From dc48c97e7c69215483beeb7d1d80e8e3adbd92c9 Mon Sep 17 00:00:00 2001 From: Jamal Hadi Salim Date: Tue, 20 Oct 2026 07:00:07 -0400 Subject: [PATCH] net/sched/sch_cake: widen buffer_used to u64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This is a follow-up to commit 4c660ee8c809 ("net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue"), which promoted the backlog + length admission sums in gred, bfifo and plug to u64. CAKE's own memory accounting has the same 32-bit wrap. The issue: cake_sched_data.buffer_used accumulates skb->truesize and is compared against memory_limit: q->buffer_used += skb->truesize; if (q->buffer_used <= q->buffer_limit) return NET_XMIT_SUCCESS; while (q->buffer_used > q->buffer_limit) drop... buffer_used is u32, so once the resident truesize passes 2^32 it wraps to a small value, the <= test passes and the drop loop is skipped even though far more than the configured limit is queued. memory_limit is a u32 attribute (TCA_CAKE_MEMORY), so buffer_limit can be as large as U32_MAX and never caps buffer_used below the wrap point; with a large memory_limit the queue then grows until the machine is out of memory. The fix: Widen buffer_used and buffer_max_used to u64, the same shape as the gred/bfifo/plug fix. buffer_limit stays u32; the comparison promotes it to u64, so the bounded queue stops at the configured limit below 2^32 and the counter can no longer wrap. buffer_used is subtracted on dequeue and drop, so the widened type flows through unchanged. The ACK-filter replacement applied the net delta as one expression, q->buffer_used += skb->truesize - ack->truesize. Both operands are unsigned int, so the subtraction is evaluated at u32 and a larger removed ACK wraps the delta; the old u32 accumulator folded that back to the correct net value, but the widened u64 accumulator would persist it as a roughly 4 GiB over-count. Apply the replacement as two exact operations on the widened counter instead; the queue already accounts ack->truesize, so the subtraction cannot underflow. Widening the counters alone is incomplete because cake_dump_stats() still exports buffer_max_used through PUT_STAT_U32(MEMORY_USED, ...), and tc.yaml declares memory-used as u32. buffer_max_used only ever rises, so once the widened counter passes 2^32 the export reports peak - 2^32 for the rest of the qdisc's life. Clamp the exported value to U32_MAX; the counter itself is unaffected. The dump path runs without the qdisc lock: tc_fill_qdisc() passes NULL to gnet_stats_start_copy_compat(), so the reader races with the enqueue-side update. READ_ONCE() is not an atomic 64-bit load on 32-bit CPUs, so a reader can observe a torn high-water mark and report a bogus MEMORY_USED. Publish the counter as u64_stats_t under the qdisc lock, which the enqueue path already holds, and take a seqcount-protected snapshot in the dump. The counter stays u64; only the exported value is bounded. Conditions to recreate the bug: CAP_NET_ADMIN (root or a user namespace, the qdisc attach is gated by netlink_net_capable(CAP_NET_ADMIN)). A TUN device is needed so the root qdisc can park packets instead of dropping them. Open /dev/net/tun, request TUNSETIFF with IFF_TUN | IFF_NO_PI | IFF_BACKPRESSURE and keep the fd open without reading it (run it in the background and stop it at teardown): with IFF_BACKPRESSURE the full tx ring calls netif_tx_stop_queue() rather than dropping, so the queue behind the qdisc can grow past 4 GiB of resident truesize. A plain 'ip tuntap add' attaches no fd and does not request IFF_BACKPRESSURE, so it cannot park the queue this way. ip link set tun0 txqueuelen 32 up ip addr add 10.99.0.1/24 dev tun0 tc qdisc add dev tun0 root handle 1: cake memlimit 4294967295 # Then push more than 4 GiB of resident truesize through the qdisc: six # UDP senders using large SO_SNDBUFFORCE buffers send 1400-byte # datagrams (the reported OOM witness used 6 workers for 100 s on a # 12 GiB guest). On the