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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , linux-kernel@vger.kernel.org (open list) Subject: [PATCH net v4 5/8] net/sched: netem: batch-transfer ready packets to avoid child re-entrancy Date: Mon, 6 Apr 2026 10:25:13 -0700 Message-ID: <20260406172627.210894-6-stephen@networkplumber.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260406172627.210894-1-stephen@networkplumber.org> References: <20260406172627.210894-1-stephen@networkplumber.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit netem_dequeue_child() previously transferred one packet from the tfifo to the child qdisc per dequeue call. Parents like HFSC that track class active/inactive state on qlen transitions could see an enqueue during dequeue, causing double-insertion into the eltree (CVE-2025-37890, CVE-2025-38001). Non-work-conserving children like TBF could also refuse to return a just-enqueued packet, making netem return NULL despite having backlog, which causes parents like DRR to incorrectly deactivate the class. Move all time-ready packets into the child before calling its dequeue. This separates the enqueue and dequeue phases so the parent sees consistent qlen transitions. Fixes: 50612537e9ab ("netem: fix classful handling") Signed-off-by: Stephen Hemminger --- net/sched/sch_netem.c | 49 +++++++++++++++++++++---------------------- 1 file changed, 24 insertions(+), 25 deletions(-) diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c index e264f7aefb97..b93f0e886a2b 100644 --- a/net/sched/sch_netem.c +++ b/net/sched/sch_netem.c @@ -734,8 +734,10 @@ static void netem_slot_account(struct netem_sched_data *q, } /* - * Transfer time-ready packets from the tfifo into the child qdisc, - * then dequeue from the child. + * Transfer all time-ready packets from the tfifo into the child qdisc, + * then dequeue from the child. Batching the transfers avoids calling + * qdisc_enqueue() inside the parent's dequeue path, which confuses + * parents that track active/inactive state on qlen transitions (HFSC). */ static struct sk_buff *netem_dequeue_child(struct Qdisc *sch) { @@ -743,31 +745,28 @@ static struct sk_buff *netem_dequeue_child(struct Qdisc *sch) u64 now = ktime_get_ns(); struct sk_buff *skb; - skb = netem_peek(q); - if (skb) { - u64 time_to_send = netem_skb_cb(skb)->time_to_send; - - if (q->slot.slot_next && q->slot.slot_next < time_to_send) - get_slot_next(q, now); - - if (time_to_send <= now && q->slot.slot_next <= now) { - struct sk_buff *to_free = NULL; - unsigned int pkt_len; - int err; + while ((skb = netem_peek(q)) != NULL) { + struct sk_buff *to_free = NULL; + unsigned int pkt_len; + int err; - skb = netem_pull_tfifo(q, sch); - netem_slot_account(q, skb, now); + if (netem_skb_cb(skb)->time_to_send > now) + break; + if (q->slot.slot_next && q->slot.slot_next > now) + break; - pkt_len = qdisc_pkt_len(skb); - err = qdisc_enqueue(skb, q->qdisc, &to_free); - kfree_skb_list(to_free); - if (err != NET_XMIT_SUCCESS) { - if (net_xmit_drop_count(err)) - qdisc_qstats_drop(sch); - sch->qstats.backlog -= pkt_len; - sch->q.qlen--; - qdisc_tree_reduce_backlog(sch, 1, pkt_len); - } + skb = netem_pull_tfifo(q, sch); + netem_slot_account(q, skb, now); + + pkt_len = qdisc_pkt_len(skb); + err = qdisc_enqueue(skb, q->qdisc, &to_free); + kfree_skb_list(to_free); + if (unlikely(err != NET_XMIT_SUCCESS)) { + if (net_xmit_drop_count(err)) + qdisc_qstats_drop(sch); + sch->qstats.backlog -= pkt_len; + sch->q.qlen--; + qdisc_tree_reduce_backlog(sch, 1, pkt_len); } } -- 2.53.0