linux-kernel - Re: [PATCH 1/2] fs/exec: allow to unshare a time namespace on vfork+exec

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Message-ID: <202206141412.2B0732FF6C@keescook>
Date:   Tue, 14 Jun 2022 14:14:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrei Vagin <avagin@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Christian Brauner <brauner@...nel.org>,
        Florian Weimer <fweimer@...hat.com>, linux-mm@...ck.org,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH 1/2] fs/exec: allow to unshare a time namespace on
 vfork+exec

On Sun, Jun 12, 2022 at 11:07:22PM -0700, Andrei Vagin wrote:
> Right now, a new process can't be forked in another time namespace
> if it shares mm with its parent. It is prohibited, because each time
> namespace has its own vvar page that is mapped into a process address
> space.
> 
> When a process calls exec, it gets a new mm and so it could be "legal"
> to switch time namespace in that case. This was not implemented and
> now if we want to do this, we need to add another clone flag to not
> break backward compatibility.
> 
> We don't have any user requests to switch times on exec except the
> vfork+exec combination, so there is no reason to add a new clone flag.
> As for vfork+exec, this should be safe to allow switching timens with
> the current clone flag. Right now, vfork (CLONE_VFORK | CLONE_VM) fails
> if a child is forked into another time namespace. With this change,
> vfork creates a new process in parent's timens, and the following exec
> does the actual switch to the target time namespace.

This seems like a very special case. None of the other namespaces do
this, do they?

How is CLONE_NEWTIME supposed to be used today?

-Kees

-- 
Kees Cook