ACPI fix for 4.20-rc5

Fix a recent regression in ACPICA releted to the Generic Serial Bus
 protocol handling and causing it to read or write too little or too
 much data in some cases, so incorrect data may be written to hardware
 as a result (Hans de Goede).
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Merge tag 'acpi-4.20-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm

Pull ACPI fix from Rafael Wysocki:
 "Fix a recent regression in ACPICA releted to the Generic Serial Bus
  protocol handling and causing it to read or write too little or too
  much data in some cases, so incorrect data may be written to hardware
  as a result (Hans de Goede)"

* tag 'acpi-4.20-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm:
  ACPICA: Fix handling of buffer-size in acpi_ex_write_data_to_field()

drivers/acpi/acpica/exserial.c

@@ -244,7 +244,6 @@ acpi_ex_write_serial_bus(union acpi_operand_object *source_desc,
 {
 	acpi_status status;
 	u32 buffer_length;
-	u32 data_length;
 	void *buffer;
 	union acpi_operand_object *buffer_desc;
 	u32 function;
@@ -282,14 +281,12 @@ acpi_ex_write_serial_bus(union acpi_operand_object *source_desc,
 	case ACPI_ADR_SPACE_SMBUS:
 
 		buffer_length = ACPI_SMBUS_BUFFER_SIZE;
-		data_length = ACPI_SMBUS_DATA_SIZE;
 		function = ACPI_WRITE | (obj_desc->field.attribute << 16);
 		break;
 
 	case ACPI_ADR_SPACE_IPMI:
 
 		buffer_length = ACPI_IPMI_BUFFER_SIZE;
-		data_length = ACPI_IPMI_DATA_SIZE;
 		function = ACPI_WRITE;
 		break;
 
@@ -310,7 +307,6 @@ acpi_ex_write_serial_bus(union acpi_operand_object *source_desc,
 		/* Add header length to get the full size of the buffer */
 
 		buffer_length += ACPI_SERIAL_HEADER_SIZE;
-		data_length = source_desc->buffer.pointer[1];
 		function = ACPI_WRITE | (accessor_type << 16);
 		break;
 
@@ -318,20 +314,6 @@ acpi_ex_write_serial_bus(union acpi_operand_object *source_desc,
 		return_ACPI_STATUS(AE_AML_INVALID_SPACE_ID);
 	}
 
-#if 0
-	OBSOLETE ?
-	    /* Check for possible buffer overflow */
-	    if (data_length > source_desc->buffer.length) {
-		ACPI_ERROR((AE_INFO,
-			    "Length in buffer header (%u)(%u) is greater than "
-			    "the physical buffer length (%u) and will overflow",
-			    data_length, buffer_length,
-			    source_desc->buffer.length));
-
-		return_ACPI_STATUS(AE_AML_BUFFER_LIMIT);
-	}
-#endif
-
 	/* Create the transfer/bidirectional/return buffer */
 
 	buffer_desc = acpi_ut_create_buffer_object(buffer_length);
@@ -342,7 +324,8 @@ acpi_ex_write_serial_bus(union acpi_operand_object *source_desc,
 	/* Copy the input buffer data to the transfer buffer */
 
 	buffer = buffer_desc->buffer.pointer;
-	memcpy(buffer, source_desc->buffer.pointer, data_length);
+	memcpy(buffer, source_desc->buffer.pointer,
+	       min(buffer_length, source_desc->buffer.length));
 
 	/* Lock entire transaction if requested */