Input: elantech - Fix stack out of bound access in elantech_change_report_id()

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]

67 more lines

diff --git a/drivers/input/mouse/elantech.c b/drivers/input/mouse/elantech.c
index 956d9cd34796..ece97f8c6a3e 100644
--- a/drivers/input/mouse/elantech.c
+++ b/drivers/input/mouse/elantech.c

@@ -1588,7 +1588,13 @@ static const struct dmi_system_id no_hw_res_dmi_table[] = {
  */
 static int elantech_change_report_id(struct psmouse *psmouse)
 {
-	unsigned char param[2] = { 0x10, 0x03 };
+	/*
+	 * NOTE: the code is expecting to receive param[] as an array of 3
+	 * items (see __ps2_command()), even if in this case only 2 are
+	 * actually needed. Make sure the array size is 3 to avoid potential
+	 * stack out-of-bound accesses.
+	 */
+	unsigned char param[3] = { 0x10, 0x03 };
 
 	if (elantech_write_reg_params(psmouse, 0x7, param) ||
 	    elantech_read_reg_params(psmouse, 0x7, param) ||

-- 
2.32.0

[    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

I wonder if the KASAN trace could be shortened, but I leave this to
Dmitry.

I like the comment, thanks!

Reviewed-by: Wolfram Sang <wsa@kernel.org>

On Tue, Nov 16, 2021 at 10:55:58AM +0100, Andrea Righi wrote:

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]

59 more quoted lines

Applied, thank you.

-- 
Dmitry