netdev - Re: [PATCH net-n] net/sched: stop qdisc_tree_reduce_backlog on TC_H

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Message-ID: <Zx0dHmOtsI6FmOeN@pop-os.localdomain>
Date: Sat, 26 Oct 2024 09:47:26 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: netdev@...r.kernel.org, markovicbudimir@...il.com, victor@...atatu.com,
	pctammela@...atatu.com, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, jiri@...nulli.us
Subject: Re: [PATCH net-n] net/sched: stop qdisc_tree_reduce_backlog on
 TC_H_ROOT

On Thu, Oct 24, 2024 at 12:55:47PM -0400, Jamal Hadi Salim wrote:
> From: Pedro Tammela <pctammela@...atatu.com>
> 
> In qdisc_tree_reduce_backlog, Qdiscs with major handle ffff: are assumed
> to be either root or ingress. This assumption is bogus since it's valid
> to create egress qdiscs with major handle ffff:
> Budimir Markovic found that for qdiscs like DRR that maintain an active
> class list, it will cause a UAF with a dangling class pointer.
> 
> In 066a3b5b2346, the concern was to avoid iterating over the ingress
> qdisc since its parent is itself. The proper fix is to stop when parent
> TC_H_ROOT is reached because the only way to retrieve ingress is when a
> hierarchy which does not contain a ffff: major handle call into
> qdisc_lookup with TC_H_MAJ(TC_H_ROOT).
> 
> In the scenario where major ffff: is an egress qdisc in any of the tree
> levels, the updates will also propagate to TC_H_ROOT, which then the
> iteration must stop.
> 
> Fixes: 066a3b5b2346 ("[NET_SCHED] sch_api: fix qdisc_tree_decrease_qlen() loop")
> Reported-by: Budimir Markovic <markovicbudimir@...il.com>
> Suggested-by: Jamal Hadi Salim <jhs@...atatu.com>
> Tested-by: Victor Nogueira <victor@...atatu.com>
> Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
> Signed-off-by: Jamal Hadi Salim <jhs@...atatu.com>
> 
>  net/sched/sch_api.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Can we also add a selftest since it is reproducible?

I am not saying you have to put it together with this patch, a separate patch is
certainly okay.

Thanks.