[PATCH v2] net: sched: tapr: remove WARN_ON() in taprio_get_start_time()

[Du Cheng <ducheng2@gmail.com>]

There is a reproducible sequence from the userland that will trigger a WARN_ON()
condition in taprio_get_start_time, which causes kernel to panic if configured
as "panic_on_warn". Remove this WARN_ON() to prevent kernel from crashing by
userland-initiated syscalls.

Reported as bug on syzkaller:
https://syzkaller.appspot.com/bug?extid=d50710fd0873a9c6b40c

Reported-by: syzbot+d50710fd0873a9c6b40c@syzkaller.appspotmail.com
Signed-off-by: Du Cheng <ducheng2@gmail.com>
---
Detailed explanation:

In net/sched/sched_taprio.c:999
The condition WARN_ON(!cycle) will be triggered if cycle == 0. Value of cycle
comes from sched->cycle_time, where sched is of type(struct sched_gate_list*).

sched->cycle_time is accumulated within `parse_taprio_schedule()` during
`taprio_init()`, in the following 2 ways:

1. from nla_get_s64(tb[TCA_TAPRIO_ATTR_SCHED_CYCLE_TIME]);
2. (if zero) from parse_sched_list(..., tb[TCA_TAPRIO_ATTR_SCHED_ENTRY_LIST], ...);

note: tb is a map parsed from netlink attributes provided via sendmsg() from the userland:

If both two attributes (TCA_TAPRIO_ATTR_SCHED_CYCLE_TIME,
TCA_TAPRIO_ATTR_SCHED_ENTRY_LIST) contain 0 values or are missing, this will result
in sched->cycle_time == 0 and hence trigger the WARN_ON(!cycle).

Reliable reproducable steps:
1. add net device team0 
2. add team_slave_0, team_slave_1
3. sendmsg(struct msghdr {
	.iov = struct nlmsghdr {
		.type = RTM_NEWQDISC,
	}
	struct tcmsg {
		.tcm_ifindex = ioctl(SIOCGIFINDEX, "team0"),
		.nlattr[] = {
			TCA_KIND: "taprio",
			TCA_OPTIONS: {
				.nlattr = {
					TCA_TAPRIO_ATTR_PRIOMAP: ...,
					TCA_TAPRIO_ATTR_SCHED_ENTRY_LIST: {0},
					TCA_TAPRIO_ATTR_SCHED_CLICKID: 0,
				}
			}
		}
	}
}

Callstack:

parse_taprio_schedule()
taprio_change()
taprio_init()
qdisc_create()
tc_modify_qdisc()
rtnetlink_rcv_msg()
...
sendmsg()

These steps are extracted from syzkaller reproducer:
https://syzkaller.appspot.com/text?tag=ReproC&x=15727cf1900000

 net/sched/sch_taprio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 8287894541e3..5f2ff0f15d5c 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -996,7 +996,7 @@ static int taprio_get_start_time(struct Qdisc *sch,
 	 * something went really wrong. In that case, we should warn about this
 	 * inconsistent state and return error.
 	 */
-	if (WARN_ON(!cycle))
+	if (!cycle)
 		return -EFAULT;
 
 	/* Schedule the start time for the beginning of the next
-- 
2.30.2