Re: Potentially undesirable interactions between vfork() and time namespaces

From: "Eric W. Biederman" <ebiederm@xmission.com>
To: Christian Brauner <brauner@kernel.org>
Cc: Andrei Vagin <avagin@gmail.com>,
	Alexey Izbyshev <izbyshev@ispras.ru>,
	Florian Weimer <fweimer@redhat.com>,
	Dmitry Safonov <0x7f454c46@gmail.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Kees Cook <keescook@chromium.org>
Subject: Re: Potentially undesirable interactions between vfork() and time namespaces
Date: Thu, 08 Sep 2022 17:13:08 -0500	[thread overview]
Message-ID: <87v8pxa51n.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <20220908081003.sjuerd5wiyge4jos@wittgenstein> (Christian Brauner's message of "Thu, 8 Sep 2022 10:10:03 +0200")

Christian Brauner <brauner@kernel.org> writes:

> On Wed, Sep 07, 2022 at 10:15:51AM -0700, Andrei Vagin wrote:
>> On Wed, Sep 07, 2022 at 08:33:20AM +0300, Alexey Izbyshev wrote:
>> > > 
>> > > That is something to be double checked.
>> > > 
>> > > I can't see where it would make sense to unshare a time namespace and
>> > > then call exec, instead of calling exit.  So I suspect we can just
>> > > change this behavior and no one will notice.
>> > > 
>> > One can imagine a helper binary that calls unshare, forks some children in
>> > new namespaces, and then calls exec to hand off actual work to another
>> > binary (which might not expect being in the new time namespace). I'm purely
>> > theorizing here, however. Keeping a special case for vfork() based only on
>> > FUD is likely a net negative, so it'd be nice to hear actual time namespace
>> > users speak up, and switch to the solution you suggested if they don't care.
>> 
>> I can speak for one tool that uses time namespaces for the right
>> reasons. It is CRIU.  When a process is restored, the monotonic and
>> boottime clocks have to be adjusted to match old values. It is for what
>> the timens was designed for. These changes doesn't affect CRIU.
>> 
>> Honestly, I haven't heard about other users of timens yet. I don't take
>> into account tools like unshare.
>
> LXC/LXD does
>
> unshare(CLONE_NEWTIME)
> // write offsets to /proc/self/timens_offsets
> timens_fd = open("/proc/self/ns/time_for_children", O_RDONLY | O_CLOEXEC)
> setns(timens_fd, CLONE_NEWTIME)
> exec(payload)
>
> so I agree don't change the uapi, please.
>
> But as you can see what we do is basically emulating changing time
> namespace during exec via the setns() prior to the exec call.

If I understand the description of lxc/lxd correctly the proposed change
will not effect lxc/lxd, as the time namespace is already installed
before exec.  If anything what is proposed would potentially allow
lxc/lxd to be simplified in the future by removing the setns.

Are you then requesting the behavior of the time namespace not change
when the proposed change will not effect lxc/lxd?

Eric