Re: [PATCH v2] leds: class: Protect brightness_show() with led_cdev->led_access mutex

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

Hi Mukesh,

On 10/25/24 19:11, Mukesh Ojha 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.
> 
> Use mutex led_cdev->led_access to protect access to led->cdev and its
> attribute inside brightness_show() and max_brightness_show() and also
> update the comment for mutex that it should be used to protect the led
> class device fields.
> 
> 	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>
> ---
> Changes in v2:
>   - Updated the comment for led_access mutex lock.
>   - Also added mutex protection for max_brightness_show().
> 
>   drivers/leds/led-class.c | 14 +++++++++++---
>   include/linux/leds.h     |  2 +-
>   2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
> index 06b97fd49ad9..f69f4e928d61 100644
> --- a/drivers/leds/led-class.c
> +++ b/drivers/leds/led-class.c
> @@ -29,11 +29,14 @@ static ssize_t brightness_show(struct device *dev,
>   		struct device_attribute *attr, char *buf)
>   {
>   	struct led_classdev *led_cdev = dev_get_drvdata(dev);
> +	unsigned int brightness;
>   
> -	/* no lock needed for this */
> +	mutex_lock(&led_cdev->led_access);
>   	led_update_brightness(led_cdev);
> +	brightness = led_cdev->brightness;
> +	mutex_unlock(&led_cdev->led_access);
>   
> -	return sprintf(buf, "%u\n", led_cdev->brightness);
> +	return sprintf(buf, "%u\n", brightness);
>   }
>   
>   static ssize_t brightness_store(struct device *dev,
> @@ -70,8 +73,13 @@ static ssize_t max_brightness_show(struct device *dev,
>   		struct device_attribute *attr, char *buf)
>   {
>   	struct led_classdev *led_cdev = dev_get_drvdata(dev);
> +	unsigned int max_brightness;
> +
> +	mutex_lock(&led_cdev->led_access);
> +	max_brightness = led_cdev->max_brightness;
> +	mutex_unlock(&led_cdev->led_access);
>   
> -	return sprintf(buf, "%u\n", led_cdev->max_brightness);
> +	return sprintf(buf, "%u\n", max_brightness);
>   }
>   static DEVICE_ATTR_RO(max_brightness);
>   
> diff --git a/include/linux/leds.h b/include/linux/leds.h
> index e5968c3ed4ae..3524634fcc47 100644
> --- a/include/linux/leds.h
> +++ b/include/linux/leds.h
> @@ -238,7 +238,7 @@ struct led_classdev {
>   	struct kernfs_node	*brightness_hw_changed_kn;
>   #endif
>   
> -	/* Ensures consistent access to the LED Flash Class device */
> +	/* Ensures consistent access to the LED Class device */

Nit: It was improper in the original comment as well:

s/Class/class/

>   	struct mutex		led_access;
>   };
>   
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>

-- 
Best regards,
Jacek Anaszewski