Message-ID: <70817d8a-4736-c788-77df-28b01517d6a7@gmail.com>
Date: Fri, 18 Oct 2024 22:10:39 +0200
From: Jacek Anaszewski <jacek.anaszewski@...il.com>
To: anish kumar <yesanishhere@...il.com>
Cc: Mukesh Ojha <quic_mojha@...cinc.com>, Pavel Machek <pavel@....cz>,
 Lee Jones <lee@...nel.org>, linux-leds@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] leds: class: Protect brightness_show() with
 led_cdev->led_access mutex

On 10/17/24 22:30, anish kumar wrote:
> On Thu, Oct 17, 2024 at 10:59 AM Jacek Anaszewski
> <jacek.anaszewski@...il.com> wrote:
>>
>>
>>
>> On 10/17/24 18:41, anish kumar wrote:
>>> On Thu, Oct 17, 2024 at 5:12 AM Jacek Anaszewski
>>> <jacek.anaszewski@...il.com> wrote:
>>>>
>>>> Hi Anish and Mukesh,
>>>>
>>>> On 10/16/24 18:37, anish kumar wrote:
>>>>> On Tue, Oct 15, 2024 at 10:45 PM Mukesh Ojha <quic_mojha@...cinc.com> wrote:
>>>>>>
>>>>>> On Tue, Oct 15, 2024 at 03:28:08PM -0700, anish kumar wrote:
>>>>>>> On Tue, Oct 15, 2024 at 12:28 PM Mukesh Ojha <quic_mojha@...cinc.com> wrote:
>>>>>>>>
>>>>>>>> On Tue, Oct 15, 2024 at 10:59:12AM -0700, anish kumar wrote:
>>>>>>>>> On Tue, Oct 15, 2024 at 9:26 AM Mukesh Ojha <quic_mojha@...cinc.com> wrote:
>>>>>>>>>>
>>>>>>>>>> There is NULL pointer issue observed if from Process A where hid device
>>>>>>>>>> being added which results in adding a led_cdev addition and later a
>>>>>>>>>> another call to access of led_cdev attribute from Process B can result
>>>>>>>>>> in NULL pointer issue.
>>>>>>>>>
>>>>>>>>> Which pointer is NULL? Call stack shows that dualshock4_led_get_brightness
>>>>>>>>> function could be culprit?
>>>>>>>>
>>>>>>>> in dualshock4_led_get_brightness()[1], led->dev is NULL here, as [2]
>>>>>>>> is not yet completed.
>>>>>>>>
>>>>>>>> [1]
>>>>>>>>     struct hid_device *hdev = to_hid_device(led->dev->parent);
>>>>>>>>
>>>>>>>> [2]
>>>>>>>> led_cdev->dev = device_create_with_groups(&leds_class, parent, 0,
>>>>>>>>                      led_cdev, led_cdev->groups, "%s", final_name);
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Use mutex led_cdev->led_access to protect access to led->cdev and its
>>>>>>>>>> attribute inside brightness_show().
>>>>>>>>>
>>>>>>>>> I don't think it is needed here because it is just calling the led driver
>>>>>>>>> callback and updating the brightness. So, why would we need to serialize
>>>>>>>>> that using mutex? Maybe the callback needs some debugging.
>>>>>>>>> I'm curious if it is ready by the time the callback is invoked.
>>>>>>>>
>>>>>>>> Because, we should not be allowed to access led_cdev->dev as it is not
>>>>>>>> completed and since, brightness_store() has this lock brightness_show()
>>>>>>>> should also have this as we are seeing the issue without it.
>>>>>>>>
>>>>>>>> I hope, above might have answered your question.
>>>>>>>>
>>>>>>>> -Mukesh
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>            Process A                               Process B
>>>>>>>>>>
>>>>>>>>>>     kthread+0x114
>>>>>>>>>>     worker_thread+0x244
>>>>>>>>>>     process_scheduled_works+0x248
>>>>>>>>>>     uhid_device_add_worker+0x24
>>>>>>>>>>     hid_add_device+0x120
>>>>>>>>>>     device_add+0x268
>>>>>>>>>>     bus_probe_device+0x94
>>>>>>>>>>     device_initial_probe+0x14
>>>>>>>>>>     __device_attach+0xfc
>>>>>>>>>>     bus_for_each_drv+0x10c
>>>>>>>>>>     __device_attach_driver+0x14c
>>>>>>>>>>     driver_probe_device+0x3c
>>>>>>>>>>     __driver_probe_device+0xa0
>>>>>>>>>>     really_probe+0x190
>>>>>>>>>>     hid_device_probe+0x130
>>>>>>>>>>     ps_probe+0x990
>>>>>>>>>>     ps_led_register+0x94
>>>>>>>>>>     devm_led_classdev_register_ext+0x58
>>>>>>>>>>     led_classdev_register_ext+0x1f8
>>>>>>>>>>     device_create_with_groups+0x48
>>>>>>>>>>     device_create_groups_vargs+0xc8
>>>>>>>>>>     device_add+0x244
>>>>>>>>>>     kobject_uevent+0x14
>>>>>>>>>>     kobject_uevent_env[jt]+0x224
>>>>>>>>>>     mutex_unlock[jt]+0xc4
>>>>>>>>>>     __mutex_unlock_slowpath+0xd4
>>>>>>>>>>     wake_up_q+0x70
>>>>>>>>>>     try_to_wake_up[jt]+0x48c
>>>>>>>>>>     preempt_schedule_common+0x28
>>>>>>>>>>     __schedule+0x628
