From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Fri, 13 Mar 2020 22:29:54 -0700
Subject: net_sched: cls_route: remove the right filter from hashtable
Git-commit: ef299cc3fa1a9e1288665a9fdc8bff55629fd359
Patch-mainline: 5.6
References: CVE-2021-3715 networking-stable-20_03_28 bsc#1190349

route4_change() allocates a new filter and copies values from
the old one. After the new filter is inserted into the hash
table, the old filter should be removed and freed, as the final
step of the update.

However, the current code mistakenly removes the new one. This
looks apparently wrong to me, and it causes double "free" and
use-after-free too, as reported by syzbot.

Reported-and-tested-by: syzbot+f9b32aaacd60305d9687@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+2f8c233f131943d6056d@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+9c2df9fd5e9445b74e01@syzkaller.appspotmail.com
Fixes: 1109c00547fc ("net: sched: RCU cls_route")
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/sched/cls_route.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/sched/cls_route.c
+++ b/net/sched/cls_route.c
@@ -528,8 +528,8 @@ static int route4_change(struct net *net
 			fp = &b->ht[h];
 			for (pfp = rtnl_dereference(*fp); pfp;
 			     fp = &pfp->next, pfp = rtnl_dereference(*fp)) {
-				if (pfp == f) {
-					*fp = f->next;
+				if (pfp == fold) {
+					rcu_assign_pointer(*fp, fold->next);
 					break;
 				}
 			}