net/sched: netem: validate slot configuration

Reject slot configurations that have no defensible meaning:

  - negative min_delay or max_delay
  - min_delay greater than max_delay
  - negative dist_delay or dist_jitter
  - negative max_packets or max_bytes

Negative or out-of-order delays underflow in get_slot_next(),
producing garbage intervals. Negative limits trip the per-slot
accounting (packets_left/bytes_left <= 0) on the first packet of
every slot, defeating the rate-limiting half of the slot feature.

Note that dist_jitter has been silently coerced to its absolute
value by get_slot() since the feature was introduced; rejecting
negatives here converts that silent coercion into -EINVAL. The
abs() can be removed in a follow-up.

Fixes: 836af83b54e3 ("netem: support delivering packets in delayed time slots")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-5-stephen@networkplumber.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 556f9747f0e7..640b51be807a 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -827,6 +827,29 @@ static int get_dist_table(struct disttable **tbl, const struct nlattr *attr)
 	return 0;
 }
 
+static int validate_slot(const struct nlattr *attr, struct netlink_ext_ack *extack)
+{
+	const struct tc_netem_slot *c = nla_data(attr);
+
+	if (c->min_delay < 0 || c->max_delay < 0) {
+		NL_SET_ERR_MSG_ATTR(extack, attr, "negative slot delay");
+		return -EINVAL;
+	}
+	if (c->min_delay > c->max_delay) {
+		NL_SET_ERR_MSG_ATTR(extack, attr, "slot min delay greater than max delay");
+		return -EINVAL;
+	}
+	if (c->dist_delay < 0 || c->dist_jitter < 0) {
+		NL_SET_ERR_MSG_ATTR(extack, attr, "negative dist delay");
+		return -EINVAL;
+	}
+	if (c->max_packets < 0 || c->max_bytes < 0) {
+		NL_SET_ERR_MSG_ATTR(extack, attr, "negative slot limit");
+		return -EINVAL;
+	}
+	return 0;
+}
+
 static void get_slot(struct netem_sched_data *q, const struct nlattr *attr)
 {
 	const struct tc_netem_slot *c = nla_data(attr);
@@ -1040,6 +1063,12 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 			goto table_free;
 	}
 
+	if (tb[TCA_NETEM_SLOT]) {
+		ret = validate_slot(tb[TCA_NETEM_SLOT], extack);
+		if (ret)
+			goto table_free;
+	}
+
 	sch_tree_lock(sch);
 	/* backup q->clg and q->loss_model */
 	old_clg = q->clg;