[PATCH v2 1/5] list: Split list_add() debug checking into separate function

[Kees Cook <keescook@chromium.org>]

Right now, __list_add() code is repeated either in list.h or in
list_debug.c, but only the debug checks are the different part. This
extracts the checking into a separate function and consolidates
__list_add(). Additionally this __list_add_debug() will stop list
manipulations if a corruption is detected, instead of allowing for further
corruption that may lead to even worse conditions.

This is slight refactoring of the same hardening done in PaX and Grsecurity.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/list.h | 22 ++++++++++++++++------
 lib/list_debug.c     | 48 +++++++++++++++++++++++-------------------------
 2 files changed, 39 insertions(+), 31 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 5183138aa932..0ed58591538e 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -28,27 +28,37 @@ static inline void INIT_LIST_HEAD(struct list_head *list)
 	list->prev = list;
 }
 
+#ifdef CONFIG_DEBUG_LIST
+extern bool __list_add_valid(struct list_head *new,
+			      struct list_head *prev,
+			      struct list_head *next);
+#else
+static inline bool __list_add_valid(struct list_head *new,
+				struct list_head *prev,
+				struct list_head *next)
+{
+	return true;
+}
+#endif
+
 /*
  * Insert a new entry between two known consecutive entries.
  *
  * This is only for internal list manipulation where we know
  * the prev/next entries already!
  */
-#ifndef CONFIG_DEBUG_LIST
 static inline void __list_add(struct list_head *new,
 			      struct list_head *prev,
 			      struct list_head *next)
 {
+	if (!__list_add_valid(new, prev, next))
+		return;
+
 	next->prev = new;
 	new->next = next;
 	new->prev = prev;
 	WRITE_ONCE(prev->next, new);
 }
-#else
-extern void __list_add(struct list_head *new,
-			      struct list_head *prev,
-			      struct list_head *next);
-#endif
 
 /**
  * list_add - add a new entry
diff --git a/lib/list_debug.c b/lib/list_debug.c
index 3859bf63561c..149dd57b583b 100644
--- a/lib/list_debug.c
+++ b/lib/list_debug.c
@@ -2,8 +2,7 @@
  * Copyright 2006, Red Hat, Inc., Dave Jones
  * Released under the General Public License (GPL).
  *
- * This file contains the linked list implementations for
- * DEBUG_LIST.
+ * This file contains the linked list validation for DEBUG_LIST.
  */
 
 #include <linux/export.h>
@@ -13,33 +12,32 @@
 #include <linux/rculist.h>
 
 /*
- * Insert a new entry between two known consecutive entries.
- *
- * This is only for internal list manipulation where we know
- * the prev/next entries already!
+ * Check that the data structures for the list manipulations are reasonably
+ * valid. Failures here indicate memory corruption (and possibly an exploit
+ * attempt).
  */
 
-void __list_add(struct list_head *new,
-			      struct list_head *prev,
-			      struct list_head *next)
+bool __list_add_valid(struct list_head *new, struct list_head *prev,
+		      struct list_head *next)
 {
-	WARN(next->prev != prev,
-		"list_add corruption. next->prev should be "
-		"prev (%p), but was %p. (next=%p).\n",
-		prev, next->prev, next);
-	WARN(prev->next != next,
-		"list_add corruption. prev->next should be "
-		"next (%p), but was %p. (prev=%p).\n",
-		next, prev->next, prev);
-	WARN(new == prev || new == next,
-	     "list_add double add: new=%p, prev=%p, next=%p.\n",
-	     new, prev, next);
-	next->prev = new;
-	new->next = next;
-	new->prev = prev;
-	WRITE_ONCE(prev->next, new);
+	if (unlikely(next->prev != prev)) {
+		WARN(1, "list_add corruption. next->prev should be prev (%p), but was %p. (next=%p).\n",
+			prev, next->prev, next);
+		return false;
+	}
+	if (unlikely(prev->next != next)) {
+		WARN(1, "list_add corruption. prev->next should be next (%p), but was %p. (prev=%p).\n",
+			next, prev->next, prev);
+		return false;
+	}
+	if (unlikely(new == prev || new == next)) {
+		WARN(1, "list_add double add: new=%p, prev=%p, next=%p.\n",
+			new, prev, next);
+		return false;
+	}
+	return true;
 }
-EXPORT_SYMBOL(__list_add);
+EXPORT_SYMBOL(__list_add_valid);
 
 void __list_del_entry(struct list_head *entry)
 {
-- 
2.7.4