linux-kernel - Re: [PATCH v1] ksmbd: Fix user namespace mapping

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Date:   Thu, 29 Sep 2022 15:08:15 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Hyunchul Lee <hyc.lee@...il.com>,
        Namjae Jeon <linkinjeon@...nel.org>,
        Steve French <smfrench@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>, linux-cifs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1] ksmbd: Fix user namespace mapping

On Thu, Sep 29, 2022 at 02:18:43PM +0200, Mickaël Salaün wrote:
> 
> On 29/09/2022 13:37, Christian Brauner wrote:
> > 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@...il.com>
> > > Cc: Namjae Jeon <linkinjeon@...nel.org>
> > > Cc: Steve French <smfrench@...il.com>
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Mickaël Salaün <mic@...ikod.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.
> 
> That would be better indeed. I guess ksmbd works whenever the netlink peer
> is not in a user namespace with mapped UID/GID, but it should result in
> obvious access bugs otherwise (which is already the case anyway). It seems
> that the netlink peer must be trusted because it is the source of truth for
> account/user mapping anyway. This change fixes the more critical side of the
> issue and it should fit well for backports.

Sorry, I also forgot,
Acked-by: Christian Brauner (Microsoft) <brauner@...nel.org>