ACPICA: Fix possible buffer overflow during a field unit read operation

Can only happen under these conditions: 1) The DSDT version is 1,
meaning integers are 32-bits.  2) The field is between 33 and 64
bits long.

It applies cleanly back to ACPICA 20100806+ (Linux v2.6.37+).

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Cc: 2.6.37+ <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/exfldio.c

diff --git a/drivers/acpi/acpica/exfldio.c b/drivers/acpi/acpica/exfldio.c
index ec7f569..c84ee95 100644
--- a/drivers/acpi/acpica/exfldio.c
+++ b/drivers/acpi/acpica/exfldio.c
@@ -720,7 +720,19 @@
 
 	if ((obj_desc->common_field.start_field_bit_offset == 0) &&
 	    (obj_desc->common_field.bit_length == access_bit_width)) {
-		status = acpi_ex_field_datum_io(obj_desc, 0, buffer, ACPI_READ);
+		if (buffer_length >= sizeof(u64)) {
+			status =
+			    acpi_ex_field_datum_io(obj_desc, 0, buffer,
+						   ACPI_READ);
+		} else {
+			/* Use raw_datum (u64) to handle buffers < 64 bits */
+
+			status =
+			    acpi_ex_field_datum_io(obj_desc, 0, &raw_datum,
+						   ACPI_READ);
+			ACPI_MEMCPY(buffer, &raw_datum, buffer_length);
+		}
+
 		return_ACPI_STATUS(status);
 	}