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posix-timers: Ensure that timer initialization is fully visible

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[ Upstream commit 2389c6efd3ad8edb3bcce0019b4edcc7d9c7de19 ]

Frederic pointed out that the memory operations to initialize the timer are
not guaranteed to be visible, when __lock_timer() observes timer::it_signal
valid under timer::it_lock:

  T0                                      T1
  ---------                               -----------
  do_timer_create()
      // A
      new_timer->.... = ....
      spin_lock(current->sighand)
      // B
      WRITE_ONCE(new_timer->it_signal, current->signal)
      spin_unlock(current->sighand)
					sys_timer_*()
					   t =  __lock_timer()
						  spin_lock(&timr->it_lock)
						  // observes B
						  if (timr->it_signal == current->signal)
						    return timr;
			                   if (!t)
					       return;
					// Is not guaranteed to observe A

Protect the write of timer::it_signal, which makes the timer valid, with
timer::it_lock as well. This guarantees that T1 must observe the
initialization A completely, when it observes the valid signal pointer
under timer::it_lock. sighand::siglock must still be taken to protect the
signal::posix_timers list.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20250308155623.507944489@linutronix.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Thomas Gleixner
2025-03-08 17:48:10 +01:00
committed by Greg Kroah-Hartman
parent d0dc233fe2
commit ae22452d15
1 changed files with 14 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files

21
kernel/time/posix-timers.c
View File

@@ -514,14 +514,21 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
if (error)
goto out;
spin_lock_irq(&current->sighand->siglock);
/* This makes the timer valid in the hash table */
WRITE_ONCE(new_timer->it_signal, current->signal);
hlist_add_head(&new_timer->list, &current->signal->posix_timers);
spin_unlock_irq(&current->sighand->siglock);
/*
* After unlocking sighand::siglock @new_timer is subject to
* concurrent removal and cannot be touched anymore
* timer::it_lock ensures that __lock_timer() observes a fully
* initialized timer when it observes a valid timer::it_signal.
*
* sighand::siglock is required to protect signal::posix_timers.
*/
scoped_guard (spinlock_irq, &new_timer->it_lock) {
guard(spinlock)(&current->sighand->siglock);
/* This makes the timer valid in the hash table */
WRITE_ONCE(new_timer->it_signal, current->signal);
hlist_add_head(&new_timer->list, &current->signal->posix_timers);
}
/*
* After unlocking @new_timer is subject to concurrent removal and
* cannot be touched anymore
*/
return 0;
out: