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From: Dmitry Vyukov <dvyukov@google.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Andrea Parri <parri.andrea@gmail.com>,
	bsingharora@gmail.com, Marco Elver <elver@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	syzbot <syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] taskstats: fix data-race
Date: Mon, 7 Oct 2019 16:08:41 +0200	[thread overview]
Message-ID: <CACT4Y+Y3oohjuM59Mkdhgpv1UJT_Z_m88vSVtkU5Eq=yRTU2eg@mail.gmail.com> (raw)
In-Reply-To: <20191007135527.qd5ibfyajnihsrsh@wittgenstein>

On Mon, Oct 7, 2019 at 3:55 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Mon, Oct 07, 2019 at 03:50:47PM +0200, Dmitry Vyukov wrote:
> > On Mon, Oct 7, 2019 at 3:18 PM Andrea Parri <parri.andrea@gmail.com> wrote:
> > >
> > > On Mon, Oct 07, 2019 at 01:01:17PM +0200, Christian Brauner wrote:
> > > > When assiging and testing taskstats in taskstats_exit() there's a race
> > > > when writing and reading sig->stats when a thread-group with more than
> > > > one thread exits:
> > > >
> > > > cpu0:
> > > > thread catches fatal signal and whole thread-group gets taken down
> > > >  do_exit()
> > > >  do_group_exit()
> > > >  taskstats_exit()
> > > >  taskstats_tgid_alloc()
> > > > The tasks reads sig->stats holding sighand lock seeing garbage.
> > >
> > > You meant "without holding sighand lock" here, right?
> > >
> > >
> > > >
> > > > cpu1:
> > > > task calls exit_group()
> > > >  do_exit()
> > > >  do_group_exit()
> > > >  taskstats_exit()
> > > >  taskstats_tgid_alloc()
> > > > The task takes sighand lock and assigns new stats to sig->stats.
> > > >
> > > > Fix this by using READ_ONCE() and smp_store_release().
> > > >
> > > > Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com
> > > > Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation")
> > > > Cc: stable@vger.kernel.org
> > > > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> > > > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > > > Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com
> > > > ---
> > > > /* v1 */
> > > > Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com
> > > >
> > > > /* v2 */
> > > > - Dmitry Vyukov <dvyukov@google.com>, Marco Elver <elver@google.com>:
> > > >   - fix the original double-checked locking using memory barriers
> > > >
> > > > /* v3 */
> > > > - Andrea Parri <parri.andrea@gmail.com>:
> > > >   - document memory barriers to make checkpatch happy
> > > > ---
> > > >  kernel/taskstats.c | 21 ++++++++++++---------
> > > >  1 file changed, 12 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> > > > index 13a0f2e6ebc2..978d7931fb65 100644
> > > > --- a/kernel/taskstats.c
> > > > +++ b/kernel/taskstats.c
> > > > @@ -554,24 +554,27 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
> > > >  static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
> > > >  {
> > > >       struct signal_struct *sig = tsk->signal;
> > > > -     struct taskstats *stats;
> > > > +     struct taskstats *stats_new, *stats;
> > > >
> > > > -     if (sig->stats || thread_group_empty(tsk))
> > > > -             goto ret;
> > > > +     /* Pairs with smp_store_release() below. */
> > > > +     stats = READ_ONCE(sig->stats);
> > >
> > > This pairing suggests that the READ_ONCE() is heading an address
> > > dependency, but I fail to identify it: what is the target memory
> > > access of such a (putative) dependency?
> >
> > I would assume callers of this function access *stats. So the
> > dependency is between loading stats and accessing *stats.
>
> Right, but why READ_ONCE() and not smp_load_acquire here?

