[PATCH] smb: client: reject userspace cifs.idmap descriptions

[Paulo Alcantara]

henrymei <ljp1205831794 at gmail.com> writes:

> From: Aohan Mei <henrymei at tencent.com>
>
> cifs.idmap key descriptions carry authority-bearing fields (owner and
> group SIDs and uid/gid values in "os:"/"gs:"/"oi:"/"gi:" form) that the
> cifs.idmap upcall helper treats as kernel-originating inputs.  Unlike
> its sibling cifs.spnego, the cifs.idmap key type has no vet_description
> hook, so userspace can create keys of this type through
> request_key(2)/add_key(2) and supply those fields without CIFS origin.
> A request_key(2) call with a non-NULL callout then drives a root
> usermodehelper upcall (/sbin/request-key -> cifs.idmap) that consumes
> the unvetted description in root context.
>
> Only accept cifs.idmap descriptions while CIFS is using its private
> root_cred to request the key.  id_to_sid()/sid_to_id() already run
> under override_creds(root_cred), so the kernel-originated path is
> unaffected.
> ...

Applied.