netdev - [PATCH net 1/1] net_sched: cls_route: disallow handle of 0

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Date:   Sun, 14 Aug 2022 11:27:58 +0000
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com
Cc:     netdev@...r.kernel.org, xiyou.wangcong@...il.com, jiri@...nulli.us,
        kuznet@....inr.ac.ru, cascardo@...onical.com,
        linux-distros@...openwall.org, security@...nel.org,
        stephen@...workplumber.org, dsahern@...il.com,
        gregkh@...uxfoundation.org, Jamal Hadi Salim <jhs@...atatu.com>
Subject: [PATCH net 1/1] net_sched: cls_route: disallow handle of 0

Follows up on:
https://lore.kernel.org/all/20220809170518.164662-1-cascardo@canonical.com/

handle of 0 implies from/to of universe realm which is not very
sensible.

Lets see what this patch will do:
$sudo tc qdisc add dev $DEV root handle 1:0 prio

//lets manufacture a way to insert handle of 0
$sudo tc filter add dev $DEV parent 1:0 protocol ip prio 100 \
route to 0 from 0 classid 1:10 action ok

//gets rejected...
Error: handle of 0 is not valid.
We have an error talking to the kernel, -1

//lets create a legit entry..
sudo tc filter add dev $DEV parent 1:0 protocol ip prio 100 route from 10 \
classid 1:10 action ok

//what did the kernel insert?
$sudo tc filter ls dev $DEV parent 1:0
filter protocol ip pref 100 route chain 0
filter protocol ip pref 100 route chain 0 fh 0x000a8000 flowid 1:10 from 10
	action order 1: gact action pass
	 random type none pass val 0
	 index 1 ref 1 bind 1

//Lets try to replace that legit entry with a handle of 0
$ sudo tc filter replace dev $DEV parent 1:0 protocol ip prio 100 \
handle 0x000a8000 route to 0 from 0 classid 1:10 action drop

Error: Replacing with handle of 0 is invalid.
We have an error talking to the kernel, -1

And last, lets run Cascardo's POC:
$ ./poc
0
0
-22
-22
-22

Signed-off-by: Jamal Hadi Salim <jhs@...atatu.com>
---
 net/sched/cls_route.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/sched/cls_route.c b/net/sched/cls_route.c
index 3f935cbbaff6..48712bc51bda 100644
--- a/net/sched/cls_route.c
+++ b/net/sched/cls_route.c
@@ -424,6 +424,11 @@ static int route4_set_parms(struct net *net, struct tcf_proto *tp,
 			return -EINVAL;
 	}
 
+	if (!nhandle) {
+		NL_SET_ERR_MSG(extack, "Replacing with handle of 0 is invalid");
+		return -EINVAL;
+	}
+
 	h1 = to_hash(nhandle);
 	b = rtnl_dereference(head->table[h1]);
 	if (!b) {
@@ -477,6 +482,11 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
 	int err;
 	bool new = true;
 
+	if (!handle) {
+		NL_SET_ERR_MSG(extack, "Creating with handle of 0 is invalid");
+		return -EINVAL;
+	}
+
 	if (opt == NULL)
 		return handle ? -EINVAL : 0;
 
-- 
2.25.1