Re: security implications of caching with virtio pmem (was Re: [PATCH v3 0/5] kvm "virtio pmem" device)

David Hildenbrand
2019-02-06T14:00:26Z

On 04.02.19 23:56, Michael S. Tsirkin wrote:
>
> On Wed, Jan 09, 2019 at 08:17:31PM +0530, Pankaj Gupta wrote:
>> This patch series has implementation for "virtio pmem".
>> "virtio pmem" is fake persistent memory(nvdimm) in guest
>> which allows to bypass the guest page cache. This also
>> implements a VIRTIO based asynchronous flush mechanism.
>
>
> At Pankaj's request I looked at information leak implications of virtio
> pmem in light of the recent page cache side channels paper
> (https://arxiv.org/pdf/1901.01161.pdf) - to see what
> kind of side channels it might create if any. TLDR - I think that
> depending on the host side implementation there could be some, but this
> might be addressable by better documentation in both code and spec.
> The fake dax approach backing the guest memory by a host page cache
> does seem to have potential issues.
>
> For clarity: we are talking about leaking information either to a VM, or
> within a VM (I did not look into leaks to hypervisor in configurations
> such as SEV) through host page cache.
>
> Leaks into a VM: It seems clear that while pmem allows memory accesses
> versus read/write with e.g. a block device, from host page cache point
> of view this doesn't matter much: reads populate cache in the same way
> as memory faults. Thus ignoring presence of information leaks (which is
> an interesting question e.g. in light of recent discard support) pmem
> doesn't seem to be any better or worse for leaking information into a
> VM.

+1, just a different way to access that cache.

Conceptually a virtio-pmem devices is from the guest view a "device with
a managed buffer". Some accesses might be faster than others. There are
no guarantees on how fast a certain access is. And yes, actions on other
guests can result in accesses being slower but not faster.

Also other storage devices have caches like that (well, the caches size
depends on the device) - thinking especially about storage systems -
which would in my opinion, also allow similar leaks. How are such
security concerns handled there? Are they different (besides eventually
access speed)?

>
> Leaks within VM: Right now pmem seems to bypass the guest page cache
> completely. Whether pmem memory is then resident in a page cache would
> be up to the device/host. Assuming that it is, the "Preventing
> Efficient Eviction while Increasing the System Performance"
> countermeasure for the page cache side channel attack would appear to
> become ineffective with pmem. What is suggested is a per-process
> management of the page cache, and host does not have visibility of
> processes within a VM. Another possible countermeasure - not discussed
> in the paper - could be modify the applications to lock the security
> relevant pages in memory. Again this becomes impractical with pmem as
> host does not have visibility into that. However note that as long
> as the only countermeasure linux uses is "Privileged Access"
> (i.e. blocking mincore) nothing can be done as guest page cache
> remains as vulnerable as host page cache.

This sounds very use-case specific. If I run a VM only with a very
specific workload (say, a container running one application), I usually
don't care about leaks within the VM. At least not leaks between
applications ;)

In contrast, to running different applications (e.g. containers from
different customers) on one system, I really care about leaks within a VM.

>
>
> Countermeasures: which host-side countermeasures can be designed would
> depend on which countermeasures are used guest-side - we would need to
> make sure they are not broken by pmem. For "Preventing Efficient
> Eviction while Increasing the System Performance" modifying the host
> implementation to ensure that pmem device bypasses the host page cache
> would seem to address the security problem.Similarly, ensuring that a
> real memory device (e.g. DAX, RAM such as hugetlbfs, pmem for nested
> virt) is used for pmem would make the memory locking countermeasure
> work. Whether with such limitations the device is still useful
> performance wise is an open question. These questions probably should
> be addressed in the documentation, spec and possible qemu code.
>
I also want to note that using a disk/file as memory backend with
NVDIMMs in QEMU essentially results in the exact same questions we have
with virtio-pmem.

E.g. kata-containers use nvdimms for the rootfile system (read-only) as
far as I am aware.

Conceptually, a virtio-pmem device is just an emulated nvdimm device
with a flush interface. And the nice thing is, that it is designed to
also work on architectures that don't speak "nvdimm".

>
> Severity of the security implications: some people argue that the
> security implications of the page cache leaks are minor. I do not have
> an opinion on this: the severity would seem to depend on the specific
> configuration.

I guess configuration and use case.

Nice summary, thanks for looking into this Michael!

--

Thanks,

David / dhildenb