leds: class: Protect brightness_show() with led_cdev->led_access mutex

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 <[email protected]>
Reviewed-by: Anish Kumar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Lee Jones <[email protected]>

Author: Mukesh Ojha

drivers/leds/led-class.c

@@ -31,11 +31,14 @@ 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,
@@ -71,8 +74,13 @@ 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);
 

include/linux/leds.h

@@ -239,7 +239,7 @@ 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 */
 	struct mutex		led_access;
 };