[PATCH 05/19] kernel: convert task_struct.stack_refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova at intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel at gmail.com>
Signed-off-by: Kees Cook <keescook at chromium.org>
Signed-off-by: David Windsor <dwindsor at gmail.com>
---
 include/linux/init_task.h | 3 ++-
 include/linux/sched.h     | 4 ++--
 kernel/fork.c             | 6 +++---
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 1c6df0b..c452c57 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -12,6 +12,7 @@
 #include <linux/securebits.h>
 #include <linux/seqlock.h>
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include <net/net_namespace.h>
 #include <linux/sched/rt.h>
 
@@ -205,7 +206,7 @@ extern struct task_group root_task_group;
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 # define INIT_TASK_TI(tsk)			\
 	.thread_info = INIT_THREAD_INFO(tsk),	\
-	.stack_refcount = ATOMIC_INIT(1),
+	.stack_refcount = REFCOUNT_INIT(1),
 #else
 # define INIT_TASK_TI(tsk)
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d760ad6..41a2e52 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1969,7 +1969,7 @@ struct task_struct {
 #endif
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/* A live task holds one reference. */
-	atomic_t stack_refcount;
+	refcount_t stack_refcount;
 #endif
 /* CPU-specific state of this task */
 	struct thread_struct thread;
@@ -3260,7 +3260,7 @@ static inline unsigned long *end_of_stack(struct task_struct *p)
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 static inline void *try_get_task_stack(struct task_struct *tsk)
 {
-	return atomic_inc_not_zero(&tsk->stack_refcount) ?
+	return refcount_inc_not_zero(&tsk->stack_refcount) ?
 		task_stack_page(tsk) : NULL;
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index b9b3296..31c887c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -331,7 +331,7 @@ static void release_task_stack(struct task_struct *tsk)
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 void put_task_stack(struct task_struct *tsk)
 {
-	if (atomic_dec_and_test(&tsk->stack_refcount))
+	if (refcount_dec_and_test(&tsk->stack_refcount))
 		release_task_stack(tsk);
 }
 #endif
@@ -349,7 +349,7 @@ void free_task(struct task_struct *tsk)
 	 * If the task had a separate stack allocation, it should be gone
 	 * by now.
 	 */
-	WARN_ON_ONCE(atomic_read(&tsk->stack_refcount) != 0);
+	WARN_ON_ONCE(refcount_read(&tsk->stack_refcount) != 0);
 #endif
 	rt_mutex_debug_task_free(tsk);
 	ftrace_graph_exit_task(tsk);
@@ -504,7 +504,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	tsk->stack_vm_area = stack_vm_area;
 #endif
 #ifdef CONFIG_THREAD_INFO_IN_TASK
-	atomic_set(&tsk->stack_refcount, 1);
+	refcount_set(&tsk->stack_refcount, 1);
 #endif
 
 	if (err)
-- 
2.7.4