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Message-ID: <20251126194513.3984722-1-xmei5@asu.edu>
Date: Wed, 26 Nov 2025 12:45:12 -0700
From: Xiang Mei <xmei5@....edu>
To: security@...nel.org
Cc: netdev@...r.kernel.org,
toke@...e.dk,
xiyou.wangcong@...il.com,
cake@...ts.bufferbloat.net,
bestswngs@...il.com,
Xiang Mei <xmei5@....edu>
Subject: [PATCH net v7 1/2] net/sched: sch_cake: Fix incorrect qlen reduction in cake_drop
In cake_drop(), qdisc_tree_reduce_backlog() is used to update the qlen
and backlog of the qdisc hierarchy. Its caller, cake_enqueue(), assumes
that the parent qdisc will enqueue the current packet. However, this
assumption breaks when cake_enqueue() returns NET_XMIT_CN: the parent
qdisc stops enqueuing current packet, leaving the tree qlen/backlog
accounting inconsistent. This mismatch can lead to a NULL dereference
(e.g., when the parent Qdisc is qfq_qdisc).
This patch computes the qlen/backlog delta in a more robust way by
observing the difference before and after the series of cake_drop()
calls, and then compensates the qdisc tree accounting if cake_enqueue()
returns NET_XMIT_CN.
To ensure correct compensation when ACK thinning is enabled, a new
variable is introduced to keep qlen unchanged.
Fixes: 15de71d06a40 ("net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit")
Signed-off-by: Xiang Mei <xmei5@....edu>
---
v2: add missing cc
v3: move qdisc_tree_reduce_backlog out of cake_drop
v4: remove redundant variable and handle ack branch correctly
v5: add the PoC as a test case
v6: split test and patch; fix the wrong drop count
v7: remove redundant comments
---
net/sched/sch_cake.c | 58 ++++++++++++++++++++++++--------------------
1 file changed, 32 insertions(+), 26 deletions(-)
diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 32bacfc314c2..d325a90cde9e 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1597,7 +1597,6 @@ static unsigned int cake_drop(struct Qdisc *sch, struct sk_buff **to_free)
qdisc_drop_reason(skb, sch, to_free, SKB_DROP_REASON_QDISC_OVERLIMIT);
sch->q.qlen--;
- qdisc_tree_reduce_backlog(sch, 1, len);
cake_heapify(q, 0);
@@ -1743,14 +1742,14 @@ static void cake_reconfigure(struct Qdisc *sch);
static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
struct sk_buff **to_free)
{
+ u32 idx, tin, prev_qlen, prev_backlog, drop_id;
struct cake_sched_data *q = qdisc_priv(sch);
- int len = qdisc_pkt_len(skb);
- int ret;
+ int len = qdisc_pkt_len(skb), ret;
struct sk_buff *ack = NULL;
ktime_t now = ktime_get();
struct cake_tin_data *b;
struct cake_flow *flow;
- u32 idx, tin;
+ bool same_flow = false;
/* choose flow to insert into */
idx = cake_classify(sch, &b, skb, q->flow_mode, &ret);
@@ -1823,6 +1822,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
consume_skb(skb);
} else {
/* not splitting */
+ int ack_pkt_len = 0;
+
cobalt_set_enqueue_time(skb, now);
get_cobalt_cb(skb)->adjusted_len = cake_overhead(q, skb);
flow_queue_add(flow, skb);
@@ -1833,13 +1834,13 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
if (ack) {
b->ack_drops++;
sch->qstats.drops++;
- b->bytes += qdisc_pkt_len(ack);
- len -= qdisc_pkt_len(ack);
+ ack_pkt_len = qdisc_pkt_len(ack);
+ b->bytes += ack_pkt_len;
q->buffer_used += skb->truesize - ack->truesize;
if (q->rate_flags & CAKE_FLAG_INGRESS)
cake_advance_shaper(q, b, ack, now, true);
- qdisc_tree_reduce_backlog(sch, 1, qdisc_pkt_len(ack));
+ qdisc_tree_reduce_backlog(sch, 1, ack_pkt_len);
consume_skb(ack);
} else {
sch->q.qlen++;
@@ -1848,11 +1849,11 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
/* stats */
b->packets++;
- b->bytes += len;
- b->backlogs[idx] += len;
- b->tin_backlog += len;
- sch->qstats.backlog += len;
- q->avg_window_bytes += len;
+ b->bytes += len - ack_pkt_len;
+ b->backlogs[idx] += len - ack_pkt_len;
+ b->tin_backlog += len - ack_pkt_len;
+ sch->qstats.backlog += len - ack_pkt_len;
+ q->avg_window_bytes += len - ack_pkt_len;
}
if (q->overflow_timeout)
@@ -1927,24 +1928,29 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
if (q->buffer_used > q->buffer_max_used)
q->buffer_max_used = q->buffer_used;
- if (q->buffer_used > q->buffer_limit) {
- bool same_flow = false;
- u32 dropped = 0;
- u32 drop_id;
+ if (q->buffer_used <= q->buffer_limit)
+ return NET_XMIT_SUCCESS;
- while (q->buffer_used > q->buffer_limit) {
- dropped++;
- drop_id = cake_drop(sch, to_free);
+ prev_qlen = sch->q.qlen;
+ prev_backlog = sch->qstats.backlog;
- if ((drop_id >> 16) == tin &&
- (drop_id & 0xFFFF) == idx)
- same_flow = true;
- }
- b->drop_overlimit += dropped;
+ while (q->buffer_used > q->buffer_limit) {
+ drop_id = cake_drop(sch, to_free);
+ if ((drop_id >> 16) == tin &&
+ (drop_id & 0xFFFF) == idx)
+ same_flow = true;
+ }
+
+ prev_qlen -= sch->q.qlen;
+ prev_backlog -= sch->qstats.backlog;
+ b->drop_overlimit += prev_qlen;
- if (same_flow)
- return NET_XMIT_CN;
+ if (same_flow) {
+ qdisc_tree_reduce_backlog(sch, prev_qlen - 1,
+ prev_backlog - len);
+ return NET_XMIT_CN;
}
+ qdisc_tree_reduce_backlog(sch, prev_qlen, prev_backlog);
return NET_XMIT_SUCCESS;
}
--
2.43.0
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