[PATCH 04/15] sched: convert numa_group.refcount to refcount_t

From: Elena Reshetova
Date: Wed Aug 30 2017 - 08:26:54 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 numa_group.refcount 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>
---
kernel/sched/fair.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a5d83ed..fe4e38c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1059,7 +1059,7 @@ unsigned int sysctl_numa_balancing_scan_size = 256;
unsigned int sysctl_numa_balancing_scan_delay = 1000;

struct numa_group {
- atomic_t refcount;
+ refcount_t refcount;

spinlock_t lock; /* nr_tasks, tasks */
int nr_tasks;
@@ -1128,7 +1128,7 @@ static unsigned int task_scan_start(struct task_struct *p)
unsigned long shared = group_faults_shared(ng);
unsigned long private = group_faults_priv(ng);

- period *= atomic_read(&ng->refcount);
+ period *= refcount_read(&ng->refcount);
period *= shared + 1;
period /= private + shared + 1;
}
@@ -1151,7 +1151,7 @@ static unsigned int task_scan_max(struct task_struct *p)
unsigned long private = group_faults_priv(ng);
unsigned long period = smax;

- period *= atomic_read(&ng->refcount);
+ period *= refcount_read(&ng->refcount);
period *= shared + 1;
period /= private + shared + 1;

@@ -2236,12 +2236,12 @@ static void task_numa_placement(struct task_struct *p)

static inline int get_numa_group(struct numa_group *grp)
{
- return atomic_inc_not_zero(&grp->refcount);
+ return refcount_inc_not_zero(&grp->refcount);
}

static inline void put_numa_group(struct numa_group *grp)
{
- if (atomic_dec_and_test(&grp->refcount))
+ if (refcount_dec_and_test(&grp->refcount))
kfree_rcu(grp, rcu);
}

@@ -2262,7 +2262,7 @@ static void task_numa_group(struct task_struct *p, int cpupid, int flags,
if (!grp)
return;

- atomic_set(&grp->refcount, 1);
+ refcount_set(&grp->refcount, 1);
grp->active_nodes = 1;
grp->max_faults_cpu = 0;
spin_lock_init(&grp->lock);
--
2.7.4