netdev - Re: [PATCH net] net/sched: sch_taprio: fix possible use-after-free

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Date:   Sun, 15 Jan 2023 16:35:56 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
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>,
        Alexander Potapenko <glider@...gle.com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: Re: [PATCH net] net/sched: sch_taprio: fix possible use-after-free

On Fri, Jan 13, 2023 at 04:48:49PM +0000, Eric Dumazet wrote:
> syzbot reported a nasty crash [1] in net_tx_action() which
> made little sense until we got a repro.
> 
> This repro installs a taprio qdisc, but providing an
> invalid TCA_RATE attribute.
> 
> qdisc_create() has to destroy the just initialized
> taprio qdisc, and taprio_destroy() is called.
> 
> However, the hrtimer used by taprio had already fired,
> therefore advance_sched() called __netif_schedule().
> 
> Then net_tx_action was trying to use a destroyed qdisc.
> 
> We can not undo the __netif_schedule(), so we must wait
> until one cpu serviced the qdisc before we can proceed.
> 

This workaround looks a bit ugly. I think we _may_ be able to make
hrtimer_start() as the last step of the initialization, IOW, move other
validations and allocations before it.

Can you share your reproducer?

Thanks,