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 <oleg@tv-sign.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 6923ad8a5983dc8ddc8772d8461e1f98fa28e183..7a15afb73ed0352d9ee31f6d03334255c716e5a9 100644 (file)
--- a/kernel/posix-timers.c
+++ b/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;