The array param[] in elantech_change_report_id() must be at least 3
bytes, because elantech_read_reg_params() is calling ps2_command() with
PSMOUSE_CMD_GETINFO, that is going to access 3 bytes from param[], but
it's defined in the stack as an array of 2 bytes, therefore we have a
potential stack out-of-bounds access here, also confirmed by KASAN:

[    6.512374] BUG: KASAN: stack-out-of-bounds in __ps2_command+0x372/0x7e0
[    6.512397] Read of size 1 at addr ffff8881024d77c2 by task kworker/2:1/118

[    6.512416] CPU: 2 PID: 118 Comm: kworker/2:1 Not tainted 5.13.0-22-generic #22+arighi20211110
[    6.512428] Hardware name: LENOVO 20T8000QGE/20T8000QGE, BIOS R1AET32W (1.08 ) 08/14/2020
[    6.512436] Workqueue: events_long serio_handle_event
[    6.512453] Call Trace:
[    6.512462]  show_stack+0x52/0x58
[    6.512474]  dump_stack+0xa1/0xd3
[    6.512487]  print_address_description.constprop.0+0x1d/0x140
[    6.512502]  ? __ps2_command+0x372/0x7e0
[    6.512516]  __kasan_report.cold+0x7d/0x112
[    6.512527]  ? _raw_write_lock_irq+0x20/0xd0
[    6.512539]  ? __ps2_command+0x372/0x7e0
[    6.512552]  kasan_report+0x3c/0x50
[    6.512564]  __asan_load1+0x6a/0x70
[    6.512575]  __ps2_command+0x372/0x7e0
[    6.512589]  ? ps2_drain+0x240/0x240
[    6.512601]  ? dev_printk_emit+0xa2/0xd3
[    6.512612]  ? dev_vprintk_emit+0xc5/0xc5
[    6.512621]  ? __kasan_check_write+0x14/0x20
[    6.512634]  ? mutex_lock+0x8f/0xe0
[    6.512643]  ? __mutex_lock_slowpath+0x20/0x20
[    6.512655]  ps2_command+0x52/0x90
[    6.512670]  elantech_ps2_command+0x4f/0xc0 [psmouse]
[    6.512734]  elantech_change_report_id+0x1e6/0x256 [psmouse]
[    6.512799]  ? elantech_report_trackpoint.constprop.0.cold+0xd/0xd [psmouse]
[    6.512863]  ? ps2_command+0x7f/0x90
[    6.512877]  elantech_query_info.cold+0x6bd/0x9ed [psmouse]
[    6.512943]  ? elantech_setup_ps2+0x460/0x460 [psmouse]
[    6.513005]  ? psmouse_reset+0x69/0xb0 [psmouse]
[    6.513064]  ? psmouse_attr_set_helper+0x2a0/0x2a0 [psmouse]
[    6.513122]  ? phys_pmd_init+0x30e/0x521
[    6.513137]  elantech_init+0x8a/0x200 [psmouse]
[    6.513200]  ? elantech_init_ps2+0xf0/0xf0 [psmouse]
[    6.513249]  ? elantech_query_info+0x440/0x440 [psmouse]
[    6.513296]  ? synaptics_send_cmd+0x60/0x60 [psmouse]
[    6.513342]  ? elantech_query_info+0x440/0x440 [psmouse]
[    6.513388]  ? psmouse_try_protocol+0x11e/0x170 [psmouse]
[    6.513432]  psmouse_extensions+0x65d/0x6e0 [psmouse]
[    6.513476]  ? psmouse_try_protocol+0x170/0x170 [psmouse]
[    6.513519]  ? mutex_unlock+0x22/0x40
[    6.513526]  ? ps2_command+0x7f/0x90
[    6.513536]  ? psmouse_probe+0xa3/0xf0 [psmouse]
[    6.513580]  psmouse_switch_protocol+0x27d/0x2e0 [psmouse]
[    6.513624]  psmouse_connect+0x272/0x530 [psmouse]
[    6.513669]  serio_driver_probe+0x55/0x70
[    6.513679]  really_probe+0x190/0x720
[    6.513689]  driver_probe_device+0x160/0x1f0
[    6.513697]  device_driver_attach+0x119/0x130
[    6.513705]  ? device_driver_attach+0x130/0x130
[    6.513713]  __driver_attach+0xe7/0x1a0
[    6.513720]  ? device_driver_attach+0x130/0x130
[    6.513728]  bus_for_each_dev+0xfb/0x150
[    6.513738]  ? subsys_dev_iter_exit+0x10/0x10
[    6.513748]  ? _raw_write_unlock_bh+0x30/0x30
[    6.513757]  driver_attach+0x2d/0x40
[    6.513764]  serio_handle_event+0x199/0x3d0
[    6.513775]  process_one_work+0x471/0x740
[    6.513785]  worker_thread+0x2d2/0x790
[    6.513794]  ? process_one_work+0x740/0x740
[    6.513802]  kthread+0x1b4/0x1e0
[    6.513809]  ? set_kthread_struct+0x80/0x80
[    6.513816]  ret_from_fork+0x22/0x30

[    6.513832] The buggy address belongs to the page:
[    6.513838] page:00000000bc35e189 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1024d7
[    6.513847] flags: 0x17ffffc0000000(node=0|zone=2|lastcpupid=0x1fffff)
[    6.513860] raw: 0017ffffc0000000 dead000000000100 dead000000000122 0000000000000000
[    6.513867] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[    6.513872] page dumped because: kasan: bad access detected

[    6.513879] addr ffff8881024d77c2 is located in stack of task kworker/2:1/118 at offset 34 in frame:
[    6.513887]  elantech_change_report_id+0x0/0x256 [psmouse]

[    6.513941] this frame has 1 object:
[    6.513947]  [32, 34) 'param'

[    6.513956] Memory state around the buggy address:
[    6.513962]  ffff8881024d7680: f2 f2 f2 f2 f2 00 00 f3 f3 00 00 00 00 00 00 00
[    6.513969]  ffff8881024d7700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    6.513976] >ffff8881024d7780: 00 00 00 00 f1 f1 f1 f1 02 f3 f3 f3 00 00 00 00
[    6.513982]                                            ^
[    6.513988]  ffff8881024d7800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    6.513995]  ffff8881024d7880: 00 f1 f1 f1 f1 03 f2 03 f2 03 f3 f3 f3 00 00 00
[    6.514000] ==================================================================

Define param[] in elantech_change_report_id() as an array of 3 bytes to
prevent the out-of-bounds access in the stack.

Fixes: e4c90627 ("Input: elantech - fix protocol errors for some trackpoints in SMBus mode")
BugLink: https://bugs.launchpad.net/bugs/1945590Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Reviewed-by: Wolfram Sang <wsa@kernel.org>
Link: https://lore.kernel.org/r/20211116095559.24395-1-andrea.righi@canonical.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>