net: openvswitch: fix nested key length validation in the set() action

It's not safe to access nla_len(ovs_key) if the data is smaller than
the netlink header.  Check that the attribute is OK first.

Fixes: ccb1352 ("net: Add Open vSwitch kernel components.")
Reported-by: syzbot+b07a9da40df1576b8048@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b07a9da40df1576b8048
Tested-by: syzbot+b07a9da40df1576b8048@syzkaller.appspotmail.com
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20250412104052.2073688-1-i.maximets@ovn.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/openvswitch/flow_netlink.c

@@ -2876,7 +2876,8 @@ static int validate_set(const struct nlattr *a,
 	size_t key_len;
 
 	/* There can be only one key in a action */
-	if (nla_total_size(nla_len(ovs_key)) != nla_len(a))
+	if (!nla_ok(ovs_key, nla_len(a)) ||
+	    nla_total_size(nla_len(ovs_key)) != nla_len(a))
 		return -EINVAL;
 
 	key_len = nla_len(ovs_key);