An issue was discovered in the Linux kernel through 4.17.10. There is a NULL pointer dereference and panic in hfsplus_lookup() in fs/hfsplus/dir.c when opening a file (that is purportedly a hard link) in an hfs+ filesystem that has malformed catalog data, and is mounted read-only without a metadata directory.
https://www.spinics.net/lists/linux-fsdevel/msg130021.html
https://www.debian.org/security/2018/dsa-4308
https://usn.ubuntu.com/4118-1/
https://usn.ubuntu.com/4094-1/
https://usn.ubuntu.com/3821-2/
https://usn.ubuntu.com/3821-1/
https://lists.debian.org/debian-lts-announce/2018/10/msg00003.html