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

From: Mukesh Ojha
Date: Tue Oct 15 2024 - 15:28:16 EST

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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> > 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 */
>
> >> also you missed this.
>
> > + mutex_lock(&led_cdev->led_access);
> > led_update_brightness(led_cdev);
> > + mutex_unlock(&led_cdev->led_access);
> >
> > return sprintf(buf, "%u\n", led_cdev->brightness);
> > }
> > --
> > 2.34.1
> >
> >