sched/core: Convert task_struct.usage to refcount_t

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.

** Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.

The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.

Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.

Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the task_struct.usage it might make a difference
in following places:

 - put_task_struct(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and control dependency on success
   vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Cc: viro@zeniv.linux.org.uk
Link: https://lkml.kernel.org/r/1547814450-18902-5-git-send-email-elena.reshetova@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e2bba022827d12ea81094dd33c2016f503a769aa..9d14d6864ca678f84e78d12a93bc7b6677435c88 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -21,6 +21,7 @@
 #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>
@@ -607,7 +608,7 @@ 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 44c6f15800ff554ebc36994106e482bb62e2bb4b..2e97a2227045ec689d21e2f3f2da8ae24a9c69b3 100644 (file)
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -88,13 +88,13 @@ 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/init/init_task.c b/init/init_task.c
index 9aa3ebc74970a569ab3ac3582fba860ffcb53c3e..aca34c89529f98102792d076d826280ae50de081 100644 (file)
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -65,7 +65,7 @@ struct task_struct init_task
 #endif
        .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/kernel/fork.c b/kernel/fork.c
index 935a42d5f8ffbea46868ab9938fbf1cad0b0404a..3f7e192e29f20fe4794efa21a95f88211aaf0873 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -717,7 +717,7 @@ 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);
@@ -896,7 +896,7 @@ 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