linux-kernel - Re: [PATCH] platform/x86: quickstart: Fix race condition when reporting input event

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Date: Mon, 8 Apr 2024 08:01:24 +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 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.

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
>>>
>>>
>