net: xfrm: fix shift-out-of-bounds in xfrm_get_default

Syzbot hit shift-out-of-bounds in xfrm_get_default. The problem was in
missing validation check for user data.

up->dirmask comes from user-space, so we need to check if this value
is less than XFRM_USERPOLICY_DIRMASK_MAX to avoid shift-out-of-bounds bugs.

Fixes: 2d151d39 ("xfrm: Add possibility to set the default to block if we have no policy")
Reported-and-tested-by: syzbot+b2be9dd8ca6f6c73ee2d@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_user.c

@@ -2005,6 +2005,11 @@ static int xfrm_get_default(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return -EMSGSIZE;
 	}
 
+	if (up->dirmask >= XFRM_USERPOLICY_DIRMASK_MAX) {
+		kfree_skb(r_skb);
+		return -EINVAL;
+	}
+
 	r_up = nlmsg_data(r_nlh);
 
 	r_up->action = ((net->xfrm.policy_default & (1 << up->dirmask)) >> up->dirmask);