netdev - Re: [PATCH net] veth: fix races around rq->rx_notify

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Date:   Thu, 10 Feb 2022 08:57:15 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] veth: fix races around rq->rx_notify_masked

On Wed, Feb 9, 2022 at 4:36 AM Toshiaki Makita
<toshiaki.makita1@...il.com> wrote:
>
> On 2022/02/09 8:28, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
>
> Thank you for handling this case.
>
> > veth being NETIF_F_LLTX enabled, we need to be more careful
> > whenever we read/write rq->rx_notify_masked.
> >
> > BUG: KCSAN: data-race in veth_xmit / veth_xmit
> >
> > w
> > value changed: 0x00 -> 0x01
>
> I'm not familiar with KCSAN.
> Does this mean rx_notify_masked value is changed while another CPU is reading it?
>

Yes.

> If so, I'm not sure there is a problem with that.

This is a problem if not annotated properly.

> At least we could call napi_schedule() twice, but that just causes one extra napi
> poll due to NAPIF_STATE_MISSED, and it happens very rarely?
>
> Toshiaki Makita

The issue might be more problematic, a compiler might play bad games,
look for load and store tearing.