ACPICA: Tables: Mechanism to handle late stage acpi_get_table() imbalance

Considering this case:

 1. A program opens a sysfs table file 65535 times, it can increase
    validation_count and first increment cause the table to be mapped:

     validation_count = 65535

 2. AML execution causes "Load" to be executed on the same
    table, this time it cannot increase validation_count, so
    validation_count remains:

      validation_count = 65535

 3. The program closes sysfs table file 65535 times, it can decrease
    validation_count and the last decrement cause the table to be
    unmapped:

     validation_count = 0

 4. AML code still accessing the loaded table, kernel crash can be
    observed.

To prevent that from happening, add a validation_count threashold.
When it is reached, the validation_count can no longer be
incremented/decremented to invalidate the table descriptor (means
preventing table unmappings)

Note that code added in acpi_tb_put_table() is actually a no-op but
changes the warning message into a "warn once" one. Lv Zheng.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[ rjw: Changelog, comments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/acpi/actbl.h

@@ -374,6 +374,20 @@ struct acpi_table_desc {
 	u16 validation_count;
 };
 
+/*
+ * Maximum value of the validation_count field in struct acpi_table_desc.
+ * When reached, validation_count cannot be changed any more and the table will
+ * be permanently regarded as validated.
+ *
+ * This is to prevent situations in which unbalanced table get/put operations
+ * may cause premature table unmapping in the OS to happen.
+ *
+ * The maximum validation count can be defined to any value, but should be
+ * greater than the maximum number of OS early stage mapping slots to avoid
+ * leaking early stage table mappings to the late stage.
+ */
+#define ACPI_MAX_TABLE_VALIDATIONS          ACPI_UINT16_MAX
+
 /* Masks for Flags field above */
 
 #define ACPI_TABLE_ORIGIN_EXTERNAL_VIRTUAL  (0)	/* Virtual address, external maintained */