repeatable boot randomness inside KVM guest

[Alexey Dobriyan]

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.