Message-ID: <CABCoZhBmmmyQ6s27s_gXrv6rR9Wf672-t+OEg-PfdxReFdr++Q@mail.gmail.com>
Date: Thu, 17 Oct 2024 13:30:18 -0700
From: anish kumar <yesanishhere@...il.com>
To: Jacek Anaszewski <jacek.anaszewski@...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 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);
"
>
> --
> Best regards,
> Jacek Anaszewski

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