linux-kernel - Re: [PATCH 2/2] random: add fork_event sysctl for polling VM forks

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Date:   Mon, 2 May 2022 18:12:03 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Lennart Poettering <mzxreary@...inter.de>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Graf <graf@...zon.com>,
        Colm MacCarthaigh <colmmacc@...zon.com>,
        Torben Hansen <htorben@...zon.co.uk>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 2/2] random: add fork_event sysctl for polling VM forks

On Mon, May 02, 2022 at 05:40:02PM +0200, Lennart Poettering wrote:
> On Mo, 02.05.22 16:06, Jason A. Donenfeld (Jason@...c4.com) wrote:
> 
> > In order to inform userspace of virtual machine forks, this commit adds
> > a "fork_event" sysctl, which does not return any data, but allows
> > userspace processes to poll() on it for notification of VM forks.
> >
> > It avoids exposing the actual vmgenid from the hypervisor to userspace,
> > in case there is any randomness value in keeping it secret. Rather,
> > userspace is expected to simply use getrandom() if it wants a fresh
> > value.
> 
> Wouldn't it make sense to expose a monotonic 64bit counter of detected
> VM forks since boot through read()? It might be interesting to know
> for userspace how many forks it missed the fork events for. Moreover it
> might be interesting to userspace to know if any fork happened so far
> *at* *all*, by checking if the counter is non-zero.

"Might be interesting" is different from "definitely useful". I'm not
going to add this without a clear use case. This feature is pretty
narrowly scoped in its objectives right now, and I intend to keep it
that way if possible. (And yes, I realize that is likely considerably
different from your development philosophy.)

> 
> (Ideally that counter file would even be mmapable...)

You missed the last year of discussion about this and why we have wound
up here as a first step. Check the archives for extensive discussion.

Jason