CVE-2022-48988: memcg: fix possible use-after-free in memcg_write_event_control()

In the Linux kernel, the following vulnerability has been resolved:

memcg: fix possible use-after-free in memcgwriteeventcontrol()

memcgwriteeventcontrol() accesses the dentry->dname of the specified control fd to route the write call. As a cgroup interface file can't be renamed, it's safe to access dname as long as the specified file is a regular cgroup file. Also, as these cgroup interface files can't be removed before the directory, it's safe to access the parent too.

Prior to 347c4a874710 ("memcg: remove cgroupevent->cft"), there was a call to filecft() which verified that the specified file is a regular cgroupfs file before further accesses. The cftype pointer returned from filecft() was no longer necessary and the commit inadvertently dropped the file type check with it allowing any file to slip through. With the invarients broken, the dname and parent accesses can now race against renames and removals of arbitrary files and cause use-after-free's.

Fix the bug by resurrecting the file type check in filecft(). Now that cgroupfs is implemented through kernfs, checking the file operations needs to go through a layer of indirection. Instead, let's check the superblock and dentry type.