netdev - Re: [PATCH] net/sched: act_skbedit: fix divide-by-zero in tcf_skbedit

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Message-ID: <CANn89iL5F=zN2t-LfBPtR6xzCQjVr8XB+bHu=LLYCvaao3Fx0Q@mail.gmail.com>
Date: Wed, 11 Feb 2026 20:07:12 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Ruitong Liu <cnitlrt@...il.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, 
	horms@...nel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
	Shuyuan Liu <L0x1c3r@...il.com>
Subject: Re: [PATCH] net/sched: act_skbedit: fix divide-by-zero in tcf_skbedit_hash()

On Wed, Feb 11, 2026 at 7:48 PM Ruitong Liu <cnitlrt@...il.com> wrote:
>
> mapping_mod is computed as:
>
>   mapping_mod = queue_mapping_max - queue_mapping + 1;
>
> mapping_mod is stored as u16, so the calculation can overflow when
> queue_mapping=0 and queue_mapping_max=0xffff. In this case the value
> wraps to 0, leading to a divide-by-zero in tcf_skbedit_hash():
>
>   queue_mapping += skb_get_hash(skb) % params->mapping_mod;
>
> Fix it by using a wider type for mapping_mod and performing the
> calculation in u32, preventing overflow to zero.
>
> Fixes: 38a6f0865796 ("net: sched: support hash selecting tx queue")
> Cc: stable@...r.kernel.org # 6.12+
> Reported-by: Ruitong Liu <cnitlrt@...il.com>
> Reported-by: Shuyuan Liu <L0x1c3r@...il.com>
> Signed-off-by: Ruitong Liu <cnitlrt@...il.com>
> ---

I do not think we want to support very large mapping_mod values, this
makes no sense.

Please reject wrong configuration instead.

diff --git a/net/sched/act_skbedit.c b/net/sched/act_skbedit.c
index 8c1d1554f657..0ab83dc776d1 100644
--- a/net/sched/act_skbedit.c
+++ b/net/sched/act_skbedit.c
@@ -126,7 +126,7 @@ static int tcf_skbedit_init(struct net *net,
struct nlattr *nla,
        struct tcf_skbedit *d;
        u32 flags = 0, *priority = NULL, *mark = NULL, *mask = NULL;
        u16 *queue_mapping = NULL, *ptype = NULL;
-       u16 mapping_mod = 1;
+       u32 mapping_mod = 1;
        bool exists = false;
        int ret = 0, err;
        u32 index;
@@ -194,6 +194,10 @@ static int tcf_skbedit_init(struct net *net,
struct nlattr *nla,
                        }

                        mapping_mod = *queue_mapping_max - *queue_mapping + 1;
+                       if (mapping_mod > 0xFFFF) {
+                               NL_SET_ERR_MSG_MOD(extack, "The range
of queue_mapping is invalid.");
+                               return -EINVAL;
+                       }
                        flags |= SKBEDIT_F_TXQ_SKBHASH;
                }
                if (*pure_flags & SKBEDIT_F_INHERITDSFIELD)