net: openvswitch: reject negative ifindex

Recent changes in net-next (commit 759ab1e ("net: store netdevs
in an xarray")) refactored the handling of pre-assigned ifindexes
and let syzbot surface a latent problem in ovs. ovs does not validate
ifindex, making it possible to create netdev ports with negative
ifindex values. It's easy to repro with YNL:

$ ./cli.py --spec netlink/specs/ovs_datapath.yaml \
         --do new \
	 --json '{"upcall-pid": 1, "name":"my-dp"}'
$ ./cli.py --spec netlink/specs/ovs_vport.yaml \
	 --do new \
	 --json '{"upcall-pid": "00000001", "name": "some-port0", "dp-ifindex":3,"ifindex":4294901760,"type":2}'

$ ip link show
-65536: some-port0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 7a:48:21:ad:0b:fb brd ff:ff:ff:ff:ff:ff
...

Validate the inputs. Now the second command correctly returns:

$ ./cli.py --spec netlink/specs/ovs_vport.yaml \
	 --do new \
	 --json '{"upcall-pid": "00000001", "name": "some-port0", "dp-ifindex":3,"ifindex":4294901760,"type":2}'

lib.ynl.NlError: Netlink error: Numerical result out of range
nl_len = 108 (92) nl_flags = 0x300 nl_type = 2
	error: -34	extack: {'msg': 'integer out of range', 'unknown': [[type:4 len:36] b'\x0c\x00\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0c\x00\x03\x00\xff\xff\xff\x7f\x00\x00\x00\x00\x08\x00\x01\x00\x08\x00\x00\x00'], 'bad-attr': '.ifindex'}

Accept 0 since it used to be silently ignored.

Fixes: 54c4ef3 ("openvswitch: allow specifying ifindex of new interfaces")
Reported-by: syzbot+7456b5dcf65111553320@syzkaller.appspotmail.com
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Link: https://lore.kernel.org/r/20230814203840.2908710-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

net/openvswitch/datapath.c

@@ -1829,7 +1829,7 @@ static int ovs_dp_cmd_new(struct sk_buff *skb, struct genl_info *info)
 	parms.port_no = OVSP_LOCAL;
 	parms.upcall_portids = a[OVS_DP_ATTR_UPCALL_PID];
 	parms.desired_ifindex = a[OVS_DP_ATTR_IFINDEX]
-		? nla_get_u32(a[OVS_DP_ATTR_IFINDEX]) : 0;
+		? nla_get_s32(a[OVS_DP_ATTR_IFINDEX]) : 0;
 
 	/* So far only local changes have been made, now need the lock. */
 	ovs_lock();
@@ -2049,7 +2049,7 @@ static const struct nla_policy datapath_policy[OVS_DP_ATTR_MAX + 1] = {
 	[OVS_DP_ATTR_USER_FEATURES] = { .type = NLA_U32 },
 	[OVS_DP_ATTR_MASKS_CACHE_SIZE] =  NLA_POLICY_RANGE(NLA_U32, 0,
 		PCPU_MIN_UNIT_SIZE / sizeof(struct mask_cache_entry)),
-	[OVS_DP_ATTR_IFINDEX] = {.type = NLA_U32 },
+	[OVS_DP_ATTR_IFINDEX] = NLA_POLICY_MIN(NLA_S32, 0),
 };
 
 static const struct genl_small_ops dp_datapath_genl_ops[] = {
@@ -2302,7 +2302,7 @@ static int ovs_vport_cmd_new(struct sk_buff *skb, struct genl_info *info)
 	parms.port_no = port_no;
 	parms.upcall_portids = a[OVS_VPORT_ATTR_UPCALL_PID];
 	parms.desired_ifindex = a[OVS_VPORT_ATTR_IFINDEX]
-		? nla_get_u32(a[OVS_VPORT_ATTR_IFINDEX]) : 0;
+		? nla_get_s32(a[OVS_VPORT_ATTR_IFINDEX]) : 0;
 
 	vport = new_vport(&parms);
 	err = PTR_ERR(vport);
@@ -2539,7 +2539,7 @@ static const struct nla_policy vport_policy[OVS_VPORT_ATTR_MAX + 1] = {
 	[OVS_VPORT_ATTR_TYPE] = { .type = NLA_U32 },
 	[OVS_VPORT_ATTR_UPCALL_PID] = { .type = NLA_UNSPEC },
 	[OVS_VPORT_ATTR_OPTIONS] = { .type = NLA_NESTED },
-	[OVS_VPORT_ATTR_IFINDEX] = { .type = NLA_U32 },
+	[OVS_VPORT_ATTR_IFINDEX] = NLA_POLICY_MIN(NLA_S32, 0),
 	[OVS_VPORT_ATTR_NETNSID] = { .type = NLA_S32 },
 	[OVS_VPORT_ATTR_UPCALL_STATS] = { .type = NLA_NESTED },
 };