Re: [PATCH for-6.1-fixes] memcg: Fix possible use-after-free in memcg_write_event_control()

[Johannes Weiner]

On Wed, Dec 07, 2022 at 04:53:15PM -1000, Tejun Heo wrote:
> 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.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Fixes: 347c4a874710 ("memcg: remove cgroup_event->cft")
> Cc: stable@xxxxxxxxxxxxxxx # v3.14+
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>