netdev - Re: [PATCH net v2 1/4] net/sched: sch_qfq: reintroduce lmax bound check for MTU

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Date: Sat, 8 Jul 2023 10:14:51 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Pedro Tammela <pctammela@...atatu.com>
Cc: netdev@...r.kernel.org, jhs@...atatu.com, xiyou.wangcong@...il.com, 
	jiri@...nulli.us, davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	shuah@...nel.org, shaozhengchao@...wei.com, victor@...atatu.com, 
	simon.horman@...igine.com, paolo.valente@...more.it
Subject: Re: [PATCH net v2 1/4] net/sched: sch_qfq: reintroduce lmax bound
 check for MTU

On Sat, Jul 8, 2023 at 12:01 AM Pedro Tammela <pctammela@...atatu.com> wrote:
>
> 25369891fcef deletes a check for the case where no 'lmax' is
> specified which 3037933448f6 previously fixed as 'lmax'
> could be set to the device's MTU without any bound checking
> for QFQ_LMAX_MIN and QFQ_LMAX_MAX. Therefore, reintroduce the check.
>
> Fixes: 25369891fcef ("net/sched: sch_qfq: refactor parsing of netlink parameters")
> Acked-by: Jamal Hadi Salim <jhs@...atatu.com>
> Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
> ---
>  net/sched/sch_qfq.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
> index dfd9a99e6257..63a5b277c117 100644
> --- a/net/sched/sch_qfq.c
> +++ b/net/sched/sch_qfq.c
> @@ -423,10 +423,17 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>         else
>                 weight = 1;
>
> -       if (tb[TCA_QFQ_LMAX])
> +       if (tb[TCA_QFQ_LMAX]) {
>                 lmax = nla_get_u32(tb[TCA_QFQ_LMAX]);
> -       else
> +       } else {
> +               /* MTU size is user controlled */
>                 lmax = psched_mtu(qdisc_dev(sch));
> +               if (lmax < QFQ_MIN_LMAX || lmax > QFQ_MAX_LMAX) {
> +                       NL_SET_ERR_MSG_MOD(extack,
> +                                          "MTU size out of bounds for qfq");
> +                       return -EINVAL;
> +               }
> +       }
>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Speaking of psched_mtu(), I see that net/sched/sch_pie.c is using it
without holding RTNL,
so dev->mtu can be changed underneath. KCSAN could issue a warning.

Feel free to submit this fix (I am currently traveling)

diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index e98aac9d5ad5737592ab7cd409c174707cd68681..15960564e0c364ef430f1e3fcdd0e835c2f94a77
100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h
@@ -134,7 +134,7 @@ extern const struct nla_policy rtm_tca_policy[TCA_MAX + 1];
  */
 static inline unsigned int psched_mtu(const struct net_device *dev)
 {
-       return dev->mtu + dev->hard_header_len;
+       return READ_ONCE(dev->mtu) + dev->hard_header_len;
 }

 static inline struct net *qdisc_net(struct Qdisc *q)