netdev - Re: [PATCH] netem: fix possible skb leak

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Message-ID: <20120430094443.3ace544c@nehalam.linuxnetplumber.net>
Date:	Mon, 30 Apr 2012 09:44:43 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...l.org>
Subject: Re: [PATCH] netem: fix possible skb leak

On Mon, 30 Apr 2012 18:40:16 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Mon, 2012-04-30 at 09:25 -0700, Stephen Hemminger wrote:
> > On Sun, 29 Apr 2012 21:08:22 +0200
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > 
> > > skb_checksum_help(skb) can return an error, we must free skb in this
> > > case. qdisc_drop(skb, sch) can also be feeded with a NULL skb (if
> > > skb_unshare() failed), so lets use this generic helper.
> > > 
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Stephen Hemminger <shemminger@...l.org>
> > > ---
> > >  net/sched/sch_netem.c |    6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> > > index 5da548f..ebd2296 100644
> > > --- a/net/sched/sch_netem.c
> > > +++ b/net/sched/sch_netem.c
> > > @@ -408,10 +408,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> > >  	if (q->corrupt && q->corrupt >= get_crandom(&q->corrupt_cor)) {
> > >  		if (!(skb = skb_unshare(skb, GFP_ATOMIC)) ||
> > >  		    (skb->ip_summed == CHECKSUM_PARTIAL &&
> > > -		     skb_checksum_help(skb))) {
> > > -			sch->qstats.drops++;
> > > -			return NET_XMIT_DROP;
> > > -		}
> > > +		     skb_checksum_help(skb)))
> > > +			return qdisc_drop(skb, sch);
> > >  
> > >  		skb->data[net_random() % skb_headlen(skb)] ^= 1<<(net_random() % 8);
> > >  	}
> > > 
> > > 
> > 
> > This would crater if skb_unshare() returned NULL. I think the conditional
> > needs to be split into two paths.
> 
> Maybe you can read the changelog where I explained this was safe ;)

Ok, I am surprised that qdisc_drop() is safe with NULL skb, but yes.

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