The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability in the given code slice is:

The code attempts to access the 'cnf' member of the 'idev' pointer without first checking if 'idev' is NULL. This can lead to a NULL pointer dereference if 'idev' is indeed NULL.

Specifically:

1. On line 4, 'idev' is assigned the result of ip6_dst_idev(skb_dst(skb)).

2. There's no check to ensure that 'idev' is not NULL before using it on line 9.

3. If ip6_dst_idev() or skb_dst() returns NULL, or if skb itself is NULL, 'idev' could be NULL.

4. The code then directly accesses 'idev->cnf.disable_ipv6' without verifying 'idev' is valid.

5. If 'idev' is NULL, this access will cause a NULL pointer dereference, potentially leading to a crash or undefined behavior.

To fix this vulnerability, a NULL check should be added for 'idev' before attempting to access its members. This would prevent the potential NULL pointer dereference and make the code more robust.