[PATCH 05/15] sched/task_struct: convert task_struct.usage to refcount_t

[Elena Reshetova]

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 task_struct.usage 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/init_task.h  | 2 +-
 include/linux/sched.h      | 3 ++-
 include/linux/sched/task.h | 4 ++--
 kernel/fork.c              | 4 ++--
 4 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 6c6f520..d312376 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -227,7 +227,7 @@ extern struct cred init_cred;
 	INIT_TASK_TI(tsk)						\
 	.state		= 0,						\
 	.stack		= init_stack,					\
-	.usage		= ATOMIC_INIT(2),				\
+	.usage		= REFCOUNT_INIT(2),				\
 	.flags		= PF_KTHREAD,					\
 	.prio		= MAX_PRIO-20,					\
 	.static_prio	= MAX_PRIO-20,					\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6110471..1eeb300 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -20,6 +20,7 @@
 #include <linux/seccomp.h>
 #include <linux/nodemask.h>
 #include <linux/rcupdate.h>
+#include <linux/refcount.h>
 #include <linux/resource.h>
 #include <linux/latencytop.h>
 #include <linux/sched/prio.h>
@@ -534,7 +535,7 @@ struct task_struct {
 	randomized_struct_fields_start
 
 	void				*stack;
-	atomic_t			usage;
+	refcount_t			usage;
 	/* Per task flags (PF_*), defined further below: */
 	unsigned int			flags;
 	unsigned int			ptrace;
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 79a2a74..2ddc9b0 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -85,13 +85,13 @@ extern void sched_exec(void);
 #define sched_exec()   {}
 #endif
 
-#define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0)
+#define get_task_struct(tsk) do { refcount_inc(&(tsk)->usage); } while(0)
 
 extern void __put_task_struct(struct task_struct *t);
 
 static inline void put_task_struct(struct task_struct *t)
 {
-	if (atomic_dec_and_test(&t->usage))
+	if (refcount_dec_and_test(&t->usage))
 		__put_task_struct(t);
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index d2e2eb0..1e54683 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -407,7 +407,7 @@ static inline void put_signal_struct(struct signal_struct *sig)
 void __put_task_struct(struct task_struct *tsk)
 {
 	WARN_ON(!tsk->exit_state);
-	WARN_ON(atomic_read(&tsk->usage));
+	WARN_ON(refcount_read(&tsk->usage));
 	WARN_ON(tsk == current);
 
 	cgroup_free(tsk);
@@ -564,7 +564,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	 * One for us, one for whoever does the "release_task()" (usually
 	 * parent)
 	 */
-	atomic_set(&tsk->usage, 2);
+	refcount_set(&tsk->usage, 2);
 #ifdef CONFIG_BLK_DEV_IO_TRACE
 	tsk->btrace_seq = 0;
 #endif
-- 
2.7.4