net/sched: cls_route: No longer copy tcf_result on update to avoid use-after-free

jira VULN-6646
cve CVE-2023-4206
commit-author valis <[email protected]>
commit b80b829

When route4_change() is called on an existing filter, the whole
tcf_result struct is always copied into the new instance of the filter.

This causes a problem when updating a filter bound to a class,
as tcf_unbind_filter() is always called on the old instance in the
success path, decreasing filter_cnt of the still referenced class
and allowing it to be deleted, leading to a use-after-free.

Fix this by no longer copying the tcf_result struct from the old filter.

Fixes: 1109c00 ("net: sched: RCU cls_route")
	Reported-by: valis <[email protected]>
	Reported-by: Bing-Jhong Billy Jheng <[email protected]>
	Signed-off-by: valis <[email protected]>
	Signed-off-by: Jamal Hadi Salim <[email protected]>
	Reviewed-by: Victor Nogueira <[email protected]>
	Reviewed-by: Pedro Tammela <[email protected]>
	Reviewed-by: M A Ramdhan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
	Signed-off-by: Jakub Kicinski <[email protected]>
(cherry picked from commit b80b829)
	Signed-off-by: Marcin Wcisło <[email protected]>

Author: pvts-mat

net/sched/cls_route.c

@@ -505,7 +505,6 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
 	if (fold) {
 		f->id = fold->id;
 		f->iif = fold->iif;
-		f->res = fold->res;
 		f->handle = fold->handle;
 
 		f->tp = fold->tp;