linux-kernel - Re: [PATCH] netem: fix return value if duplicate enqueue fails

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Message-ID: <20240820132250.28bd507b@hermes.local>
Date: Tue, 20 Aug 2024 13:22:50 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, Budimir Markovic <markovicbudimir@...il.com>,
 Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>,
 Jiri Pirko <jiri@...nulli.us>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>, Sheng Lan <lansheng@...wei.com>, open list
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] netem: fix return value if duplicate enqueue fails

On Tue, 20 Aug 2024 16:42:13 +0100
Simon Horman <horms@...nel.org> wrote:

> On Mon, Aug 19, 2024 at 10:56:45AM -0700, Stephen Hemminger wrote:
> > There is a bug in netem_enqueue() introduced by
> > commit 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> > that can lead to a use-after-free.
> > 
> > This commit made netem_enqueue() always return NET_XMIT_SUCCESS
> > when a packet is duplicated, which can cause the parent qdisc's q.qlen to be
> > mistakenly incremented. When this happens qlen_notify() may be skipped on the
> > parent during destruction, leaving a dangling pointer for some classful qdiscs
> > like DRR.
> > 
> > There are two ways for the bug happen:
> > 
> > - If the duplicated packet is dropped by rootq->enqueue() and then the original
> >   packet is also dropped.
> > - If rootq->enqueue() sends the duplicated packet to a different qdisc and the
> >   original packet is dropped.
> > 
> > In both cases NET_XMIT_SUCCESS is returned even though no packets are enqueued
> > at the netem qdisc.
> > 
> > The fix is to defer the enqueue of the duplicate packet until after the
> > original packet has been guaranteed to return NET_XMIT_SUCCESS.
> > 
> > Fixes: 5845f706388a ("net: netem: fix skb length BUG_ON in __skb_to_sgvec")
> > Reported-by: Budimir Markovic <markovicbudimir@...il.com>
> > Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>  
> 
> Thanks Stephen,
> 
> The code changes all make sense to me.
> 
> Reviewed-by: Simon Horman <horms@...nel.org>

Reported by doesn't really do enough credit here. The commit log is from
the original bug report which had more detail. Fixing it was the easy part.