linux-kernel - Re: [PATCH 1/2] fs/exec: switch timens when a task gets a new mm

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Date:   Wed, 21 Sep 2022 20:29:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrei Vagin <avagin@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Alexey Izbyshev <izbyshev@...ras.ru>,
        Christian Brauner <brauner@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH 1/2] fs/exec: switch timens when a task gets a new mm

On Tue, Sep 20, 2022 at 05:31:19PM -0700, Andrei Vagin wrote:
> From: Andrei Vagin <avagin@...il.com>
> 
> Changing a time namespace requires remapping a vvar page, so we don't want
> to allow doing that if any other tasks can use the same mm.
> 
> Currently, we install a time namespace when a task is created with a new
> vm. exec() is another case when a task gets a new mm and so it can switch
> a time namespace safely, but it isn't handled now.
> 
> One more issue of the current interface is that clone() with CLONE_VM isn't
> allowed if the current task has unshared a time namespace
> (timens_for_children doesn't match the current timens).
> 
> Both these issues make some inconvenience for users. For example, Alexey
> and Florian reported that posix_spawn() uses vfork+exec and this pattern
> doesn't work with time namespaces due to the both described issues.
> LXC needed to workaround the exec() issue by calling setns.
> 
> In the commit 133e2d3e81de5 ("fs/exec: allow to unshare a time namespace on
> vfork+exec"), we tried to fix these issues with minimal impact on UAPI. But
> it adds extra complexity and some undesirable side effects. Eric suggested
> fixing the issues properly because here are all the reasons to suppose that
> there are no users that depend on the old behavior.
> 
> Cc: Alexey Izbyshev <izbyshev@...ras.ru>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Dmitry Safonov <0x7f454c46@...il.com>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Cc: Florian Weimer <fweimer@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> Suggested-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> Origin-author: "Eric W. Biederman" <ebiederm@...ssion.com>
> Signed-off-by: Andrei Vagin <avagin@...il.com>

This looks good -- my intention is for this to go into -next after the
v6.1 merge window closes. Does that match everyone's expectations?

Thanks!

-Kees

-- 
Kees Cook