[PATCH bpf-next/net v2 1/5] bpf: net_sched: Fix bpf qdisc init prologue when set as default qdisc

[Amery Hung <ameryhung@gmail.com>]

Allow .init to proceed if qdisc_lookup() returns NULL as it only happens
when called by qdisc_create_dflt() in mq/mqprio_init and the parent qdisc
has not been added to qdisc_hash yet. In qdisc_create(), the caller,
__tc_modify_qdisc(), would have made sure the parent qdisc already exist.

In addition, call qdisc_watchdog_init() whether .init succeeds or not to
prevent null-pointer dereference. In qdisc_create() and
qdisc_create_dflt(), if .init fails, .destroy will be called. As a
result, the destroy epilogue could call qdisc_watchdog_cancel() with an
uninitialized timer, causing null-pointer deference in hrtimer_cancel().

Fixes: c8240344956e ("bpf: net_sched: Support implementation of Qdisc_ops in bpf")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
---
 net/sched/bpf_qdisc.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/sched/bpf_qdisc.c b/net/sched/bpf_qdisc.c
index 9f32b305636f..a8efc3ff2b7e 100644
--- a/net/sched/bpf_qdisc.c
+++ b/net/sched/bpf_qdisc.c
@@ -234,18 +234,20 @@ __bpf_kfunc int bpf_qdisc_init_prologue(struct Qdisc *sch,
 	struct net_device *dev = qdisc_dev(sch);
 	struct Qdisc *p;
 
+	qdisc_watchdog_init(&q->watchdog, sch);
+
 	if (sch->parent != TC_H_ROOT) {
+		/* If qdisc_lookup() returns NULL, it means .init is called by
+		 * qdisc_create_dflt() in mq/mqprio_init and the parent qdisc
+		 * has not been added to qdisc_hash yet.
+		 */
 		p = qdisc_lookup(dev, TC_H_MAJ(sch->parent));
-		if (!p)
-			return -ENOENT;
-
-		if (!(p->flags & TCQ_F_MQROOT)) {
+		if (p && !(p->flags & TCQ_F_MQROOT)) {
 			NL_SET_ERR_MSG(extack, "BPF qdisc only supported on root or mq");
 			return -EINVAL;
 		}
 	}
 
-	qdisc_watchdog_init(&q->watchdog, sch);
 	return 0;
 }
 
-- 
2.47.1