ACPICA: Fix handling of buffer-size in acpi_ex_write_data_to_field()

Generic Serial Bus transfers use a data struct like this:

struct gsb_buffer {
        u8      status;
        u8      len;
        u8      data[0];
};

acpi_ex_write_data_to_field() copies the data which is to be written from
the source-buffer to a temp-buffer. This is done because the OpReg-handler
overwrites the status field and some transfers do a write + read-back.

Commit f99b89e ("ACPICA: Update for generic_serial_bus and
attrib_raw_process_bytes protocol") acpi_ex_write_data_to_field()
introduces a number of problems with this:

 1) It drops a "length += 2" statement used to calculate the temp-buffer
 size causing the temp-buffer to only be 1/2 bytes large for byte/word
 transfers while it should be 3/4 bytes (taking the status and len field
 into account). This is already fixed in commit e324e10 ("ACPICA:
 Update for field unit access") which refactors the code.

The ACPI 6.0 spec (ACPI_6.0.pdf) "5.5.2.4.5.2 Declaring and Using a
GenericSerialBusData Buffer" (page 232) states that the GenericSerialBus
Data Buffer Length field is only valid when doing a Read/Write Block
(AttribBlock) transfer, but since the troublesome commit we unconditionally
use the len field to determine how much data to copy from the source-buffer
into the temp-buffer passed to the OpRegion.

This causes 3 further issues:

 2) This may lead to not copying enough data to the temp-buffer causing the
 OpRegion handler for the serial-bus to write garbage to the hardware.

 3) The temp-buffer passed to the OpRegion is allocated to the size
 returned by acpi_ex_get_serial_access_length(), which may be as little
 as 1, so potentially this may lead to a write overflow of the temp-buffer.

 4) Commit e324e10 ("ACPICA: Update for field unit access") drops a
 length check on the source-buffer, leading to a potential read overflow
 of the source-buffer.

This commit fixes all 3 remaining issues by not looking at the len field at
all (the interpretation of this field is left up to the OpRegion handler),
and copying the minimum of the source- and temp-buffer sizes from the
source-buffer to the temp-buffer.

This fixes e.g. an Acer S1003 no longer booting since the troublesome
commit.

Fixes: f99b89e (ACPICA: Update for generic_serial_bus and ...)
Fixes: e324e10 (ACPICA: Update for field unit access)
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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;
 
@@ -310,28 +307,13 @@ 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;
 
 	default:
 		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 */