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

From: Simon Horman <[email-masked]>
Date: Tue, 20 Aug 2024 16:42:13 +0100

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]>