Possible invalid sequence?
>> input_report_key(dev, keycode, 1);
>> input_sync();
>> input_sync();
>> input_report_key(dev, keycode, 0);
>> input_sync();
>>
>>
>> I think this input sequence would be invalid, so we need the locking.
> The:
>
> input_report_key(dev, keycode, 0);    // Possible invalid sequence?
> input_report_key(dev, keycode, 1);
> input_sync();
>
> Part is equivalent of:
>
> input_report_key(dev, keycode, 1);
> input_sync();
>
> Since there is no sync() after the release event it will
> never reach userspace.
>
> With that said, I'm still happy to merge this if you
> prefer to have the locking in place.
>
> Either way please let me know how you want to proceed.
>
> Regards,
>
> Hans

I would prefer to have the locking in place, just in case.

Thanks,
Armin Wolf

>> Thanks,
>> Armin Wolf
>>
>>>>> Fixes: afd66f2a739e ("platform/x86: Add ACPI quickstart button (PNP0C32) driver")
>>>>> Signed-off-by: Armin Wolf <W_Armin@....de>
>>>>> ---
>>>>> This applies on the branch "review-hans". Maybe we could somehow
>>>>> document the concurrency rules for ACPI notify handlers?
>>>>> ---
>>>>>     drivers/platform/x86/quickstart.c | 17 +++++++++++++++++
>>>>>     1 file changed, 17 insertions(+)
>>>>>
>>>>> diff --git a/drivers/platform/x86/quickstart.c b/drivers/platform/x86/quickstart.c
>>>>> index ba3a7a25dda7..e40f852d42c1 100644
>>>>> --- a/drivers/platform/x86/quickstart.c
>>>>> +++ b/drivers/platform/x86/quickstart.c
>>>>> @@ -18,6 +18,7 @@
>>>>>     #include <linux/input/sparse-keymap.h>
>>>>>     #include <linux/kernel.h>
>>>>>     #include <linux/module.h>
>>>>> +#include <linux/mutex.h>
>>>>>     #include <linux/platform_device.h>
>>>>>     #include <linux/sysfs.h>
>>>>>     #include <linux/types.h>
>>>>> @@ -35,6 +36,7 @@
>>>>>
>>>>>     struct quickstart_data {
>>>>>         struct device *dev;
>>>>> +    struct mutex input_lock;    /* Protects input sequence during notify */
>>>>>         struct input_dev *input_device;
>>>>>         char input_name[32];
>>>>>         char phys[32];
>>>>> @@ -73,7 +75,10 @@ static void quickstart_notify(acpi_handle handle, u32 event, void *context)
>>>>>
>>>>>         switch (event) {
>>>>>         case QUICKSTART_EVENT_RUNTIME:
>>>>> +        mutex_lock(&data->input_lock);
>>>>>             sparse_keymap_report_event(data->input_device, 0x1, 1, true);
>>>>> +        mutex_unlock(&data->input_lock);
>>>>> +
>>>>>             acpi_bus_generate_netlink_event(DRIVER_NAME, dev_name(data->dev), event, 0);
>>>>>             break;
>>>>>         default:
>>>>> @@ -147,6 +152,13 @@ static void quickstart_notify_remove(void *context)
>>>>>         acpi_remove_notify_handler(handle, ACPI_DEVICE_NOTIFY, quickstart_notify);
>>>>>     }
>>>>>
>>>>> +static void quickstart_mutex_destroy(void *data)
>>>>> +{
>>>>> +    struct mutex *lock = data;
>>>>> +
>>>>> +    mutex_destroy(lock);
>>>>> +}
>>>>> +
>>>>>     static int quickstart_probe(struct platform_device *pdev)
>>>>>     {
>>>>>         struct quickstart_data *data;
>>>>> @@ -165,6 +177,11 @@ static int quickstart_probe(struct platform_device *pdev)
>>>>>         data->dev = &pdev->dev;
>>>>>         dev_set_drvdata(&pdev->dev, data);
>>>>>
>>>>> +    mutex_init(&data->input_lock);
>>>>> +    ret = devm_add_action_or_reset(&pdev->dev, quickstart_mutex_destroy, &data->input_lock);
>>>>> +    if (ret < 0)
>>>>> +        return ret;
>>>>> +
>>>>>         /* We have to initialize the device wakeup before evaluating GHID because
>>>>>          * doing so will notify the device if the button was used to wake the machine
>>>>>          * from S5.
>>>>> --
>>>>> 2.39.2
>>>>>
>>>>>
>

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