rtnetlink: Reject negative ifindexes in RTM_NEWLINK

Negative ifindexes are illegal, but the kernel does not validate the
ifindex in the ancillary header of RTM_NEWLINK messages, resulting in
the kernel generating a warning [1] when such an ifindex is specified.

Fix by rejecting negative ifindexes.

[1]
WARNING: CPU: 0 PID: 5031 at net/core/dev.c:9593 dev_index_reserve+0x1a2/0x1c0 net/core/dev.c:9593
[...]
Call Trace:
 <TASK>
 register_netdevice+0x69a/0x1490 net/core/dev.c:10081
 br_dev_newlink+0x27/0x110 net/bridge/br_netlink.c:1552
 rtnl_newlink_create net/core/rtnetlink.c:3471 [inline]
 __rtnl_newlink+0x115e/0x18c0 net/core/rtnetlink.c:3688
 rtnl_newlink+0x67/0xa0 net/core/rtnetlink.c:3701
 rtnetlink_rcv_msg+0x439/0xd30 net/core/rtnetlink.c:6427
 netlink_rcv_skb+0x16b/0x440 net/netlink/af_netlink.c:2545
 netlink_unicast_kernel net/netlink/af_netlink.c:1342 [inline]
 netlink_unicast+0x536/0x810 net/netlink/af_netlink.c:1368
 netlink_sendmsg+0x93c/0xe40 net/netlink/af_netlink.c:1910
 sock_sendmsg_nosec net/socket.c:728 [inline]
 sock_sendmsg+0xd9/0x180 net/socket.c:751
 ____sys_sendmsg+0x6ac/0x940 net/socket.c:2538
 ___sys_sendmsg+0x135/0x1d0 net/socket.c:2592
 __sys_sendmsg+0x117/0x1e0 net/socket.c:2621
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fixes: 38f7b870 ("[RTNETLINK]: Link creation API")
Reported-by: syzbot+5ba06978f34abb058571@syzkaller.appspotmail.com
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230823064348.2252280-1-idosch@nvidia.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/rtnetlink.c

@@ -3561,6 +3561,9 @@ static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	if (ifm->ifi_index > 0) {
 		link_specified = true;
 		dev = __dev_get_by_index(net, ifm->ifi_index);
+	} else if (ifm->ifi_index < 0) {
+		NL_SET_ERR_MSG(extack, "ifindex can't be negative");
+		return -EINVAL;
 	} else if (tb[IFLA_IFNAME] || tb[IFLA_ALT_IFNAME]) {
 		link_specified = true;
 		dev = rtnl_dev_get(net, tb);