From 8f735d64382dcf162f4276d6699d03ad2f859c0b Mon Sep 17 00:00:00 2001 From: Jamal Hadi Salim Date: Tue, 25 Aug 2026 04:14:03 -0400 Subject: [PATCH] net/sched: bound qdisc_pkt_len to prevent qdisc soft lockup qdisc_get_stab() accepts a user-supplied size table, and __qdisc_calculate_pkt_len() amplifies qdisc_pkt_len() through the overhead, the size-table data (u16), and size_log (up to STAB_SIZE_LOG_MAX). A crafted stab can therefore set qdisc_pkt_len() to ~1 GiB for an ordinary skb. Per-flow deficit schedulers such as DRR and ETS replenish one quantum per loop iteration; with a tiny quantum (1) they spin billions of times under the qdisc lock, producing a soft lockup / RCU stall as illustrated by vega@nebusec.ai. Cap the final qdisc_pkt_len() to QDISC_PKT_LEN_MAX so the size-table amplification cannot drive deficit schedulers into an unbounded loop. A legitimate size table (e.g. qfq's overhead 999999999, which is handled by dropping) is still accepted. Introduce cap QDISC_PKT_LEN_MAX (1 << 20) = 1 MiB which is well above any legitimate single-skb wire length: the largest current skb->len is GSO_MAX_SIZE (524280), and an ATM-style size table (53/48 cell tax) amplifies that to ~578 KB, both comfortably below 1 MiB. At the same time, 1 MiB bounds the deficit refill loop to ~1M iterations per packet with quantum=1, which completes in a few milliseconds well under the demonstrated softlockup threshold (~10^9 iterations). Conditions to recreate the bug: - CONFIG_NET_SCHED=y, CONFIG_NET_SCH_DRR=y (or CONFIG_NET_SCH_ETS=y). - Attach a DRR (or ETS) root qdisc with a crafted TCA_STAB that amplifies qdisc_pkt_len to ~1 GiB (e.g. size_log=15, data=[32768]). - Add a class with a tiny quantum of 1 and send one small packet; the deficit loop spins billions of times under the qdisc lock and trips the softlockup detector (panic with kernel.softlockup_panic=1). - Reachable as root or from an unprivileged user in a fresh user+net namespace (unshare -Urn) with namespace-local CAP_NET_ADMIN. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: vega@nebusec.ai Tested-by: Victor Nogueira Signed-off-by: Jamal Hadi Salim Link: https://patch.msgid.link/20260825081403.133992-1-jhs@mojatatu.com Signed-off-by: Jakub Kicinski --- include/net/pkt_sched.h | 1 + net/sched/sch_api.c | 7 +++++-- 2 files changed, 6 insertions(+), 2 deletions(-) diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h index 18a419cd9d94..90d3e7943b19 100644 --- a/include/net/pkt_sched.h +++ b/include/net/pkt_sched.h @@ -12,6 +12,7 @@ #define DEFAULT_TX_QUEUE_LEN 1000 #define STAB_SIZE_LOG_MAX 30 +#define QDISC_PKT_LEN_MAX (1 << 20) /* 1 MiB */ struct qdisc_walker { int stop; diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c index 65b35528d125..90503e59e6e3 100644 --- a/net/sched/sch_api.c +++ b/net/sched/sch_api.c @@ -610,8 +610,11 @@ void __qdisc_calculate_pkt_len(struct sk_buff *skb, pkt_len <<= stab->szopts.size_log; out: - if (unlikely(pkt_len < 1)) - pkt_len = 1; + /* A size table can inflate qdisc_pkt_len() beyond any real packet + * (via overhead, the data table, or size_log); cap it so deficit + * schedulers such as DRR/ETS terminate their refill loops. + */ + pkt_len = clamp_t(int, pkt_len, 1, QDISC_PKT_LEN_MAX); qdisc_skb_cb(skb)->pkt_len = pkt_len; }