[PATCH 2/4] posix-timers: fix creation race

[Oleg Nesterov <oleg@tv-sign.ru>]

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 <oleg@tv-sign.ru>

--- t/kernel/posix-timers.c~2_CREATE	2007-08-12 17:59:17.000000000 +0400
+++ t/kernel/posix-timers.c	2007-08-12 18:11:33.000000000 +0400
@@ -547,13 +547,12 @@ sys_timer_create(const clockid_t which_c
 				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);
 			} else {
-				spin_unlock_irqrestore(&process->sighand->siglock, flags);
 				process = NULL;
 			}
+			spin_unlock_irqrestore(&process->sighand->siglock, flags);
 		}
 		read_unlock(&tasklist_lock);
 		if (!process) {