netdev - Re: Very Minor Patch to loopback.c

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Message-ID: <CAGVrzcaxhQfV_BRWew8omfQhDWzzFUpe6c7yhdjR=qSTX1zXvA@mail.gmail.com>
Date:	Thu, 12 Jun 2014 13:27:35 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Zachary <zacharyw09264@...il.com>
Cc:	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Very Minor Patch to loopback.c

2014-06-12 12:53 GMT-07:00 Zachary <zacharyw09264@...il.com>:
> Very minor patch to loopback.c that I noticed while comparing the
> dummy.c code to loopback.c code (dummy.c was based on loopback.c).  I
> did not see a maintainer in MAINTAINERS in my cursory glance, so I
> apologize if this is to the wrong place.  I am testing this patch
> right now on my laptop and it appears to be working without issues.
> It also compiled without warnings.

Your patch introduces a potential use after free type of bug. As soon
as the skb pointer has been passed to netif_rx(), the network stack is
allowed to free this skb as soon as it has determined it is unused,
this is the reason why this 'len' variable exists in the first place.

>
> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> index bb96409..f9f88d7 100644
> --- a/drivers/net/loopback.c
> +++ b/drivers/net/loopback.c
> @@ -22,6 +22,7 @@
>   *                                      interface.
>   *             Alexey Kuznetsov:       Potential hang under some extreme
>   *                                     cases removed.
> + *             Zachary Winnerman:  Extremely minor patch in loopback_xmit
>   *
>   *             This program is free software; you can redistribute it and/or
>   *             modify it under the terms of the GNU General Public License
> @@ -72,7 +73,6 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
>                                  struct net_device *dev)
>  {
>         struct pcpu_lstats *lb_stats;
> -       int len;
>
>         skb_orphan(skb);
>
> @@ -86,10 +86,9 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
>         /* it's OK to use per_cpu_ptr() because BHs are off */
>         lb_stats = this_cpu_ptr(dev->lstats);
>
> -       len = skb->len;
>         if (likely(netif_rx(skb) == NET_RX_SUCCESS)) {
>                 u64_stats_update_begin(&lb_
> stats->syncp);
> -               lb_stats->bytes += len;
> +               lb_stats->bytes += skb->len;
>                 lb_stats->packets++;
>                 u64_stats_update_end(&lb_stats->syncp);
>         }
> --
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-- 
Florian
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