The ext4_iget function in fs/ext4/inode.c in the Linux kernel through 4.15.15 mishandles the case of a root directory with a zero i_links_count, which allows attackers to cause a denial of service (ext4_process_freed_data NULL pointer dereference and OOPS) via a crafted ext4 image.
http://openwall.com/lists/oss-security/2018/03/29/1
https://access.redhat.com/errata/RHSA-2018:2948
https://access.redhat.com/errata/RHSA-2018:3083
https://access.redhat.com/errata/RHSA-2018:3096
https://bugzilla.kernel.org/show_bug.cgi?id=199179
https://bugzilla.kernel.org/show_bug.cgi?id=199275
https://bugzilla.redhat.com/show_bug.cgi?id=1560777
https://lists.debian.org/debian-lts-announce/2018/05/msg00000.html
https://usn.ubuntu.com/3676-1/
https://usn.ubuntu.com/3676-2/
https://usn.ubuntu.com/3677-1/
https://usn.ubuntu.com/3677-2/
https://usn.ubuntu.com/3678-1/
https://usn.ubuntu.com/3678-2/
https://usn.ubuntu.com/3678-3/
https://usn.ubuntu.com/3678-4/
https://usn.ubuntu.com/3754-1/