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Subject: Re: [PATCH] xfrm : lock input tasklet skb queue
To:     Tom Rix <trix@redhat.com>, steffen.klassert@secunet.com,
        herbert@gondor.apana.org.au, davem@davemloft.net,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <CACVy4SVuw0Qbjiv6PLRn1symoxGzyBMZx2F5O23+jGZG6WHuYA@mail.gmail.com>
From:   Joerg Vehlow <lkml@jv-coder.de>
Message-ID: <ac5f327b-d693-31cb-089f-b0880a4e298b@jv-coder.de>
Date:   Tue, 22 Oct 2019 10:58:44 +0200
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Hi,

I already send a patch on 2019-09-09 to this mailing list with a similar 
issue[1].
Sadly no replies, although this is a huge bug in the rt kernel.
I fixed it a bit differently, using smaller locked regions.
You have also propably a bug in your patch, because trans->queue.lock is
no initialized by __skb_queue_head_init (in xfrm_input_init)

Jörg

[1] https://lkml.org/lkml/2019/9/9/111

Am 20.10.2019 um 17:46 schrieb Tom Rix:
> On PREEMPT_RT_FULL while running netperf, a corruption
> of the skb queue causes an oops.
>
> This appears to be caused by a race condition here
>          __skb_queue_tail(&trans->queue, skb);
>          tasklet_schedule(&trans->tasklet);
> Where the queue is changed before the tasklet is locked by
> tasklet_schedule.
>
> The fix is to use the skb queue lock.
>
> Signed-off-by: Tom Rix <trix@redhat.com>
> ---
>   net/xfrm/xfrm_input.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
> index 9b599ed66d97..226dead86828 100644
> --- a/net/xfrm/xfrm_input.c
> +++ b/net/xfrm/xfrm_input.c
> @@ -758,12 +758,16 @@ static void xfrm_trans_reinject(unsigned long data)
>       struct xfrm_trans_tasklet *trans = (void *)data;
>       struct sk_buff_head queue;
>       struct sk_buff *skb;
> +    unsigned long flags;
>
>       __skb_queue_head_init(&queue);
> +    spin_lock_irqsave(&trans->queue.lock, flags);
>       skb_queue_splice_init(&trans->queue, &queue);
>
>       while ((skb = __skb_dequeue(&queue)))
>           XFRM_TRANS_SKB_CB(skb)->finish(dev_net(skb->dev), NULL, skb);
> +
> +    spin_unlock_irqrestore(&trans->queue.lock, flags);
>   }
>
>   int xfrm_trans_queue(struct sk_buff *skb,
> @@ -771,15 +775,20 @@ int xfrm_trans_queue(struct sk_buff *skb,
>                      struct sk_buff *))
>   {
>       struct xfrm_trans_tasklet *trans;
> +    unsigned long flags;
>
>       trans = this_cpu_ptr(&xfrm_trans_tasklet);
> +    spin_lock_irqsave(&trans->queue.lock, flags);
>
> -    if (skb_queue_len(&trans->queue) >= netdev_max_backlog)
> +    if (skb_queue_len(&trans->queue) >= netdev_max_backlog) {
> +        spin_unlock_irqrestore(&trans->queue.lock, flags);
>           return -ENOBUFS;
> +    }
>
>       XFRM_TRANS_SKB_CB(skb)->finish = finish;
>       __skb_queue_tail(&trans->queue, skb);
>       tasklet_schedule(&trans->tasklet);
> +    spin_unlock_irqrestore(&trans->queue.lock, flags);
>       return 0;
>   }
>   EXPORT_SYMBOL(xfrm_trans_queue);