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.stack_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>
---
include/linux/init_task.h | 3 ++-
include/linux/sched.h | 2 +-
include/linux/sched/task_stack.h | 2 +-
kernel/fork.c | 6 +++---
4 files changed, 7 insertions(+), 6 deletions(-)
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 64d86ec..d3bc6ac 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 <linux/sched/autogroup.h>
#include <net/net_namespace.h>
#include <linux/sched/rt.h>
@@ -206,7 +207,7 @@ extern struct cred init_cred;
#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 47f1101..4eb19f9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1091,7 +1091,7 @@ 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
#ifdef CONFIG_LIVEPATCH
int patch_state;
diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
index df6ea66..aab3809 100644
--- a/include/linux/sched/task_stack.h
+++ b/include/linux/sched/task_stack.h
@@ -60,7 +60,7 @@ 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 68cc7a0..b7b26d5a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -362,7 +362,7 @@ 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
@@ -380,7 +380,7 @@ 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);
@@ -795,7 +795,7 @@ 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
