Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 21 Oct 2019 14:00:30 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     syzbot <syzbot+492a4acccd8fc75ddfd0@syzkaller.appspotmail.com>,
        akpm@linux-foundation.org, arnd@arndb.de, christian@brauner.io,
        deepa.kernel@gmail.com, ebiederm@xmission.com, elver@google.com,
        guro@fb.com, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, will@kernel.org
Subject: Re: KCSAN: data-race in exit_signals / prepare_signal
Message-ID: <20191021120029.GA24935@redhat.com>
References: <0000000000003b1e8005956939f1@google.com>
 <20191021111920.frmc3njkha4c3a72@wittgenstein>
MIME-Version: 1.0
In-Reply-To: <20191021111920.frmc3njkha4c3a72@wittgenstein>
User-Agent: Mutt/1.5.24 (2015-08-30)
Content-Type: text/plain; charset=WINDOWS-1252
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10/21, Christian Brauner wrote:
>
> This traces back to Oleg fixing a race between a group stop and a thread
> exiting before it notices that it has a pending signal or is in the middl=
e of
> do_exit() already, causing group stop to get wacky.
> The original commit to fix this race is
> commit d12619b5ff56 ("fix group stop with exit race") which took sighand
> lock before setting PF_EXITING on the thread.

Not really... sig_task_ignored() didn't check task->flags until the recent
33da8e7c81 ("signal: Allow cifs and drbd to receive their terminating signa=
ls").
But I think this doesn't matter, see below.

> If the race really matters and given how tsk->flags is currently accessed
> everywhere the simple fix for now might be:
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index c4da1ef56fdf..cf61e044c4cc 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2819,7 +2819,9 @@ void exit_signals(struct task_struct *tsk)
>         cgroup_threadgroup_change_begin(tsk);
>
>         if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) {
> +               spin_lock_irq(&tsk->sighand->siglock);
>                 tsk->flags |=3D PF_EXITING;
> +               spin_unlock_irq(&tsk->sighand->siglock);

Well, exit_signals() tries to avoid ->siglock in this case....

But this doesn't matter. IIUC the problem is not that exit_signals() sets
PF_EXITING, the problem is that it writes to tsk->flags and kasan detects
the data race.

For example, freezable_schedule() which sets PF_FREEZER_SKIP can equally
"race" with sig_task_ignored() or with ANY other code which checks this
task's flags.

I think this is WONTFIX.

Oleg.