From: Oleg Nesterov <oleg@redhat.com>
To: Kirill Tkhai <ktkhai@parallels.com>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>,
Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use after free
Date: Wed, 15 Oct 2014 21:40:44 +0200 [thread overview]
Message-ID: <20141015194044.GA4557@redhat.com> (raw)
In-Reply-To: <20141015150641.GA2755@redhat.com>
On 10/15, Oleg Nesterov wrote:
>
> On 10/15, Kirill Tkhai wrote:
> >
> > Regarding to scheduler this may be a reason of use-after-free.
> >
> > task_numa_compare() schedule()
> > rcu_read_lock() ...
> > cur = ACCESS_ONCE(dst_rq->curr) ...
> > ... rq->curr = next;
> > ... context_switch()
> > ... finish_task_switch()
> > ... put_task_struct()
> > ... __put_task_struct()
> > ... free_task_struct()
> > task_numa_assign() ...
> > get_task_struct() ...
>
> Agreed. I don't understand this code (will try to take another look later),
> but at first glance this looks wrong.
>
> At least the code like
>
> rcu_read_lock();
> get_task_struct(foreign_rq->curr);
> rcu_read_unlock();
>
> is certainly wrong. And _probably_ the problem should be fixed here. Perhaps
> we can add try_to_get_task_struct() which does atomic_inc_not_zero() ...
Yes, but perhaps in this particular case another simple fix makes more
sense. The patch below needs a comment to explain that we check PF_EXITING
because:
1. It doesn't make sense to migrate the exiting task. Although perhaps
we could check ->mm == NULL instead.
But let me repeat that I do not understand this code, I am not sure
we can equally treat is_idle_task() and PF_EXITING here...
2. If PF_EXITING is not set (or ->mm != NULL) then delayed_put_task_struct()
won't be called until we drop rcu_read_lock(), and thus get_task_struct()
is safe.
And. it seems that there is another problem? Can't task_h_load(cur) race
with itself if 2 CPU's call task_numa_migrate() and inspect the same rq
in parallel? Again, I don't understand this code, but update_cfs_rq_h_load()
doesn't look "atomic". In fact I am not even sure about task_h_load(env->p),
p == current but we do not disable preemption.
What do you think?
Oleg.
--- x/kernel/sched/fair.c
+++ x/kernel/sched/fair.c
@@ -1165,7 +1165,7 @@ static void task_numa_compare(struct tas
rcu_read_lock();
cur = ACCESS_ONCE(dst_rq->curr);
- if (cur->pid == 0) /* idle */
+ if (is_idle_task(cur) || (curr->flags & PF_EXITING))
cur = NULL;
/*
next prev parent reply other threads:[~2014-10-15 19:44 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-15 12:31 [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use after free Kirill Tkhai
2014-10-15 15:06 ` Oleg Nesterov
2014-10-15 19:40 ` Oleg Nesterov [this message]
2014-10-15 21:46 ` Kirill Tkhai
2014-10-15 22:02 ` Kirill Tkhai
2014-10-16 7:59 ` Peter Zijlstra
2014-10-16 8:16 ` Kirill Tkhai
2014-10-16 9:43 ` Peter Zijlstra
2014-10-16 9:50 ` Kirill Tkhai
2014-10-16 9:51 ` Kirill Tkhai
2014-10-16 10:04 ` Kirill Tkhai
2014-10-17 21:34 ` Oleg Nesterov
2014-10-16 7:56 ` Peter Zijlstra
2014-10-16 8:01 ` Peter Zijlstra
2014-10-16 22:05 ` Oleg Nesterov
2014-10-17 21:36 ` [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Oleg Nesterov
2014-10-18 8:15 ` Kirill Tkhai
2014-10-18 8:33 ` Kirill Tkhai
2014-10-18 19:36 ` Peter Zijlstra
2014-10-18 21:18 ` Oleg Nesterov
2014-10-19 8:20 ` Kirill Tkhai
2014-10-18 20:56 ` Oleg Nesterov
2014-10-18 23:13 ` Kirill Tkhai
2014-10-19 19:24 ` Oleg Nesterov
2014-10-19 19:37 ` Oleg Nesterov
2014-10-19 19:43 ` Oleg Nesterov
2014-10-20 9:03 ` Kirill Tkhai
2014-10-20 9:13 ` Peter Zijlstra
2014-10-20 10:36 ` Kirill Tkhai
2014-10-20 9:00 ` Kirill Tkhai
2014-10-19 21:38 ` Peter Zijlstra
2014-10-20 8:56 ` Kirill Tkhai
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