sch_htb: Fix offload cleanup in htb_destroy on htb_init failure

htb_init may fail to do the offload if it's not supported or if a
runtime error happens when allocating direct qdiscs. In those cases
TC_HTB_CREATE command is not sent to the driver, however, htb_destroy
gets called anyway and attempts to send TC_HTB_DESTROY.

It shouldn't happen, because the driver didn't receive TC_HTB_CREATE,
and also because the driver may not support ndo_setup_tc at all, while
q->offload is true, and htb_destroy mistakenly thinks the offload is
supported. Trying to call ndo_setup_tc in the latter case will lead to a
NULL pointer dereference.

This commit fixes the issues with htb_destroy by deferring assignment of
q->offload until after the TC_HTB_CREATE command. The necessary cleanup
of the offload entities is already done in htb_init.

Reported-by: syzbot+b53a709f04722ca12a3c@syzkaller.appspotmail.com
Fixes: d03b195b5a ("sch_htb: Hierarchical QoS hardware offload")
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_htb.c

@@ -1020,6 +1020,7 @@ static int htb_init(struct Qdisc *sch, struct nlattr *opt,
 	struct nlattr *tb[TCA_HTB_MAX + 1];
 	struct tc_htb_glob *gopt;
 	unsigned int ntx;
+	bool offload;
 	int err;
 
 	qdisc_watchdog_init(&q->watchdog, sch);
@@ -1044,9 +1045,9 @@ static int htb_init(struct Qdisc *sch, struct nlattr *opt,
 	if (gopt->version != HTB_VER >> 16)
 		return -EINVAL;
 
-	q->offload = nla_get_flag(tb[TCA_HTB_OFFLOAD]);
+	offload = nla_get_flag(tb[TCA_HTB_OFFLOAD]);
 
-	if (q->offload) {
+	if (offload) {
 		if (sch->parent != TC_H_ROOT)
 			return -EOPNOTSUPP;
 
@@ -1076,7 +1077,7 @@ static int htb_init(struct Qdisc *sch, struct nlattr *opt,
 		q->rate2quantum = 1;
 	q->defcls = gopt->defcls;
 
-	if (!q->offload)
+	if (!offload)
 		return 0;
 
 	for (ntx = 0; ntx < q->num_direct_qdiscs; ntx++) {
@@ -1107,12 +1108,14 @@ static int htb_init(struct Qdisc *sch, struct nlattr *opt,
 	if (err)
 		goto err_free_qdiscs;
 
+	/* Defer this assignment, so that htb_destroy skips offload-related
+	 * parts (especially calling ndo_setup_tc) on errors.
+	 */
+	q->offload = true;
+
 	return 0;
 
 err_free_qdiscs:
-	/* TC_HTB_CREATE call failed, avoid any further calls to the driver. */
-	q->offload = false;
-
 	for (ntx = 0; ntx < q->num_direct_qdiscs && q->direct_qdiscs[ntx];
 	     ntx++)
 		qdisc_put(q->direct_qdiscs[ntx]);