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

In the Linux kernel, the following vulnerability has been resolved:

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