[PATCH] ACPICA: Fix dereference in acpi_ev_address_space_dispatch()

[George Rurikov <grurikov@gmail.com>]

When support for  PCC Opregion was added, validation of field_obj
was missed.
Based on the acpi_ev_address_space_dispatch function description,
field_obj can be NULL, and also when acpi_ev_address_space_dispatch
is called in the acpi_ex_region_read() NULL is passed as field_obj.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 0acf24ad7e10 ("ACPICA: Add support for PCC Opregion special context data")
Cc: stable@vger.kernel.org
Signed-off-by: George Rurikov <grurikov@gmail.com>
---
 drivers/acpi/acpica/evregion.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/acpi/acpica/evregion.c b/drivers/acpi/acpica/evregion.c
index cf53b9535f18..03e8b6f186af 100644
--- a/drivers/acpi/acpica/evregion.c
+++ b/drivers/acpi/acpica/evregion.c
@@ -164,13 +164,17 @@ acpi_ev_address_space_dispatch(union acpi_operand_object *region_obj,
 		}
 
 		if (region_obj->region.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
-			struct acpi_pcc_info *ctx =
-			    handler_desc->address_space.context;
-
-			ctx->internal_buffer =
-			    field_obj->field.internal_pcc_buffer;
-			ctx->length = (u16)region_obj->region.length;
-			ctx->subspace_id = (u8)region_obj->region.address;
+			if (field_obj != NULL) {
+				struct acpi_pcc_info *ctx =
+					handler_desc->address_space.context;
+
+				ctx->internal_buffer =
+					field_obj->field.internal_pcc_buffer;
+				ctx->length = (u16)region_obj->region.length;
+				ctx->subspace_id = (u8)region_obj->region.address;
+			} else {
+				return_ACPI_STATUS(AE_ERROR);
+			}
 		}
 
 		if (region_obj->region.space_id ==
-- 
2.34.1