[PATCH 02/19] kernel: convert signal_struct.sigcnt 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.reshet...@intel.com>
Signed-off-by: Hans Liljestrand <ishkam...@gmail.com>
Signed-off-by: Kees Cook <keesc...@chromium.org>
Signed-off-by: David Windsor <dwind...@gmail.com>
---
 include/linux/sched.h | 2 +-
 kernel/fork.c         | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a0430aa..6aad885 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -659,7 +659,7 @@ struct autogroup;
  * 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 9488f01..b432d1c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -376,7 +376,7 @@ 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);
 }
 
@@ -1347,7 +1347,7 @@ 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);
@@ -1826,7 +1826,7 @@ 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