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From:   Jann Horn <jannh@google.com>
Date:   Tue, 19 Apr 2022 18:37:54 +0200
Message-ID: <CAG48ez3amS6=omb8XVDEz9H2bk3MxTEK_XPjD=ZO-cXcDqz-cg@mail.gmail.com>
Subject: Re: [PATCH] random: add fork_event sysctl for polling VM forks
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        Alexander Graf <graf@amazon.com>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Colm MacCarthaigh <colmmacc@amazon.com>
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On Tue, Apr 19, 2022 at 6:04 PM Jason A. Donenfeld <Jason@zx2c4.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.
>
> For example, the following snippet can be used to print a message every
> time a VM forks, after the RNG has been reseeded:
>
>   struct pollfd fd = { .fd = open("/proc/sys/kernel/random/fork_event", O_RDONLY)  };
>   assert(fd.fd >= 0);
>   for (;;) {
>     assert(poll(&fd, 1, -1) > 0);
>     puts("vm fork detected");
>   }

This is a bit of a weird API, because normally .poll is supposed to be
level-triggered rather than edge-triggered... and AFAIK things like
epoll also kinda assume that ->poll() doesn't modify state (but that
only _really_ matters in weird cases). But at the same time, it looks
like the existing proc_sys_poll() already goes against that? So I
don't know what the right thing to do there is...