From 7f4a5ec6258fd7c92633ec4b0493fc51166d9398 Mon Sep 17 00:00:00 2001 From: Jamal Hadi Salim Date: Sat, 12 Sep 2026 14:08:30 -0400 Subject: net/sched: codel: bound the dropping loop per dequeue call MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The CoDel control law schedules the next drop one interval/sqrt(count) after the previous drop, using the configured interval (codel_params.interval). For very small intervals the scheduled step rounds down to zero, so the dropping loop in codel_dequeue() never advances and drains the entire backlog under the qdisc lock in one call - an unprivileged user can trigger a soft lockup this way. Fix in the shared codel code used by both codel and fq_codel: 1. Make the control-law step at least 1 tick so the dropping loop always moves forward. 2. Cap the dropping loop at CODEL_MAX_DROPS_PER_DEQUEUE (256) drops per codel_dequeue() call, resyncing drop_next to now when the cap is hit: the catch-up owed to the loop grows with the idle gap and the backlog, which no interval threshold can bound. This is a deliberate behaviour change after long idle gaps. The cap applies to fq_codel (4b549a2ef4be) and the mac80211 TXQ path (fixed interval, cap only). The target sojourn delay (codel_params.target) is not validated: it does not feed the control law, so a sub-tick value is aggressive rather than deadlock-prone. Conditions to recreate the bug: - tc qdisc add dev lo root handle 1: tbf rate 1kbit burst 2kb limit 1000000 - tc qdisc add dev lo parent 1:1 handle 10: codel interval 2us target 1ms noecn limit 1000000 (same for fq_codel) - unpatched kernel: tc accepts it; a UDP flood under the 1kbit tbf soft-lockups (watchdog: BUG: soft lockup) while one codel_dequeue() call drops the backlog under the qdisc lock - patched kernel: same setup, at most 256 drops per dequeue call, no soft lockup Testing: claim reproducer and interval 2us/3us variants run clean; tdc qdisc category passes (see the selftests patch). Fixes: 76e3cc126bb2 ("codel: Controlled Delay AQM") Reported-by: Vega Reviewed-by: Eric Dumazet Tested-by: Victor Nogueira Signed-off-by: Jamal Hadi Salim Reviewed-by: Toke Høiland-Jørgensen Link: https://patch.msgid.link/QDISC-1L5H.v1.20260912080102@mojatatu.com Signed-off-by: Jakub Kicinski --- include/net/codel.h | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'include/net/codel.h') diff --git a/include/net/codel.h b/include/net/codel.h index aa80f744826c..183d43c2bd43 100644 --- a/include/net/codel.h +++ b/include/net/codel.h @@ -140,6 +140,11 @@ struct codel_vars { /* needed shift to get a Q0.32 number from rec_inv_sqrt */ #define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS) +/* Cap on drops per codel_dequeue() call: the loop's work depends on the + * idle gap and backlog, both outside our control; resync when exceeded. + */ +#define CODEL_MAX_DROPS_PER_DEQUEUE 256 + /** * struct codel_stats - contains codel shared variables and stats * @maxpacket: largest packet we've seen so far -- cgit v1.2.3