linux-kernel - Re: KCSAN: data-race in exit_signals / prepare

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Date:   Mon, 21 Oct 2019 14:00:30 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     syzbot <syzbot+492a4acccd8fc75ddfd0@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, arnd@...db.de, christian@...uner.io,
        deepa.kernel@...il.com, ebiederm@...ssion.com, elver@...gle.com,
        guro@...com, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, will@...nel.org
Subject: Re: KCSAN: data-race in exit_signals / prepare_signal

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 middle 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 signals").
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 |= 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.