Re: [PATCH net] net/sched: cls_matchall: fix crash when used with classful qdisc

this script, edited from Linux Advanced Routing and Traffic Control guide

tc q a dev en0 root handle 1: htb default a
tc c a dev en0 parent 1:  classid 1:1 htb rate 6mbit burst 15k
tc c a dev en0 parent 1:1 classid 1:a htb rate 5mbit ceil 6mbit burst 15k
tc c a dev en0 parent 1:1 classid 1:b htb rate 1mbit ceil 6mbit burst 15k
tc f a dev en0 parent 1:0 prio 1 $clsname $clsargs classid 1:b
ping $address -c1
tc -s c s dev en0

classifies traffic to 1:b or 1:a, depending on whether the packet matches
or not the pattern $clsargs of filter $clsname. However, when $clsname is
'matchall', a systematic crash can be observed in htb_classify(). HTB and
classful qdiscs don't assign initial value to struct tcf_result, but then
they expect it to contain valid values after filters have been run. Thus,
current 'matchall' ignores the TCA_MATCHALL_CLASSID attribute, configured
by user, and makes HTB (and classful qdiscs) dereference random pointers.

By assigning head->res to *res in mall_classify(), before the actions are
invoked, we fix this crash and enable TCA_MATCHALL_CLASSID functionality,
that had no effect on 'matchall' classifier since its first introduction.

BugLink: https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.redhat.com%2Fshow_bug.cgi%3Fid%3D1460213&data=02%7C01%7Cyotamg%40mellanox.com%7C399f6ff50cb148cbd0d408d4fcfad4c7%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636411601630363571&sdata=PSkkBrWNXkTxsvXrTmK6Dx9iKZMq61MAKlTcdVcPj8w%3D&reserved=0
Reported-by: Jiri Benc <redacted>
Fixes: b87f7936a932 ("net/sched: introduce Match-all classifier")
Signed-off-by: Davide Caratti <redacted>
---
 net/sched/cls_matchall.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/sched/cls_matchall.c b/net/sched/cls_matchall.c
index 21cc45caf842..eeac606c95ab 100644
--- a/net/sched/cls_matchall.c
+++ b/net/sched/cls_matchall.c

@@ -32,6 +32,7 @@ static int mall_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 	if (tc_skip_sw(head->flags))
 		return -1;
 
+	*res = head->res;
 	return tcf_exts_exec(skb, &head->exts, res);
 }