unfixed kernel buffer_used wraps, the admission # test passes, the drop loop is bypassed and the guest runs out of # memory; with the fix the counter passes the limit without wrapping and # the excess is dropped. # The ACK-filter over-count is a separate corner: it needs ack-filter # enabled and a queued pure ACK whose truesize is larger than the # replacement ACK. Use a fresh interface (or 'tc qdisc change') rather # than a second root 'tc qdisc add' on an interface that already has a # qdisc: tc qdisc change dev tun0 root cake ack-filter CONFIG_NET_SCH_CAKE=y and CONFIG_TUN=y are required, and the memory-residency recipe needs a root-run sender envelope with enough guest RAM to hold the queued data. Reported-by: Sashiko (gemini) Closes: https://sashiko.dev/#/patchset/20260818095927.15901-1-jhs@mojatatu.com Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260818095927.15901-1-jhs@mojatatu.com Link: https://lore.kernel.org/netdev/20260818095927.15901-1-jhs@mojatatu.com/ Reviewed-by: Victor Nogueira Signed-off-by: Jamal Hadi Salim Acked-by: Toke Høiland-Jørgensen Link: https://patch.msgid.link/QDISC-5RTM.v3.r0.20261020053711-3@mojatatu.com Signed-off-by: Jakub Kicinski --- net/sched/sch_cake.c | 27 +++++++++++++++++++++------ 1 file changed, 21 insertions(+), 6 deletions(-) diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c index 05b5f307884d..e74f85fc373f 100644 --- a/net/sched/sch_cake.c +++ b/net/sched/sch_cake.c @@ -63,6 +63,7 @@ #include #include #include +#include #include #include #include @@ -234,8 +235,10 @@ struct cake_sched_data { u16 tin_cnt; /* resource tracking */ - u32 buffer_used; - u32 buffer_max_used; + + struct u64_stats_sync buffer_max_used_sync; + u64_stats_t buffer_max_used; + u64 buffer_used; u32 buffer_limit; /* indices for dequeue */ @@ -1850,7 +1853,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, qdisc_qstats_drop(sch); ack_pkt_len = qdisc_pkt_len(ack); WRITE_ONCE(b->bytes, b->bytes + ack_pkt_len); - q->buffer_used += skb->truesize - ack->truesize; + q->buffer_used += skb->truesize; + q->buffer_used -= ack->truesize; if (q->config->rate_flags & CAKE_FLAG_INGRESS) cake_advance_shaper(q, b, ack, now, true); @@ -1941,8 +1945,11 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, cake_inc_dsthost_bulk_flow_count(b, flow, q->config->flow_mode); } - if (q->buffer_used > q->buffer_max_used) - WRITE_ONCE(q->buffer_max_used, q->buffer_used); + if (q->buffer_used > u64_stats_read(&q->buffer_max_used)) { + u64_stats_update_begin(&q->buffer_max_used_sync); + u64_stats_set(&q->buffer_max_used, q->buffer_used); + u64_stats_update_end(&q->buffer_max_used_sync); + } if (q->buffer_used <= q->buffer_limit) return NET_XMIT_SUCCESS; @@ -2862,6 +2869,7 @@ static int cake_init(struct Qdisc *sch, struct nlattr *opt, qd->cur_flow = 0; qd->config = q; + u64_stats_init(&qd->buffer_max_used_sync); qdisc_watchdog_init(&qd->watchdog, sch); if (opt) { @@ -3012,6 +3020,8 @@ static int cake_dump_stats(struct Qdisc *sch, struct gnet_dump *d) struct nlattr *stats = nla_nest_start_noflag(d->skb, TCA_STATS_APP); struct cake_sched_data *q = qdisc_priv(sch); struct nlattr *tstats, *ts; + u64 buffer_max_used; + unsigned int start; int i; if (!stats) @@ -3027,9 +3037,14 @@ static int cake_dump_stats(struct Qdisc *sch, struct gnet_dump *d) goto nla_put_failure; \ } while (0) + do { + start = u64_stats_fetch_begin(&q->buffer_max_used_sync); + buffer_max_used = u64_stats_read(&q->buffer_max_used); + } while (u64_stats_fetch_retry(&q->buffer_max_used_sync, start)); + PUT_STAT_U64(CAPACITY_ESTIMATE64, READ_ONCE(q->avg_peak_bandwidth)); PUT_STAT_U32(MEMORY_LIMIT, READ_ONCE(q->buffer_limit)); - PUT_STAT_U32(MEMORY_USED, READ_ONCE(q->buffer_max_used)); + PUT_STAT_U32(MEMORY_USED, min_t(u64, buffer_max_used, U32_MAX)); PUT_STAT_U32(AVG_NETOFF, ((READ_ONCE(q->avg_netoff) + 0x8000) >> 16)); PUT_STAT_U32(MAX_NETLEN, READ_ONCE(q->max_netlen)); PUT_STAT_U32(MAX_ADJLEN, READ_ONCE(q->max_adjlen));