Re: [PATCH] net:sched fix array-index-out-of-bounds in taprio_change

On 8/11/21 10:44 AM, Eric Dumazet wrote:

On 8/11/21 7:10 AM, tcs.kernel@gmail.com wrote:

quoted

From: Haimin Zhang <redacted>

syzbot report an array-index-out-of-bounds in taprio_change
index 16 is out of range for type '__u16 [16]'
that's because mqprio->num_tc is lager than TC_MAX_QUEUE,so we check
the return value of netdev_set_num_tc.

Reported-by: syzbot+2b3e5fb6c7ef285a94f6@syzkaller.appspotmail.com
Signed-off-by: Haimin Zhang <redacted>
---
 net/sched/sch_taprio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 9c79374..1ab2fc9 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c

@@ -1513,7 +1513,9 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 	taprio_set_picos_per_byte(dev, q);
 
 	if (mqprio) {
-		netdev_set_num_tc(dev, mqprio->num_tc);
+		err = netdev_set_num_tc(dev, mqprio->num_tc);
+		if (err)
+			goto free_sched;
 		for (i = 0; i < mqprio->num_tc; i++)
 			netdev_set_tc_queue(dev, i,
 					    mqprio->count[i],

When was the bug added ?

Hint: Please provide a Fixes: tag

taprio_parse_mqprio_opt() already checks :

/* Verify num_tc is not out of max range */
if (qopt->num_tc > TC_MAX_QUEUE) {
     NL_SET_ERR_MSG(extack, "Number of traffic classes is outside valid range");
     return -EINVAL;
}

So what is happening exactly ?

Hi, Eric!

I've looked into this bug, but I decided to write a reproducer for it 
first. Unfortunately, I didn't finish it yesterday, but I have an idea 
about what happened:

taprio_parse_mqprio_opt() may return 0 before qopt->num_tc check:

	/* If num_tc is already set, it means that the user already
	 * configured the mqprio part
	 */
	if (dev->num_tc)
		return 0;

Then taprio_mqprio_cmp() fails here:

	if (!mqprio || mqprio->num_tc != dev->num_tc)
		return -1;

That's why we won't get shift-out-of-bound in taprio_mqprio_cmp().

And finally taprio_change() gets to buggy for with wrong mqprio->num_tc.
I don't know how to reproduce it, but I'll try to finish my reproducer 
this evening.


Does above makes any sense?



With regards,
Pavel Skripkin