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

[Simon Horman <horms@kernel.org>]

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@gmail.com>
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Thanks Stephen,

The code changes all make sense to me.

Reviewed-by: Simon Horman <horms@kernel.org>