sch_netem: fix issues in netem_change() vs get_dist_table()

In blamed commit, I missed that get_dist_table() was allocating
memory using GFP_KERNEL, and acquiring qdisc lock to perform
the swap of newly allocated table with current one.

In this patch, get_dist_table() is allocating memory and
copy user data before we acquire the qdisc lock.

Then we perform swap operations while being protected by the lock.

Note that after this patch netem_change() no longer can do partial changes.
If an error is returned, qdisc conf is left unchanged.

Fixes: 2174a08db8 ("sch_netem: acquire qdisc lock in netem_change()")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230622181503.2327695-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_netem.c

@@ -773,12 +773,10 @@ static void dist_free(struct disttable *d)
  * signed 16 bit values.
  */
 
-static int get_dist_table(struct Qdisc *sch, struct disttable **tbl,
+static int get_dist_table(struct disttable **tbl, const struct nlattr *attr)
-			  const struct nlattr *attr)
 {
 	size_t n = nla_len(attr)/sizeof(__s16);
 	const __s16 *data = nla_data(attr);
-	spinlock_t *root_lock;
 	struct disttable *d;
 	int i;
 
@@ -793,13 +791,7 @@ static int get_dist_table(struct Qdisc *sch, struct disttable **tbl,
 	for (i = 0; i < n; i++)
 		d->table[i] = data[i];
 
-	root_lock = qdisc_root_sleeping_lock(sch);
+	*tbl = d;
-
-	spin_lock_bh(root_lock);
-	swap(*tbl, d);
-	spin_unlock_bh(root_lock);
-
-	dist_free(d);
 	return 0;
 }
 
@@ -956,6 +948,8 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 {
 	struct netem_sched_data *q = qdisc_priv(sch);
 	struct nlattr *tb[TCA_NETEM_MAX + 1];
+	struct disttable *delay_dist = NULL;
+	struct disttable *slot_dist = NULL;
 	struct tc_netem_qopt *qopt;
 	struct clgstate old_clg;
 	int old_loss_model = CLG_RANDOM;
@@ -966,6 +960,18 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 	if (ret < 0)
 		return ret;
 
+	if (tb[TCA_NETEM_DELAY_DIST]) {
+		ret = get_dist_table(&delay_dist, tb[TCA_NETEM_DELAY_DIST]);
+		if (ret)
+			goto table_free;
+	}
+
+	if (tb[TCA_NETEM_SLOT_DIST]) {
+		ret = get_dist_table(&slot_dist, tb[TCA_NETEM_SLOT_DIST]);
+		if (ret)
+			goto table_free;
+	}
+
 	sch_tree_lock(sch);
 	/* backup q->clg and q->loss_model */
 	old_clg = q->clg;
@@ -975,26 +981,17 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 		ret = get_loss_clg(q, tb[TCA_NETEM_LOSS]);
 		if (ret) {
 			q->loss_model = old_loss_model;
+			q->clg = old_clg;
 			goto unlock;
 		}
 	} else {
 		q->loss_model = CLG_RANDOM;
 	}
 
-	if (tb[TCA_NETEM_DELAY_DIST]) {
+	if (delay_dist)
-		ret = get_dist_table(sch, &q->delay_dist,
+		swap(q->delay_dist, delay_dist);
-				     tb[TCA_NETEM_DELAY_DIST]);
+	if (slot_dist)
-		if (ret)
+		swap(q->slot_dist, slot_dist);
-			goto get_table_failure;
-	}
-
-	if (tb[TCA_NETEM_SLOT_DIST]) {
-		ret = get_dist_table(sch, &q->slot_dist,
-				     tb[TCA_NETEM_SLOT_DIST]);
-		if (ret)
-			goto get_table_failure;
-	}
-
 	sch->limit = qopt->limit;
 
 	q->latency = PSCHED_TICKS2NS(qopt->latency);
@@ -1044,17 +1041,11 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt,
 
 unlock:
 	sch_tree_unlock(sch);
+
+table_free:
+	dist_free(delay_dist);
+	dist_free(slot_dist);
 	return ret;
-
-get_table_failure:
-	/* recover clg and loss_model, in case of
-	 * q->clg and q->loss_model were modified
-	 * in get_loss_clg()
-	 */
-	q->clg = old_clg;
-	q->loss_model = old_loss_model;
-
-	goto unlock;
 }
 
 static int netem_init(struct Qdisc *sch, struct nlattr *opt,