net/sched: codel: bound the dropping loop per dequeue call

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 <vega@nebusec.ai>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/QDISC-1L5H.v1.20260912080102@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions