sched/fair: Add tmp_alone_branch assertion

CVE-2018-20784

The magic in list_add_leaf_cfs_rq() requires that at the end of
enqueue_task_fair():

  rq->tmp_alone_branch == &rq->lead_cfs_rq_list

If this is violated, list integrity is compromised for list entries
and the tmp_alone_branch pointer might dangle.

Also, reflow list_add_leaf_cfs_rq() while there. This looses one
indentation level and generates a form that's convenient for the next
patch.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(backported from commit 5d299eab)
[ Connor Kuehl: Instead of backporting the SCHED_WARN_ON macro, just use
  the macro it wraps around (WARN_ON_ONCE) for assert_list_leaf_cfs_rq. ]
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Sultan Alsawaf <sultan.alsawaf@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

kernel/sched/fair.c

@@ -285,64 +285,69 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
 
 static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 {
-	if (!cfs_rq->on_list) {
-		struct rq *rq = rq_of(cfs_rq);
-		int cpu = cpu_of(rq);
+	struct rq *rq = rq_of(cfs_rq);
+	int cpu = cpu_of(rq);
+
+	if (cfs_rq->on_list)
+		return;
+
+	cfs_rq->on_list = 1;
+
+	/*
+	 * Ensure we either appear before our parent (if already
+	 * enqueued) or force our parent to appear after us when it is
+	 * enqueued. The fact that we always enqueue bottom-up
+	 * reduces this to two cases and a special case for the root
+	 * cfs_rq. Furthermore, it also means that we will always reset
+	 * tmp_alone_branch either when the branch is connected
+	 * to a tree or when we reach the top of the tree
+	 */
+	if (cfs_rq->tg->parent &&
+	    cfs_rq->tg->parent->cfs_rq[cpu]->on_list) {
 		/*
-		 * Ensure we either appear before our parent (if already
-		 * enqueued) or force our parent to appear after us when it is
-		 * enqueued. The fact that we always enqueue bottom-up
-		 * reduces this to two cases and a special case for the root
-		 * cfs_rq. Furthermore, it also means that we will always reset
-		 * tmp_alone_branch either when the branch is connected
-		 * to a tree or when we reach the beg of the tree
+		 * If parent is already on the list, we add the child
+		 * just before. Thanks to circular linked property of
+		 * the list, this means to put the child at the tail
+		 * of the list that starts by parent.
 		 */
-		if (cfs_rq->tg->parent &&
-		    cfs_rq->tg->parent->cfs_rq[cpu]->on_list) {
-			/*
-			 * If parent is already on the list, we add the child
-			 * just before. Thanks to circular linked property of
-			 * the list, this means to put the child at the tail
-			 * of the list that starts by parent.
-			 */
-			list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
-				&(cfs_rq->tg->parent->cfs_rq[cpu]->leaf_cfs_rq_list));
-			/*
-			 * The branch is now connected to its tree so we can
-			 * reset tmp_alone_branch to the beginning of the
-			 * list.
-			 */
-			rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
-		} else if (!cfs_rq->tg->parent) {
-			/*
-			 * cfs rq without parent should be put
-			 * at the tail of the list.
-			 */
-			list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
-				&rq->leaf_cfs_rq_list);
-			/*
-			 * We have reach the beg of a tree so we can reset
-			 * tmp_alone_branch to the beginning of the list.
-			 */
-			rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
-		} else {
-			/*
-			 * The parent has not already been added so we want to
-			 * make sure that it will be put after us.
-			 * tmp_alone_branch points to the beg of the branch
-			 * where we will add parent.
-			 */
-			list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
-				rq->tmp_alone_branch);
-			/*
-			 * update tmp_alone_branch to points to the new beg
-			 * of the branch
-			 */
-			rq->tmp_alone_branch = &cfs_rq->leaf_cfs_rq_list;
-		}
+		list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
+			&(cfs_rq->tg->parent->cfs_rq[cpu]->leaf_cfs_rq_list));
+		/*
+		 * The branch is now connected to its tree so we can
+		 * reset tmp_alone_branch to the beginning of the
+		 * list.
+		 */
+		rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
+		return;
+	}
 
-		cfs_rq->on_list = 1;
+	if (!cfs_rq->tg->parent) {
+		/*
+		 * cfs rq without parent should be put
+		 * at the tail of the list.
+		 */
+		list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
+			&rq->leaf_cfs_rq_list);
+		/*
+		 * We have reach the top of a tree so we can reset
+		 * tmp_alone_branch to the beginning of the list.
+		 */
+		rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
+		return;
 	}
+
+	/*
+	 * The parent has not already been added so we want to
+	 * make sure that it will be put after us.
+	 * tmp_alone_branch points to the begin of the branch
+	 * where we will add parent.
+	 */
+	list_add_rcu(&cfs_rq->leaf_cfs_rq_list, rq->tmp_alone_branch);
+	/*
+	 * update tmp_alone_branch to points to the new begin
+	 * of the branch
+	 */
+	rq->tmp_alone_branch = &cfs_rq->leaf_cfs_rq_list;
 }
 
 static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
@@ -353,7 +358,12 @@ static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 	}
 }
 
-/* Iterate through all leaf cfs_rq's on a runqueue: */
+static inline void assert_list_leaf_cfs_rq(struct rq *rq)
+{
+	WARN_ON_ONCE((rq->tmp_alone_branch != &rq->leaf_cfs_rq_list));
+}
+
+/* Iterate through all cfs_rq's on a runqueue in bottom-up order */
 #define for_each_leaf_cfs_rq(rq, cfs_rq) \
 	list_for_each_entry_rcu(cfs_rq, &rq->leaf_cfs_rq_list, leaf_cfs_rq_list)
 
@@ -448,6 +458,10 @@ static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 {
 }
 
+static inline void assert_list_leaf_cfs_rq(struct rq *rq)
+{
+}
+
 #define for_each_leaf_cfs_rq(rq, cfs_rq)	\
 		for (cfs_rq = &rq->cfs; cfs_rq; cfs_rq = NULL)
 
@@ -4400,6 +4414,8 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (!se)
 		sub_nr_running(rq, 1);
 
+	assert_list_leaf_cfs_rq(rq);
+
 	hrtick_update(rq);
 }