[bug report] xfrm: Add possibility to set the default to block if we have no policy

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Steffen Klassert,

The patch 2d151d39073a: "xfrm: Add possibility to set the default to
block if we have no policy" from Jul 18, 2021, leads to the following
static checker warning:

	net/xfrm/xfrm_user.c:1969 xfrm_set_default()
	error: undefined (user controlled) shift '1 << up->dirmask'

net/xfrm/xfrm_user.c
    1964 static int xfrm_set_default(struct sk_buff *skb, struct nlmsghdr *nlh,
    1965 			    struct nlattr **attrs)
    1966 {
    1967 	struct net *net = sock_net(skb->sk);
    1968 	struct xfrm_userpolicy_default *up = nlmsg_data(nlh);
--> 1969 	u8 dirmask = (1 << up->dirmask) & XFRM_POL_DEFAULT_MASK;
                              ^^^^^^^^^^^^^^^^
I believe this does come from the user so it can shift wrap leading to
a syzbot/UBSan warning.  I don't know that the shift wrapping will have
any other real world impact.

    1970 	u8 old_default = net->xfrm.policy_default;
    1971 
    1972 	net->xfrm.policy_default = (old_default & (0xff ^ dirmask))
    1973 				    | (up->action << up->dirmask);
    1974 
    1975 	rt_genid_bump_all(net);
    1976 
    1977 	return 0;
    1978 }

regards,
dan carpenter