Hi Peter,
On 10/14/19 3:58 PM, Peter Zijlstra wrote:
On Mon, Oct 14, 2019 at 02:59:11PM +0200, Peter Zijlstra wrote:
On Sat, Oct 12, 2019 at 07:49:55AM +0200, Manfred Spraul wrote:
for (;;) {
+ /* memory barrier not required, we hold info->lock */
__set_current_state(TASK_INTERRUPTIBLE);
spin_unlock(&info->lock);
time = schedule_hrtimeout_range_clock(timeout, 0,
HRTIMER_MODE_ABS, CLOCK_REALTIME);
+ if (READ_ONCE(ewp->state) == STATE_READY) {
+ /*
+ * Pairs, together with READ_ONCE(), with
+ * the barrier in __pipelined_op().
+ */
+ smp_acquire__after_ctrl_dep();
retval = 0;
goto out;
}
spin_lock(&info->lock);
+
+ /* we hold info->lock, so no memory barrier required */
+ if (READ_ONCE(ewp->state) == STATE_READY) {
retval = 0;
goto out_unlock;
}
@@ -925,14 +933,12 @@ static inline void __pipelined_op(struct wake_q_head
*wake_q,
list_del(&this->list);
wake_q_add(wake_q, this->task);
/*
+ * The barrier is required to ensure that the refcount increase
+ * inside wake_q_add() is completed before the state is updated.
fails to explain *why* this is important.
+ *
+ * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep().
*/
+ smp_store_release(&this->state, STATE_READY);
You retained the whitespace damage.
And I'm terribly confused by this code, probably due to the lack of
'why' as per the above. What is this trying to do?
Are we worried about something like:
A B C
wq_sleep()
schedule_...();
/* spuriuos
wakeup */
wake_up_process(B)
wake_q_add(A)
if (cmpxchg()) // success
->state = STATE_READY (reordered)
if (READ_ONCE() == STATE_READY)
goto out;
exit();
get_task_struct() // UaF
Can we put the exact and full race in the comment please?
Yes, I'll do that. Actually, two threads are sufficient:
A B
WRITE_ONCE(wait.state, STATE_NONE);
schedule_hrtimeout()
wake_q_add(A)
if (cmpxchg()) // success
->state = STATE_READY (reordered)
<timeout returns>
if (wait.state == STATE_READY) return;
sysret to user space
sys_exit()
get_task_struct() // UaF
Like Davidlohr already suggested, elsewhere we write it like so:
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -930,15 +930,10 @@ static inline void __pipelined_op(struct
struct mqueue_inode_info *info,
struct ext_wait_queue *this)
{
+ get_task_struct(this->task);
list_del(&this->list);
- wake_q_add(wake_q, this->task);
- /*
- * The barrier is required to ensure that the refcount increase
- * inside wake_q_add() is completed before the state is updated.
- *
- * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep().
- */
- smp_store_release(&this->state, STATE_READY);
+ smp_store_release(&this->state, STATE_READY);
+ wake_q_add_safe(wake_q, this->task);
}
/* pipelined_send() - send a message directly to the task waiting in
Much better, I'll rewrite it and then resend the series.
--
Manfred
