[PATCH 5.10 542/545] net_sched: cls_route: disallow handle of 0

[Greg Kroah-Hartman]

From: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>

commit 02799571714dc5dd6948824b9d080b44a295f695 upstream.

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

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@xxxxxxxxxxxx>
Acked-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/sched/cls_route.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/net/sched/cls_route.c
+++ b/net/sched/cls_route.c
@@ -424,6 +424,11 @@ static int route4_set_parms(struct net *
 			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
 	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;