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

From: Elena Reshetova
Date: Fri Oct 20 2017 - 08:16:13 EST


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 b40fbf7..856d957 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 ab4ddc9..869850b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -642,7 +642,7 @@ 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);
}

@@ -1443,7 +1443,7 @@ 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);
@@ -1950,7 +1950,7 @@ 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