Re: [PATCH] leds: class: Protect brightness_show() with led_cdev->led_access mutex

[Jacek Anaszewski <jacek.anaszewski@gmail.com>]

On 10/17/24 18:41, anish kumar wrote:
> On Thu, Oct 17, 2024 at 5:12 AM Jacek Anaszewski
> <jacek.anaszewski@gmail.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@quicinc.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@quicinc.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@quicinc.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@quicinc.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?

-- 
Best regards,
Jacek Anaszewski