ACPI fixes for v4.12-rc6

- Revert a 4.11 ACPICA change that made assumptions which are not
    satisfied on some systems and caused the enumeration of resources
    to fail on them (Rafael Wysocki).
 
  - Add a mechanism to prevent tables from being unmapped prematurely
    due to reference counter overflows (Lv Zheng).
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Merge tag 'acpi-4.12-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm

Pull ACPI fixes from Rafael Wysocki:
 "These revert an ACPICA commit from the 4.11 cycle that causes problems
  to happen on some systems and add a protection against possible kernel
  crashes due to table reference counter imbalance.

  Specifics:

   - Revert a 4.11 ACPICA change that made assumptions which are not
     satisfied on some systems and caused the enumeration of resources
     to fail on them (Rafael Wysocki).

   - Add a mechanism to prevent tables from being unmapped prematurely
     due to reference counter overflows (Lv Zheng)"

* tag 'acpi-4.12-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm:
  ACPICA: Tables: Mechanism to handle late stage acpi_get_table() imbalance
  Revert "ACPICA: Disassembler: Enhance resource descriptor detection"

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) {
 

drivers/acpi/acpica/utresrc.c

@@ -474,15 +474,6 @@ acpi_ut_walk_aml_resources(struct acpi_walk_state *walk_state,
 				return_ACPI_STATUS(AE_AML_NO_RESOURCE_END_TAG);
 			}
 
-			/*
-			 * The end_tag opcode must be followed by a zero byte.
-			 * Although this byte is technically defined to be a checksum,
-			 * in practice, all ASL compilers set this byte to zero.
-			 */
-			if (*(aml + 1) != 0) {
-				return_ACPI_STATUS(AE_AML_NO_RESOURCE_END_TAG);
-			}
-
 			/* Return the pointer to the end_tag if requested */
 
 			if (!user_function) {

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 */