netdev - Re: [PATCH net] net: hsr: remove seqnr

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Message-ID: <CANn89i+K8SSmsnzVQB8D_cKNk1p_WLwxipUjGT0C6YU+G+5mbw@mail.gmail.com>
Date: Thu, 5 Sep 2024 14:26:30 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net: hsr: remove seqnr_lock

On Thu, Sep 5, 2024 at 2:17 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2024-09-04 13:37:25 [+0000], Eric Dumazet wrote:
> > syzbot found a new splat [1].
> >
> > Instead of adding yet another spin_lock_bh(&hsr->seqnr_lock) /
> > spin_unlock_bh(&hsr->seqnr_lock) pair, remove seqnr_lock
> > and use atomic_t for hsr->sequence_nr and hsr->sup_sequence_nr.
> >
> > This also avoid a race in hsr_fill_info().
>
> You obtain to sequence nr without locking so two CPUs could submit skbs
> at the same time. Wouldn't this allow the race I described in commit
>    06afd2c31d338 ("hsr: Synchronize sending frames to have always incremented outgoing seq nr.")
>
> to happen again? Then one skb would be dropped while sending because it
> has lower sequence nr but in fact it was not yet sent.
>

A network protocol unable to cope with reorders can not live really.

If this is an issue, this should be fixed at the receiving side.