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Mon, 07 Oct 2019 03:40:46 -0700 (PDT) Date: Mon, 7 Oct 2019 12:40:39 +0200 From: Andrea Parri To: Christian Brauner Cc: bsingharora@gmail.com, elver@google.com, linux-kernel@vger.kernel.org, syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, Dmitry Vyukov Subject: Re: [PATCH] taskstats: fix data-race Message-ID: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> References: <20191005112806.13960-1-christian.brauner@ubuntu.com> <20191006235216.7483-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191006235216.7483-1-christian.brauner@ubuntu.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Christian, On Mon, Oct 07, 2019 at 01:52:16AM +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. > > 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 > Cc: Dmitry Vyukov > Signed-off-by: Christian Brauner FYI, checkpatch.pl says: WARNING: memory barrier without comment #62: FILE: kernel/taskstats.c:568: + smp_store_release(&sig->stats, stats_new); Maybe you can make checkpatch.pl happy ;-) and add a comment to stress the 'pairing' between this barrier and the added READ_ONCE() (as Dmitry was alluding to in a previous post)? Thanks, Andrea > --- > /* v1 */ > Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com > > /* v2 */ > - Dmitry Vyukov , Marco Elver : > - fix the original double-checked locking using memory barriers > --- > kernel/taskstats.c | 19 ++++++++++--------- > 1 file changed, 10 insertions(+), 9 deletions(-) > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > index 13a0f2e6ebc2..8ee046e8a792 100644 > --- a/kernel/taskstats.c > +++ b/kernel/taskstats.c > @@ -554,24 +554,25 @@ 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; > + stats = READ_ONCE(sig->stats); > + if (stats || thread_group_empty(tsk)) > + return stats; > > /* No problem if kmem_cache_zalloc() fails */ > - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > spin_lock_irq(&tsk->sighand->siglock); > if (!sig->stats) { > - sig->stats = stats; > - stats = NULL; > + smp_store_release(&sig->stats, stats_new); > + stats_new = NULL; > } > spin_unlock_irq(&tsk->sighand->siglock); > > - if (stats) > - kmem_cache_free(taskstats_cache, stats); > -ret: > + if (stats_new) > + kmem_cache_free(taskstats_cache, stats_new); > + > return sig->stats; > } > > -- > 2.23.0 >