[Cake] [PATCH net] sch_cake: Return __NET_XMIT_STOLEN when consuming enqueued skb

[Eric Dumazet]

On Wed, Aug 31, 2022 at 2:25 AM Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>
> When the GSO splitting feature of sch_cake is enabled, GSO superpackets
> will be broken up and the resulting segments enqueued in place of the
> original skb. In this case, CAKE calls consume_skb() on the original skb,
> but still returns NET_XMIT_SUCCESS. This can confuse parent qdiscs into
> assuming the original skb still exists, when it really has been freed. Fix
> this by adding the __NET_XMIT_STOLEN flag to the return value in this case.
>

I think you forgot to give credits to the team who discovered this issue.

Something like this

Reported-by: zdi-disclosures at trendmicro.com # ZDI-CAN-18231



> Fixes: 0c850344d388 ("sch_cake: Conditionally split GSO segments")
> Signed-off-by: Toke Høiland-Jørgensen <toke at toke.dk>
> ---
>  net/sched/sch_cake.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
> index a43a58a73d09..a04928082e4a 100644
> --- a/net/sched/sch_cake.c
> +++ b/net/sched/sch_cake.c
> @@ -1713,6 +1713,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>         }
>         idx--;
>         flow = &b->flows[idx];
> +       ret = NET_XMIT_SUCCESS;
>
>         /* ensure shaper state isn't stale */
>         if (!b->tin_backlog) {
> @@ -1771,6 +1772,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>
>                 qdisc_tree_reduce_backlog(sch, 1-numsegs, len-slen);
>                 consume_skb(skb);
> +               ret |= __NET_XMIT_STOLEN;
>         } else {
>                 /* not splitting */
>                 cobalt_set_enqueue_time(skb, now);
> @@ -1904,7 +1906,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>                 }
>                 b->drop_overlimit += dropped;
>         }
> -       return NET_XMIT_SUCCESS;
> +       return ret;
>  }
>
>  static struct sk_buff *cake_dequeue_one(struct Qdisc *sch)
> --
> 2.37.2
>