ASA-2019-00658 – Linux kernel: Mounting a crafted btrfs filesystem image can lead to a use-after-free through syncfs system call

Mounting a crafted btrfs filesystem image, performing some operations, and then making a syncfs system call can lead to a use-after-free in try_merge_free_space in fs/btrfs/free-space-cache.c because the pointer to a left data structure can be the same as the pointer to a right data structure.