Subject: Re: [PATCH] netem: fix return value if duplicate enqueue fails

From: Stephen Hemminger <[email-masked]>
Date: Tue, 20 Aug 2024 13:22:50 -0700

On Tue, 20 Aug 2024 16:42:13 +0100
Simon Horman <[email-masked]> 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 <[email-masked]>
> > Signed-off-by: Stephen Hemminger <[email-masked]>  
> 
> Thanks Stephen,
> 
> The code changes all make sense to me.
> 
> Reviewed-by: Simon Horman <[email-masked]>

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.