posix-timers: fix creation race

sys_timer_create() sets ->it_process and unlocks ->siglock, then checks
tmr->it_sigev_notify to define if get_task_struct() is needed.

We already passed ->it_id to the caller, another thread can delete this timer
and free its memory in between.

As a minimal fix, move this code under ->siglock, sys_timer_delete() takes it
too before calling release_posix_timer().  A proper serialization would be to
take ->it_lock, we add a partly initialized timer on posix_timers_id, not
good.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/posix-timers.c

@@ -547,9 +547,9 @@ sys_timer_create(const clockid_t which_clock,
 				new_timer->it_process = process;
 				list_add(&new_timer->list,
 					 &process->signal->posix_timers);
-				spin_unlock_irqrestore(&process->sighand->siglock, flags);
 				if (new_timer->it_sigev_notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
 					get_task_struct(process);
+				spin_unlock_irqrestore(&process->sighand->siglock, flags);
 			} else {
 				spin_unlock_irqrestore(&process->sighand->siglock, flags);
 				process = NULL;