linux-kernel - Re: repeatable boot randomness inside KVM guest

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Date:   Sat, 14 Apr 2018 15:41:42 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, "Ted Ts'o" <tytso@....edu>,
        kvm list <kvm@...r.kernel.org>, security@...nel.org
Subject: Re: repeatable boot randomness inside KVM guest

On Sat, Apr 14, 2018 at 12:59 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
> SLAB allocators got CONFIG_SLAB_FREELIST_RANDOM option which randomizes
> allocation pattern inside a slab:
>
>
>         #ifdef CONFIG_SLAB_FREELIST_RANDOM
>         /* Pre-initialize the random sequence cache */
>         static int init_cache_random_seq(struct kmem_cache *s)
>         {
>                 ...
>
> Then I printed actual random sequences for each kmem cache.
> Turned out they were all the same for most of the caches and
> they didn't vary across guest reboots.
>
>         int cache_random_seq_create(struct kmem_cache *cachep, unsigned int count, gfp_t gfp)
>         {
>                 ...
>                 /* Get best entropy at this stage of boot */
>                 prandom_seed_state(&state, get_random_long());
>
> Then I searched internet and turned out KVM can pass randomness via
> virtio-rng or something. So I linked /dev/urandom.
>
> And it didn't help!
>
> The only way to get randomness for SLAB is to enable RDRAND inside guest.
>
> Is it KVM bug?
>
> For the record I'm using qemu 2.11.1-r2 and whatever F27 ships now.

virtio-rng doesn't really do that.  I have an ancient patch set to do
exactly what you want, and I should dust it off.