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Date:   Mon, 2 May 2022 21:27:05 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Alexander Graf <graf@amazon.com>
Cc:     Lennart Poettering <mzxreary@0pointer.de>,
        linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Colm MacCarthaigh <colmmacc@amazon.com>,
        Torben Hansen <htorben@amazon.co.uk>,
        Jann Horn <jannh@google.com>,
        "Michael Kelley (LINUX)" <mikelley@microsoft.com>
Subject: Re: [PATCH 2/2] random: add fork_event sysctl for polling VM forks
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On Mon, May 02, 2022 at 08:56:05PM +0200, Alexander Graf wrote:
> 
> On 02.05.22 20:46, Jason A. Donenfeld wrote:
> > On Mon, May 02, 2022 at 08:34:38PM +0200, Alexander Graf wrote:
> >> Michael, since we already changed the CID in the spec, can we add a
> >> property to the device that indicates the first 4 bytes of the UUID will
> >> always be different between parent and child?
> >>
> >> That should give us the ability to mmap the vmgenid directly to user
> >> space and act based on a simple u32 compare for clone notification, no?
> > That is not a good idea. We want an _additional_ 4 bytes, so that we can
> > keep the first 16 bytes (128 bits) as a kernel space secret.
> 
> 
> An additional 4 bytes would be an additional 4kb (or 64kb on ARM) page. 
> Do we really rely on these 16 bytes to reseed after clone? If so, we'd 
> need to bite the bullet and provide an additional page, yes.
 
Ugh, you're right; memory mapping is pages. The other option would be
relying on RDRAND (both existing and being trusted by the user etc), but
generally people aren't too jazzed about that. We pretty much have to
assume that the existing pool is compromised, since people share cloned
VMs casually. The 128-bit vmgenid is a nice input to have.