[PATCH net v3 3/9] net/sched: sfq: clamp quantum in change path

[Jamal Hadi Salim <jhs@mojatatu.com>]

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: e4650d7ae425 ("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>
---
 net/sched/sch_sfq.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 187d3ed578f2..8bbcfc9e85d9 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -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) {
-- 
2.43.0