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net/sched: sfq: clamp quantum in change path

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sfq_change() accepts any non-negative quantum (only rejects
(int)ctl->quantum < 0). With a crafted size table qdisc_pkt_len reaches
~2 GiB, so quantum=1 makes the deficit-refill loop spin ~2^31 times
under the qdisc lock (a soft lockup / denial of service).

Add max(256U, ...) matching fq_codel_change(). Reject quantum > 1<<20
with -EINVAL, matching fq_codel_change() and the init clamp.

Conditions to recreate the bug:
  CONFIG_NET_SCH_SFQ=y. Requires CAP_NET_ADMIN (namespace-local via
  unshare -Urn suffices).

  tc qdisc add dev dummy0 root sfq
  tc qdisc change dev dummy0 root sfq quantum 1 stab data 32768 size_log 15 cell_log 0

Fixes: e4650d7ae4 ("net_sched: sch_sfq: handle bigger packets")
Reported-by: Vega <vega@nebusec.ai>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/QDISC-0CFC.v3.20260901204856@mojatatu.com.4
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jamal Hadi Salim 2026-09-01 17:39:24 -04:00 committed by Jakub Kicinski
parent 4864f58c53
commit fb9f88a33c
1 changed files with 6 additions and 1 deletions
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7
net/sched/sch_sfq.c
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@ -660,6 +660,11 @@ static int sfq_change(struct Qdisc *sch, struct nlattr *opt,
return -EINVAL;
}
if (ctl->quantum > 1 << 20) {
NL_SET_ERR_MSG_MOD(extack, "quantum too large");
return -EINVAL;
}
if (ctl->perturb_period < 0 ||
ctl->perturb_period > INT_MAX / HZ) {
NL_SET_ERR_MSG_MOD(extack, "invalid perturb period");
@ -688,7 +693,7 @@ static int sfq_change(struct Qdisc *sch, struct nlattr *opt,
/* update and validate configuration */
if (ctl->quantum)
quantum = ctl->quantum;
quantum = max(256U, ctl->quantum);
if (ctl->flows)
maxflows = min_t(u32, ctl->flows, SFQ_MAX_FLOWS);
if (ctl->divisor) {