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>

1 files changed, 25 insertions, 9 deletions

diff --git a/drivers/acpi/acpica/tbutils.c b/drivers/acpi/acpica/tbutils.c
index 7abe66505739..0d2e98920069 100644
--- a/drivers/acpi/acpica/tbutils.c
+++ b/drivers/acpi/acpica/tbutils.c
@@ -416,9 +416,18 @@ acpi_tb_get_table(struct acpi_table_desc *table_desc,
 		}
 	}
 
-	table_desc->validation_count++;
-	if (table_desc->validation_count == 0) {
-		table_desc->validation_count--;
+	if (table_desc->validation_count < ACPI_MAX_TABLE_VALIDATIONS) {
+		table_desc->validation_count++;
+
+		/*
+		 * Detect validation_count overflows to ensure that the warning
+		 * message will only be printed once.
+		 */
+		if (table_desc->validation_count >= ACPI_MAX_TABLE_VALIDATIONS) {
+			ACPI_WARNING((AE_INFO,
+				      "Table %p, Validation count overflows\n",
+				      table_desc));
+		}
 	}
 
 	*out_table = table_desc->pointer;
@@ -445,13 +454,20 @@ void acpi_tb_put_table(struct acpi_table_desc *table_desc)
 
 	ACPI_FUNCTION_TRACE(acpi_tb_put_table);
 
-	if (table_desc->validation_count == 0) {
-		ACPI_WARNING((AE_INFO,
-			      "Table %p, Validation count is zero before decrement\n",
-			      table_desc));
-		return_VOID;
+	if (table_desc->validation_count < ACPI_MAX_TABLE_VALIDATIONS) {
+		table_desc->validation_count--;
+
+		/*
+		 * Detect validation_count underflows to ensure that the warning
+		 * message will only be printed once.
+		 */
+		if (table_desc->validation_count >= ACPI_MAX_TABLE_VALIDATIONS) {
+			ACPI_WARNING((AE_INFO,
+				      "Table %p, Validation count underflows\n",
+				      table_desc));
+			return_VOID;
+		}
 	}
-	table_desc->validation_count--;
 
 	if (table_desc->validation_count == 0) {