Message-ID: <ZxtscPfiH4fhHQvH@hu-mojha-hyd.qualcomm.com>
Date: Fri, 25 Oct 2024 15:31:20 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Jacek Anaszewski <jacek.anaszewski@...il.com>
CC: anish kumar <yesanishhere@...il.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 Fri, Oct 18, 2024 at 10:10:39PM +0200, Jacek Anaszewski wrote:
> 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.

Yes, max_brightness_show() would also need one.
Will send v2.

-Mukesh
> 
> -- 
> Best regards,
> Jacek Anaszewski

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