[PATCH] acpi: acpica: dsutils: fix an off-by-one index

[Seraphime Kirkovski]

This was detected by UBSAN.
Fix it by checking whether there are any arguments prior to indexing the array.

[    0.222775] UBSAN: Undefined behaviour in drivers/acpi/acpica/dsutils.c:640:16
[    0.222778] index -1 is out of range for type 'acpi_operand_object*[9]'
[    0.222781] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.12.0-rc4-dirty #32
[    0.222782] Hardware name: Hewlett-Packard HP EliteBook 2560p/162B, BIOS 68SSU Ver. F.02 07/26/2011
[    0.222783] Call Trace:
[    0.222790]  dump_stack+0x4e/0x78
[    0.222792]  ubsan_epilogue+0xd/0x40
[    0.222794]  __ubsan_handle_out_of_bounds+0x68/0x80
[    0.222795]  ? perf_trace_sys_exit+0x12d/0x170
[    0.222797]  ? kmem_cache_alloc+0x1e6/0x2e0
[    0.222800]  acpi_ds_create_operand+0x20b/0x2a6
[    0.222801]  acpi_ds_eval_data_object_operands+0x58/0x16c
[    0.222803]  acpi_ds_exec_end_op+0x424/0x582
[    0.222805]  acpi_ps_parse_loop+0x730/0x792
[    0.222807]  acpi_ps_parse_aml+0xac/0x2eb
[    0.222809]  acpi_ds_execute_arguments+0x129/0x14d
[    0.222810]  acpi_ds_get_buffer_arguments+0x62/0x65
[    0.222812]  acpi_ns_init_one_object+0xe0/0x13b
[    0.222814]  acpi_ns_walk_namespace+0x121/0x1ef
[    0.222815]  ? acpi_ns_exec_module_code_list+0x1b7/0x1b7
[    0.222817]  ? acpi_ns_exec_module_code_list+0x1b7/0x1b7
[    0.222818]  acpi_walk_namespace+0xa0/0xd5
[    0.222820]  acpi_ns_initialize_objects+0x37/0x5a
[    0.222823]  acpi_initialize_objects+0x34/0x54
[    0.222824]  ? acpi_sleep_proc_init+0x2d/0x2d
[    0.222826]  acpi_init+0xcb/0x34d
[    0.222828]  ? __class_create+0x54/0x80
[    0.222830]  ? fbmem_init+0x7f/0xd4
[    0.222831]  ? acpi_sleep_proc_init+0x2d/0x2d
[    0.222832]  do_one_initcall+0x52/0x1d0
[    0.222835]  kernel_init_freeable+0x314/0x3a6
[    0.222837]  ? rest_init+0x80/0x80
[    0.222838]  kernel_init+0xf/0x120
[    0.222839]  ? rest_init+0x80/0x80
[    0.222841]  ret_from_fork+0x22/0x30

Signed-off-by: Seraphime Kirkovski <kirkseraph@xxxxxxxxx>
---
 drivers/acpi/acpica/dsutils.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/acpi/acpica/dsutils.c b/drivers/acpi/acpica/dsutils.c
index 406edec20de7..b66eddd3df6e 100644
--- a/drivers/acpi/acpica/dsutils.c
+++ b/drivers/acpi/acpica/dsutils.c
@@ -636,11 +636,10 @@ acpi_ds_create_operand(struct acpi_walk_state *walk_state,
 			ACPI_DEBUG_PRINT((ACPI_DB_DISPATCH,
 					  "Argument previously created, already stacked\n"));
 
-			acpi_db_display_argument_object(walk_state->
-							operands[walk_state->
-								 num_operands -
-								 1],
-							walk_state);
+			if (walk_state->num_operands)
+				acpi_db_display_argument_object(walk_state->
+					operands[walk_state->num_operands - 1],
+					walk_state);
 
 			/*
 			 * Use value that was already previously returned
-- 
2.11.0