Re: [PATCH v6 4/8] KVM: Extend the memslot to support fd-based private memory

Chao Peng
2022-05-30T13:47:28Z

On Mon, May 23, 2022 at 03:22:32PM +0000, Sean Christopherson wrote:
> On Mon, May 23, 2022, Chao Peng wrote:
> > On Fri, May 20, 2022 at 06:31:02PM +0000, Sean Christopherson wrote:
> > > On Fri, May 20, 2022, Andy Lutomirski wrote:
> > > > The alternative would be to have some kind of separate table or bitmap (part
> > > > of the memslot?) that tells KVM whether a GPA should map to the fd.
> > > >
> > > > What do you all think?
> > >
> > > My original proposal was to have expolicit shared vs. private memslots, and punch
> > > holes in KVM's memslots on conversion, but due to the way KVM (and userspace)
> > > handle memslot updates, conversions would be painfully slow. That's how we ended
> > > up with the current propsoal.
> > >
> > > But a dedicated KVM ioctl() to add/remove shared ranges would be easy to implement
> > > and wouldn't necessarily even need to interact with the memslots. It could be a
> > > consumer of memslots, e.g. if we wanted to disallow registering regions without an
> > > associated memslot, but I think we'd want to avoid even that because things will
> > > get messy during memslot updates, e.g. if dirty logging is toggled or a shared
> > > memory region is temporarily removed then we wouldn't want to destroy the tracking.
> >
> > Even we don't tight that to memslots, that info can only be effective
> > for private memslot, right? Setting this ioctl to memory ranges defined
> > in a traditional non-private memslots just makes no sense, I guess we can
> > comment that in the API document.
>
> Hrm, applying it universally would be funky, e.g. emulated MMIO would need to be
> declared "shared". But, applying it selectively would arguably be worse, e.g.
> letting userspace map memory into the guest as shared for a region that's registered
> as private...
>
> On option to that mess would be to make memory shared by default, and so userspace
> must declare regions that are private. Then there's no weirdness with emulated MMIO
> or "legacy" memslots.
>
> On page fault, KVM does a lookup to see if the GPA is shared or private. If the
> GPA is private, but there is no memslot or the memslot doesn't have a private fd,
> KVM exits to userspace. If there's a memslot with a private fd, the shared/private
> flag is used to resolve the
>
> And to handle the ioctl(), KVM can use kvm_zap_gfn_range(), which will bump the
> notifier sequence, i.e. force the page fault to retry if the GPA may have been
> (un)registered between checking the type and acquiring mmu_lock.

Yeah, that makes sense.

>
> > > I don't think we'd want to use a bitmap, e.g. for a well-behaved guest, XArray
> > > should be far more efficient.
> >
> > What about the mis-behaved guest? I don't want to design for the worst
> > case, but people may raise concern on the attack from such guest.
>
> That's why cgroups exist. E.g. a malicious/broken L1 can similarly abuse nested
> EPT/NPT to generate a large number of shadow page tables.

I havn't seen we had that in KVM. Is there any plan/discussion to add that?

>
> > > One benefit to explicitly tracking this in KVM is that it might be useful for
> > > software-only protected VMs, e.g. KVM could mark a region in the XArray as "pending"
> > > based on guest hypercalls to share/unshare memory, and then complete the transaction
> > > when userspace invokes the ioctl() to complete the share/unshare.
> >
> > OK, then this can be another field of states/flags/attributes. Let me
> > dig up certain level of details:
> >
> > First, introduce below KVM ioctl
> >
> > KVM_SET_MEMORY_ATTR
>
> Actually, if the semantics are that userspace declares memory as private, then we
> can reuse KVM_MEMORY_ENCRYPT_REG_REGION and KVM_MEMORY_ENCRYPT_UNREG_REGION. It'd
> be a little gross because we'd need to slightly redefine the semantics for TDX, SNP,
> and software-protected VM types, e.g. the ioctls() currently require a pre-exisitng
> memslot. But I think it'd work...

These existing ioctls looks good for TDX and probably SNP as well. For
softrware-protected VM types, it may not be enough. Maybe for the first
step we can reuse this for all hardware based solutions and invent new
interface when software-protected solution gets really supported.

There is semantics difference for fd-based private memory. Current above
two ioctls() use userspace addreess(hva) while for fd-based it should be
fd+offset, and probably it's better to use gpa in this case. Then we
will need change existing semantics and break backward-compatibility.

Chao

>
> I'll think more on this...