btrfs_root_node in fs/btrfs/ctree.c in the Linux kernel through 5.3.12 allows a NULL pointer dereference because rcu_dereference(root->node) can be zero.
https://usn.ubuntu.com/4439-1/
https://usn.ubuntu.com/4414-1/
https://security.netapp.com/advisory/ntap-20191205-0001/
https://github.com/bobfuzzer/CVE/tree/master/CVE-2019-19036
http://lists.opensuse.org/opensuse-security-announce/2020-03/msg00021.html