Re: [PATCH] taskstats: fix data-race

From: Christian Brauner <christian.brauner@ubuntu.com>
To: Andrea Parri <parri.andrea@gmail.com>
Cc: bsingharora@gmail.com, elver@google.com,
	linux-kernel@vger.kernel.org,
	syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com,
	syzkaller-bugs@googlegroups.com,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH] taskstats: fix data-race
Date: Mon, 7 Oct 2019 12:50:45 +0200	[thread overview]
Message-ID: <20191007105044.x7srxpcedfo3o5m4@wittgenstein> (raw)
In-Reply-To: <20191007104039.GA16085@andrea.guest.corp.microsoft.com>

On Mon, Oct 07, 2019 at 12:40:39PM +0200, Andrea Parri wrote:
> 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 <dvyukov@google.com>
> > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> 
> 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)?

Of course. I totally forgot the memory barrier documentation requirement.

Christian