[PATCH 02/15] sched: convert signal_struct.sigcnt to refcount_t

[Elena Reshetova <elena.reshetova@xxxxxxxxx>]

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable signal_struct.sigcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
---
 include/linux/sched/signal.h | 2 +-
 kernel/fork.c                | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 4d5cdf1..c5f1a67 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -77,7 +77,7 @@ struct thread_group_cputimer {
  * the locking of signal_struct.
  */
 struct signal_struct {
-	atomic_t		sigcnt;
+	refcount_t		sigcnt;
 	atomic_t		live;
 	int			nr_threads;
 	struct list_head	thread_head;
diff --git a/kernel/fork.c b/kernel/fork.c
index 60bd7bf..337904a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -399,7 +399,7 @@ static inline void free_signal_struct(struct signal_struct *sig)
 
 static inline void put_signal_struct(struct signal_struct *sig)
 {
-	if (atomic_dec_and_test(&sig->sigcnt))
+	if (refcount_dec_and_test(&sig->sigcnt))
 		free_signal_struct(sig);
 }
 
@@ -1379,7 +1379,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 
 	sig->nr_threads = 1;
 	atomic_set(&sig->live, 1);
-	atomic_set(&sig->sigcnt, 1);
+	refcount_set(&sig->sigcnt, 1);
 
 	/* list_add(thread_node, thread_head) without INIT_LIST_HEAD() */
 	sig->thread_head = (struct list_head)LIST_HEAD_INIT(tsk->thread_node);
@@ -1888,7 +1888,7 @@ static __latent_entropy struct task_struct *copy_process(
 		} else {
 			current->signal->nr_threads++;
 			atomic_inc(&current->signal->live);
-			atomic_inc(&current->signal->sigcnt);
+			refcount_inc(&current->signal->sigcnt);
 			list_add_tail_rcu(&p->thread_group,
 					  &p->group_leader->thread_group);
 			list_add_tail_rcu(&p->thread_node,
-- 
2.7.4