Re: repeatable boot randomness inside KVM guest

[Alexey Dobriyan]

On Sat, Apr 14, 2018 at 03:41:42PM -0700, Andy Lutomirski wrote:
> On Sat, Apr 14, 2018 at 12:59 PM, Alexey Dobriyan <adobriyan@xxxxxxxxx> 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.

Please, do. Here is a list of caches which aren't exactly randomly
randomized with my setup. Many important ones are there :-(

XXX name 'dma-kmalloc-96', r b1e6718e2e7147d4
XXX name 'dma-kmalloc-192', r a7664a0d69968019
XXX name 'dma-kmalloc-8', r 662c2e986443235c
XXX name 'dma-kmalloc-16', r 770a9b620ae4cd62
XXX name 'dma-kmalloc-32', r 2e200073d5fa9f46
XXX name 'dma-kmalloc-64', r d8538fda83c74168
XXX name 'dma-kmalloc-128', r 9e4b956d09dd7d44
XXX name 'dma-kmalloc-256', r 8b14bcb58f9e18f5
XXX name 'dma-kmalloc-512', r 2bbace4b7120624a
XXX name 'dma-kmalloc-1024', r 7cdf44406db52f5b
XXX name 'dma-kmalloc-2048', r 18fe0ebf6bcfdf43
XXX name 'dma-kmalloc-4096', r 9f1a5eee118facf7
XXX name 'dma-kmalloc-8192', r f514d72a1cc441a2
XXX name 'kmalloc-8192', r 14843df817b556cc
XXX name 'kmalloc-4096', r 52ed85fa9c691bbe
XXX name 'kmalloc-2048', r fa81aa9222ff65a7
XXX name 'kmalloc-1024', r ae355c02d31f21d3
XXX name 'kmalloc-512', r 5fe0d22aaf2ef8d9
XXX name 'kmalloc-256', r 336d07a06917b95
XXX name 'kmalloc-192', r 6b6cd5399dd06d95
XXX name 'kmalloc-128', r 893b9e85369964ab
XXX name 'kmalloc-96', r 179e185395d2612
XXX name 'kmalloc-64', r 29cf688b37eccea7
XXX name 'kmalloc-32', r fb7b4e7dca6de00a
XXX name 'kmalloc-16', r a2a441fdc499d0c7
XXX name 'kmalloc-8', r e5454c7095ddd2be
XXX name 'kmem_cache_node', r 500dc6126a47b229
XXX name 'kmem_cache', r 816c8c7bcde08372
XXX name 'task_group', r c09c4d1c1436ce97
XXX name 'radix_tree_node', r 4dd9540b830a4ea8
XXX name 'pool_workqueue', r 88b1e9d9a1f0b570
XXX name 'Acpi-Namespace', r 3e34d55f8f1cb140
XXX name 'Acpi-State', r b94e04635e77b48a
XXX name 'Acpi-Parse', r d5374863b90f2a4c
XXX name 'Acpi-ParseExt', r eefb2fff892f64a9
XXX name 'Acpi-Operand', r ce51949bcc80af13
XXX name 'pid', r cd6d8ee9e5209156
XXX name 'anon_vma', r c3a9273a68127ac7
XXX name 'anon_vma_chain', r a7cec15033c31a9b
XXX name 'cred_jar', r fe4cc38c6d99cf63
XXX name 'task_struct', r eecb8895c6b7dbdb
XXX name 'sighand_cache', r e5243c5eb2ce3a63
XXX name 'signal_cache', r 88b2e108d8ef81c7
XXX name 'files_cache', r ee29814e58dc909c
XXX name 'fs_cache', r bc700a5f8fc28ff8
XXX name 'mm_struct', r f5230f99c7447359
XXX name 'vm_area_struct', r e30f3f8e648a9f88
XXX name 'nsproxy', r ae7c08b524a0f4d4
XXX name 'uts_namespace', r 6b1266178968ed99
XXX name 'buffer_head', r b24c10679dc55a11
XXX name 'names_cache', r 2e023b54e3ca5b8f
XXX name 'dentry', r 83cc18634fbd74e8
XXX name 'inode_cache', r ff9a0ff3b4665cf5
XXX name 'filp', r 4fdad214b7ca7fc1
XXX name 'mnt_cache', r 8e726d32470b23e0
XXX name 'kernfs_node_cache', r 929c5f56778d365d
XXX name 'bdev_cache', r 8a5520036bd0a464
XXX name 'sigqueue', r 2cf75c4d16191efb
XXX name 'seq_file', r ec3ba1fe514524d5
XXX name 'proc_inode_cache', r b0c76cbbda5bb41f
XXX name 'pde_opener', r 5f82f8e7100a517c
XXX name 'proc_dir_entry', r ebabc4e93b52d7b8
XXX name 'shmem_inode_cache', r 2b25a3eb9aa32973
XXX name 'net_namespace', r 95793a7eae08a33f