>>>>>>>>>>     __switch_to+0x174
>>>>>>>>>>                                                    el0t_64_sync+0x1a8/0x1ac
>>>>>>>>>>                                                    el0t_64_sync_handler+0x68/0xbc
>>>>>>>>>>                                                    el0_svc+0x38/0x68
>>>>>>>>>>                                                    do_el0_svc+0x1c/0x28
>>>>>>>>>>                                                    el0_svc_common+0x80/0xe0
>>>>>>>>>>                                                    invoke_syscall+0x58/0x114
>>>>>>>>>>                                                    __arm64_sys_read+0x1c/0x2c
>>>>>>>>>>                                                    ksys_read+0x78/0xe8
>>>>>>>>>>                                                    vfs_read+0x1e0/0x2c8
>>>>>>>>>>                                                    kernfs_fop_read_iter+0x68/0x1b4
>>>>>>>>>>                                                    seq_read_iter+0x158/0x4ec
>>>>>>>>>>                                                    kernfs_seq_show+0x44/0x54
>>>>>>>>>>                                                    sysfs_kf_seq_show+0xb4/0x130
>>>>>>>>>>                                                    dev_attr_show+0x38/0x74
>>>>>>>>>>                                                    brightness_show+0x20/0x4c
>>>>>>>>>>                                                    dualshock4_led_get_brightness+0xc/0x74
>>>>>>>>>>
>>>>>>>>>> [ 3313.874295][ T4013] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000060
>>>>>>>>>> [ 3313.874301][ T4013] Mem abort info:
>>>>>>>>>> [ 3313.874303][ T4013]   ESR = 0x0000000096000006
>>>>>>>>>> [ 3313.874305][ T4013]   EC = 0x25: DABT (current EL), IL = 32 bits
>>>>>>>>>> [ 3313.874307][ T4013]   SET = 0, FnV = 0
>>>>>>>>>> [ 3313.874309][ T4013]   EA = 0, S1PTW = 0
>>>>>>>>>> [ 3313.874311][ T4013]   FSC = 0x06: level 2 translation fault
>>>>>>>>>> [ 3313.874313][ T4013] Data abort info:
>>>>>>>>>> [ 3313.874314][ T4013]   ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000
>>>>>>>>>> [ 3313.874316][ T4013]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
>>>>>>>>>> [ 3313.874318][ T4013]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>>>>>>>>>> [ 3313.874320][ T4013] user pgtable: 4k pages, 39-bit VAs, pgdp=00000008f2b0a000
>>>>>>>>>> ..
>>>>>>>>>>
>>>>>>>>>> [ 3313.874332][ T4013] Dumping ftrace buffer:
>>>>>>>>>> [ 3313.874334][ T4013]    (ftrace buffer empty)
>>>>>>>>>> ..
>>>>>>>>>> ..
>>>>>>>>>> [ dd3313.874639][ T4013] CPU: 6 PID: 4013 Comm: InputReader
>>>>>>>>>> [ 3313.874648][ T4013] pc : dualshock4_led_get_brightness+0xc/0x74
>>>>>>>>>> [ 3313.874653][ T4013] lr : led_update_brightness+0x38/0x60
>>>>>>>>>> [ 3313.874656][ T4013] sp : ffffffc0b910bbd0
>>>>>>>>>> ..
>>>>>>>>>> ..
>>>>>>>>>> [ 3313.874685][ T4013] Call trace:
>>>>>>>>>> [ 3313.874687][ T4013]  dualshock4_led_get_brightness+0xc/0x74
>>>>>>>>>> [ 3313.874690][ T4013]  brightness_show+0x20/0x4c
>>>>>>>>>> [ 3313.874692][ T4013]  dev_attr_show+0x38/0x74
>>>>>>>>>> [ 3313.874696][ T4013]  sysfs_kf_seq_show+0xb4/0x130
>>>>>>>>>> [ 3313.874700][ T4013]  kernfs_seq_show+0x44/0x54
>>>>>>>>>> [ 3313.874703][ T4013]  seq_read_iter+0x158/0x4ec
>>>>>>>>>> [ 3313.874705][ T4013]  kernfs_fop_read_iter+0x68/0x1b4
>>>>>>>>>> [ 3313.874708][ T4013]  vfs_read+0x1e0/0x2c8
>>>>>>>>>> [ 3313.874711][ T4013]  ksys_read+0x78/0xe8
>>>>>>>>>> [ 3313.874714][ T4013]  __arm64_sys_read+0x1c/0x2c
>>>>>>>>>> [ 3313.874718][ T4013]  invoke_syscall+0x58/0x114
>>>>>>>>>> [ 3313.874721][ T4013]  el0_svc_common+0x80/0xe0
>>>>>>>>>> [ 3313.874724][ T4013]  do_el0_svc+0x1c/0x28
>>>>>>>>>> [ 3313.874727][ T4013]  el0_svc+0x38/0x68
>>>>>>>>>> [ 3313.874730][ T4013]  el0t_64_sync_handler+0x68/0xbc
>>>>>>>>>> [ 3313.874732][ T4013]  el0t_64_sync+0x1a8/0x1ac
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
>>>>>>>>>> ---
>>>>>>>>>>     drivers/leds/led-class.c | 3 ++-
>>>>>>>>>>     1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
>>>>>>>>>> index 06b97fd49ad9..e3cb93f19c06 100644
>>>>>>>>>> --- a/drivers/leds/led-class.c
>>>>>>>>>> +++ b/drivers/leds/led-class.c
