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

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: memcg: fix possible use-after-free in memcg_write_event_control() memcg_write_event_control() accesses the dentry->d_name of the specified control fd to route the write call. As a cgroup interface file can't be renamed, it's safe to access d_name 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 cgroup_event->cft"), there was a call to __file_cft() which verified that the specified file is a regular cgroupfs file before further accesses. The cftype pointer returned from __file_cft() 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 d_name 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 __file_cft(). 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.

References

• https://git.kernel.org/stable/c/b77600e26fd48727a95ffd50ba1e937efb548125
• https://git.kernel.org/stable/c/e1ae97624ecf400ea56c238bff23e5cd139df0b8
• https://git.kernel.org/stable/c/35963b31821920908e397146502066f6b032c917
• https://git.kernel.org/stable/c/f1f7f36cf682fa59db15e2089039a2eeb58ff2ad
• https://git.kernel.org/stable/c/aad8bbd17a1d586005feb9226c2e9cfce1432e13
• https://git.kernel.org/stable/c/0ed074317b835caa6c03bcfa8f133365324673dc
• https://git.kernel.org/stable/c/4a7ba45b1a435e7097ca0f79a847d0949d0eb088