[PATCH 1/2] kthread: add barriers to set_kthread_struct() and to_kthread()

Tejun Heo
2017-03-15T23:19:01Z

Until now, all to_kthread() users are interlocked with kthread
creation and there's no need to have explicit barriers when setting
the kthread pointer or dereferencing it.

However, There is a race condition where userland can interfere with a
kthread while it's being initialized. To close it, to_kthread() needs
to be used from an unsynchronized context.

This patch moves struct kthread initialization before
set_kthread_struct() and adds matching barriers in
set_kthread_struct() and to_kthread(), so that dereferencing
to_kthread() always returns initialized fields.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Mason <clm@fb.com>
Cc: stable@vger.kernel.org # v4.3+ (we can't close the race < v4.3)
---
kernel/kthread.c | 30 +++++++++++++++++++++++-------
1 file changed, 23 insertions(+), 7 deletions(-)

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -57,6 +57,9 @@ enum KTHREAD_BITS {

static inline void set_kthread_struct(void *kthread)
{
+ /* paired with smp_read_data_barrier_depends() in to_kthread() */
+ smp_wmb();
+
/*
* We abuse ->set_child_tid to avoid the new member and because it
* can't be wrongly copied by copy_process(). We also rely on fact
@@ -67,8 +70,19 @@ static inline void set_kthread_struct(vo

static inline struct kthread *to_kthread(struct task_struct *k)
{
+ void *ptr;
+
WARN_ON(!(k->flags & PF_KTHREAD));
- return (__force void *)k->set_child_tid;
+
+ ptr = (__force void *)k->set_child_tid;
+
+ /*
+ * Paired with smp_wmb() in set_kthread_struct() and ensures that
+ * the caller sees initialized content of the returned kthread.
+ */
+ smp_read_barrier_depends();
+
+ return ptr;
}

void free_kthread_struct(struct task_struct *k)
@@ -196,6 +210,14 @@ static int kthread(void *_create)
int ret;

self = kmalloc(sizeof(*self), GFP_KERNEL);
+ if (self) {
+ self->flags = 0;
+ self->data = data;
+ init_completion(&self->exited);
+ init_completion(&self->parked);
+ current->vfork_done = &self->exited;
+ }
+
set_kthread_struct(self);

/* If user was SIGKILLed, I release the structure. */
@@ -211,12 +233,6 @@ static int kthread(void *_create)
do_exit(-ENOMEM);
}

- self->flags = 0;
- self->data = data;
- init_completion(&self->exited);
- init_completion(&self->parked);
- current->vfork_done = &self->exited;
-
/* OK, tell user we're spawned, wait for stop or wakeup */
__set_current_state(TASK_UNINTERRUPTIBLE);
create->result = current;