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.usage is used as pure reference counter. Convert it to refcount_t and fix up the operations. Suggested-by: Kees Cook <keesc...@chromium.org> Reviewed-by: David Windsor <dwind...@gmail.com> Reviewed-by: Hans Liljestrand <ishkam...@gmail.com> Signed-off-by: Elena Reshetova <elena.reshet...@intel.com> --- include/linux/init_task.h | 2 +- include/linux/sched.h | 3 ++- include/linux/sched/task.h | 4 ++-- kernel/fork.c | 4 ++-- 4 files changed, 7 insertions(+), 6 deletions(-) diff --git a/include/linux/init_task.h b/include/linux/init_task.h index a85376e..64d86ec 100644 --- a/include/linux/init_task.h +++ b/include/linux/init_task.h @@ -226,7 +226,7 @@ extern struct cred init_cred; INIT_TASK_TI(tsk) \ .state = 0, \ .stack = init_stack, \ - .usage = ATOMIC_INIT(2), \ + .usage = REFCOUNT_INIT(2), \ .flags = PF_KTHREAD, \ .prio = MAX_PRIO-20, \ .static_prio = MAX_PRIO-20, \ diff --git a/include/linux/sched.h b/include/linux/sched.h index 0f897df..47f1101 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -20,6 +20,7 @@ #include <linux/seccomp.h> #include <linux/nodemask.h> #include <linux/rcupdate.h> +#include <linux/refcount.h> #include <linux/resource.h> #include <linux/latencytop.h> #include <linux/sched/prio.h> @@ -536,7 +537,7 @@ struct task_struct { randomized_struct_fields_start void *stack; - atomic_t usage; + refcount_t usage; /* Per task flags (PF_*), defined further below: */ unsigned int flags; unsigned int ptrace; diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index a5e6f09..ac4317d 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -85,13 +85,13 @@ extern void sched_exec(void); #define sched_exec() {} #endif -#define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0) +#define get_task_struct(tsk) do { refcount_inc(&(tsk)->usage); } while(0) extern void __put_task_struct(struct task_struct *t); static inline void put_task_struct(struct task_struct *t) { - if (atomic_dec_and_test(&t->usage)) + if (refcount_dec_and_test(&t->usage)) __put_task_struct(t); } diff --git a/kernel/fork.c b/kernel/fork.c index 869850b..68cc7a0 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -649,7 +649,7 @@ static inline void put_signal_struct(struct signal_struct *sig) void __put_task_struct(struct task_struct *tsk) { WARN_ON(!tsk->exit_state); - WARN_ON(atomic_read(&tsk->usage)); + WARN_ON(refcount_read(&tsk->usage)); WARN_ON(tsk == current); cgroup_free(tsk); @@ -824,7 +824,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) * One for us, one for whoever does the "release_task()" (usually * parent) */ - atomic_set(&tsk->usage, 2); + refcount_set(&tsk->usage, 2); #ifdef CONFIG_BLK_DEV_IO_TRACE tsk->btrace_seq = 0; #endif -- 2.7.4