The ext4_valid_block_bitmap function in fs/ext4/balloc.c in the Linux kernel through 4.15.15 allows attackers to cause a denial of service (out-of-bounds read and system crash) via a crafted ext4 image because balloc.c and ialloc.c do not validate bitmap block numbers.
https://www.debian.org/security/2018/dsa-4188
https://usn.ubuntu.com/3754-1/
https://usn.ubuntu.com/3752-3/
https://usn.ubuntu.com/3752-2/
https://usn.ubuntu.com/3752-1/
https://usn.ubuntu.com/3676-2/