Re: [PATCH v2 1/9] KVM: x86: Add AMD SEV specific Hypercall3

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 07, 2021, Ashish Kalra wrote:
> Hello Steve,
> 
> On Wed, Jan 06, 2021 at 05:01:33PM -0800, Steve Rutherford wrote:
> > Avoiding an rbtree for such a small (but unstable) list seems correct.
> > 
> > For the unencrypted region list strategy, the only questions that I
> > have are fairly secondary.
> > - How should the kernel upper bound the size of the list in the face
> > of malicious guests, but still support large guests? (Something
> > similar to the size provided in the bitmap API would work).
> 
> I am thinking of another scenario, where a malicious guest can make
> infinite/repetetive hypercalls and DOS attack the host. 
> 
> But probably this is a more generic issue, this can be done by any guest
> and under any hypervisor, i don't know what kind of mitigations exist
> for such a scenario ?
> 
> Potentially, the guest memory donation model can handle such an attack,
> because in this model, the hypervisor will expect only one hypercall,
> any repetetive hypercalls can make the hypervisor disable the guest ?

KVM doesn't need to explicitly bound its tracking structures, it just needs to
use GFP_KERNEL_ACCOUNT when allocating kernel memory for the structures so that
the memory will be accounted to the task/process/VM.  Shadow MMU pages are the
only exception that comes to mind; they're still accounted properly, but KVM
also explicitly limits them for a variety of reasons.

The size of the list will naturally be bounded by the size of the guest; and
assuming KVM proactively merges adjancent regions, that upper bound is probably
reasonably low compared to other allocations, e.g. the aforementioned MMU pages.

And, using a list means a malicious guest will get automatically throttled as
the latency of walking the list (to merge/delete existing entries) will increase
with the size of the list.