[PATCH v1] ksmbd: Fix user namespace mapping

[Christian Brauner]

On Thu, Sep 29, 2022 at 12:04:47PM +0200, Mickaël Salaün wrote:
> A kernel daemon should not rely on the current thread, which is unknown
> and might be malicious.  Before this security fix,
> ksmbd_override_fsids() didn't correctly override FS UID/GID which means
> that arbitrary user space threads could trick the kernel to impersonate
> arbitrary users or groups for file system access checks, leading to
> file system access bypass.
> 
> This was found while investigating truncate support for Landlock:
> https://lore.kernel.org/r/CAKYAXd8fpMJ7guizOjHgxEyyjoUwPsx3jLOPZP=wPYcbhkVXqA@mail.gmail.com
> 
> Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
> Cc: Hyunchul Lee <hyc.lee at gmail.com>
> Cc: Namjae Jeon <linkinjeon at kernel.org>
> Cc: Steve French <smfrench at gmail.com>
> Cc: stable at vger.kernel.org
> Signed-off-by: Mickaël Salaün <mic at digikod.net>
> Link: https://lore.kernel.org/r/20220929100447.108468-1-mic@digikod.net
> ---

I think this is ok. The alternative would probably be to somehow use a
relevant userns when struct ksmbd_user is created when the session is
established. But these are deeper ksmbd design questions. The fix
proposed here itself seems good.