Because if all memory accesses we need to order have data dependency
between them, READ_ONCE is enough and is cheaper on some archs (e.g.
ARM).
In our case there is a data dependency between loading of stats and
accessing *stats (only Alpha could reorder that, other arches can't
load via a pointer before loading the pointer itself (sic!)).

  reply	other threads:[~2019-10-07 14:08 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-05  4:26 KCSAN: data-race in taskstats_exit / taskstats_exit syzbot
2019-10-05  4:29 ` Dmitry Vyukov
2019-10-05 11:29   ` Christian Brauner
2019-10-05 11:28 ` [PATCH] taskstats: fix data-race Christian Brauner
2019-10-05 13:33   ` Marco Elver
2019-10-05 14:15     ` Christian Brauner
2019-10-05 14:34       ` Marco Elver
2019-10-06 10:00   ` Dmitry Vyukov
2019-10-06 10:59     ` Christian Brauner
2019-10-06 23:52   ` Christian Brauner
2019-10-07  7:31     ` Dmitry Vyukov
2019-10-07  9:29       ` Christian Brauner
2019-10-07 10:40     ` Andrea Parri
2019-10-07 10:50       ` Christian Brauner
2019-10-07 11:01       ` [PATCH v2] " Christian Brauner
2019-10-07 13:18         ` Andrea Parri
2019-10-07 13:28           ` Christian Brauner
2019-10-07 13:50           ` Dmitry Vyukov
2019-10-07 13:55             ` Christian Brauner
2019-10-07 14:08               ` Dmitry Vyukov [this message]
2019-10-07 14:10                 ` Christian Brauner
2019-10-07 14:14             ` Andrea Parri
2019-10-07 14:18               ` Dmitry Vyukov
2019-10-08 14:20                 ` Andrea Parri
2019-10-08 14:24                   ` Christian Brauner
2019-10-08 15:26                     ` Andrea Parri
2019-10-08 15:35                       ` Christian Brauner
2019-10-08 15:44                         ` Andrea Parri
2019-10-09 11:31                           ` [PATCH] " Christian Brauner
2019-10-09 11:40                             ` Christian Brauner
2019-10-09 11:48                             ` [PATCH v5] " Christian Brauner
2019-10-09 12:08                               ` Andrea Parri
2019-10-09 13:26                               ` Christian Brauner
2019-10-21 11:33                               ` [PATCH v6] " Christian Brauner
2019-10-21 12:19                                 ` Rasmus Villemoes
2019-10-21 13:04                                   ` Christian Brauner
2019-11-29 17:56                                     ` Will Deacon
2019-11-30 15:08                                       ` Christian Brauner
2019-10-23 12:16                                 ` Andrea Parri
2019-10-23 12:39                                   ` Dmitry Vyukov
2019-10-23 13:11                                     ` Christian Brauner
2019-10-23 13:20                                       ` Dmitry Vyukov
2019-10-24 11:31                                     ` Andrea Parri
2019-10-24 11:51                                       ` Dmitry Vyukov
2019-10-24 13:05                                         ` Andrea Parri
2019-10-24 13:13                                           ` Dmitry Vyukov
2019-10-24 13:21                                             ` Christian Brauner
2019-10-24 13:34                                               ` Dmitry Vyukov
2019-10-24 13:43                                             ` Andrea Parri
2019-10-24 13:58                                               ` Dmitry Vyukov
2019-10-24 14:40                                                 ` Andrea Parri
2019-10-24 14:49                                                   ` Dmitry Vyukov
2019-11-29 17:57                                 ` Will Deacon
2019-10-09 11:48                             ` [PATCH] " Marco Elver
2019-10-09 11:53                               ` Christian Brauner
2019-11-06  0:27   ` Balbir Singh
2019-11-06  0:09 ` KCSAN: data-race in taskstats_exit / taskstats_exit Balbir Singh
2019-11-06 10:23   ` Marco Elver
2019-11-07 10:39     ` Balbir Singh
2019-11-08  0:54     ` Balbir Singh
2019-11-08  8:55       ` Dmitry Vyukov
2019-11-09  3:42         ` Balbir Singh

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