>>>>>>>>>> @@ -30,8 +30,9 @@ static ssize_t brightness_show(struct device *dev,
>>>>>>>>>>     {
>>>>>>>>>>            struct led_classdev *led_cdev = dev_get_drvdata(dev);
>>>>>>>>>>
>>>>>>>>>> -       /* no lock needed for this */
>>>>>>>
>>>>>>> just get rid of the above comment then.
>>>>>>
>>>>>> If you notice, it is already removed (-) .
>>>>>>
>>>>>>>
>>>>>>> Also, the comment below in file leds.h
>>>>>>> needs an update as originally the idea for this mutex lock was to
>>>>>>> provide quick feedback to userspace based on this commit
>>>>>>> https://github.com/torvalds/linux/commit/acd899e4f3066b6662f6047da5b795cc762093cb
>>>>>>>
>>>>>>> Basically a comment somewhere so that when a new attribute
>>>>>>> gets added, it doesn't make the same mistake of not using the mutex
>>>>>>> and run into the same issue.
>>>>>>>
>>>>>>> /* Ensures consistent access to the LED Flash Class device */
>>>>>>> struct mutex led_access;
>>>>>>
>>>>>> Thanks for accepting that it is an issue.
>>>>>> I think, comment is very obvious actually the patch you mentioned should
>>>>>> be in fixes tag as it introduced the lock but did not protect the show
>>>>>> while it does it for store.
>>>>>
>>>>> Yes, but that patch was added for supporting flash class
>>>>> device and wasn't explicitly to take care of the scenario that you
>>>>> are trying to handle and the above comment in leds.h states the same.
>>>>
>>>> Correct. led_access mutex was introduced to add support for preventing
>>>> any LED class device state changes originating from sysfs while
>>>> v4l2_flash wrapper owns the device.
>>>>
>>>> Since the inception of LED subsystem all the locking was deemed to be
>>>> the responsibility of every single LED class driver and initially sysfs
>>>> attr callbacks didn't have any locking. After some time when LED core
>>>> started to grow it turned out that it was required to lock the LED class
>>>> initialization sequence, so as not to give the userspace an opportunity
>>>> to set LED brightness on not fully initialized device, which was
>>>> introduced in [0]. led_access mutex was already in place so it was used.
>>>> However as you noticed, it is not used consistently across all LED class
>>>> sysfs attrs callbacks.
>>>>
>>>> Since brightness_show() does not acquire led_access mutex it is still
>>>> possible to call brightness_get op when LED class initialization
>>>> sequence is not yet finished.
>>>>
>>>> Still, I'd propose to first narrow down the issue and figure out what
>>>> actually causes NULL pointer dereference, as it apparently
>>>> originates from dualshock4_led_get_brightness and not from LED core.
>>>
>>> Mukesh already explained the issue in earlier emails but here is the gist
>>> anyway.
>>>
>>> led_cdev->dev = device_create_with_groups(&leds_class, parent, 0,
>>>     led_cdev, led_cdev->groups, "%s", final_name);
>>>
>>> If you look at the above code, device_create_with_groups function
>>> can create all the sysfs and before it returns and assigns led_cdev->dev
>>> pointer, those sysfs callback can get triggered and if the callback
>>> accesses led_cdev->dev this variable, it will crash as it is not yet
>>> assigned.
>>
>> Your trace ends in dualshock4_led_get_brightness(). Did you confirm that
>> NULL pointer dereference is caused by accessing led_cdev->dev there?
> 
> Based on the comment from mukesh, he confirmed that.
> 
> Relevant comment from him:
> "
> in dualshock4_led_get_brightness()[1], led->dev is NULL here, as [2]
> is not yet completed.
> 
> [1]
> struct hid_device *hdev = to_hid_device(led->dev->parent);
> 
> [2]
> led_cdev->dev = device_create_with_groups(&leds_class, parent, 0,
>                         led_cdev, led_cdev->groups, "%s", final_name);
> "

Ah, right, I missed that. So yes, this change in general is justified,
but as you mentioned we need to adjust the comment next to the
led_access mutex definition.

Probably just change s/LED Flash Class/LED class/.

And also add the locking to max_brightness_show() to be consistent.

-- 
Best regards,
Jacek Anaszewski

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