Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory

[Quentin Perret]

On Monday 28 Mar 2022 at 18:58:35 (+0000), Sean Christopherson wrote:
> On Mon, Mar 28, 2022, Quentin Perret wrote:
> > Hi Sean,
> > 
> > Thanks for the reply, this helps a lot.
> > 
> > On Monday 28 Mar 2022 at 17:13:10 (+0000), Sean Christopherson wrote:
> > > On Thu, Mar 24, 2022, Quentin Perret wrote:
> > > > For Protected KVM (and I suspect most other confidential computing
> > > > solutions), guests have the ability to share some of their pages back
> > > > with the host kernel using a dedicated hypercall. This is necessary
> > > > for e.g. virtio communications, so these shared pages need to be mapped
> > > > back into the VMM's address space. I'm a bit confused about how that
> > > > would work with the approach proposed here. What is going to be the
> > > > approach for TDX?
> > > > 
> > > > It feels like the most 'natural' thing would be to have a KVM exit
> > > > reason describing which pages have been shared back by the guest, and to
> > > > then allow the VMM to mmap those specific pages in response in the
> > > > memfd. Is this something that has been discussed or considered?
> > > 
> > > The proposed solution is to exit to userspace with a new exit reason, 
> > > KVM_EXIT_MEMORY_ERROR,
> > > when the guest makes the hypercall to request conversion[1].  The private 
> > > fd itself
> > > will never allow mapping memory into userspace, instead userspace will 
> > > need to punch
> > > a hole in the private fd backing store.  The absense of a valid mapping 
> > > in the private
> > > fd is how KVM detects that a pfn is "shared" (memslots without a private 
> > > fd are always
> > > shared)[2].
> > 
> > Right. I'm still a bit confused about how the VMM is going to get the
> > shared page mapped in its page-table. Once it has punched a hole into
> > the private fd, how is it supposed to access the actual physical page
> > that the guest shared?
> 
> The guest doesn't share a _host_ physical page, the guest shares a _guest_ 
> physical
> page.  Until host userspace converts the gfn to shared and thus maps the 
> gfn=>hva
> via mmap(), the guest is blocked and can't read/write/exec the memory.  
> AFAIK, no
> architecture allows in-place decryption of guest private memory.  s390 allows 
> a
> page to be "made accessible" to the host for the purposes of swap, and other
> architectures will have similar behavior for migrating a protected VM, but 
> those
> scenarios are not sharing the page (and they also make the page inaccessible 
> to
> the guest).

I see. FWIW, since pKVM is entirely MMU-based, we are in fact capable of
doing in-place sharing, which also means it can retain the content of
the page as part of the conversion.

Also, I'll ask the Arm CCA developers to correct me if this is wrong, but
I _believe_ it should be technically possible to do in-place sharing for
them too.

> > Is there an assumption somewhere that the VMM should have this page mapped 
> > in
> > via an alias that it can legally access only once it has punched a hole at
> > the corresponding offset in the private fd or something along those lines?
> 
> Yes, the VMM must have a completely separate VMA.  The VMM doesn't haven't to
> wait until the conversion to mmap() the shared variant, though obviously it 
> will
> potentially consume double the memory if the VMM actually populates both the
> private and shared backing stores.

Gotcha. This is what confused me I think -- in this approach private and
shared pages are in fact entirely different.

In which scenario could you end up with both the private and shared
pages live at the same time? Would this be something like follows?

 - userspace creates a private fd, fallocates into it, and associates
   the <fd, offset, size> tuple with a private memslot;

 - userspace then mmaps anonymous memory (for ex.), and associates it
   with a standard memslot, which happens to be positioned at exactly
   the right offset w.r.t to the private memslot (with this offset
   defined by the bit that is set for the private addresses in the gpa
   space);

 - the guest runs, and accesses both 'aliases' of the page without doing
   an explicit share hypercall.

Is there another option?

Is implicit sharing a thing? E.g., if a guest makes a memory access in
the shared gpa range at an address that doesn't have a backing memslot,
will KVM check whether there is a corresponding private memslot at the
right offset with a hole punched and report a KVM_EXIT_MEMORY_ERROR? Or
would that just generate an MMIO exit as usual?

> > > The key point is that KVM never decides to convert between shared and 
> > > private, it's
> > > always a userspace decision.  Like normal memslots, where userspace has 
> > > full control
> > > over what gfns are a valid, this gives userspace full control over 
> > > whether a gfn is
> > > shared or private at any given time.
> > 
> > I'm understanding this as 'the VMM is allowed to punch holes in the
> > private fd whenever it wants'. Is this correct?
> 
> From the kernel's perspective, yes, the VMM can punch holes at any time.  
> From a
> "do I want to DoS my guest" perspective, the VMM must honor its contract with 
> the
> guest and not spuriously unmap private memory.
> 
> > What happens if it does so for a page that a guest hasn't shared back?
> 
> When the hole is punched, KVM will unmap the corresponding private SPTEs.  If 
> the
> guest is still accessing the page as private, the next access will fault and 
> KVM
> will exit to userspace with KVM_EXIT_MEMORY_ERROR.  Of course the guest is 
> probably
> hosed if the hole punch was truly spurious, as at least hardware-based 
> protected VMs
> effectively destroy data when a private page is unmapped from the guest 
> private SPTEs.
>
> E.g. Linux guests for TDX and SNP will panic/terminate in such a scenario as 
> they
> will get a fault (injected by trusted hardware/firmware) saying that the 
> guest is
> trying to access an unaccepted/unvalidated page (TDX and SNP require the 
> guest to
> explicit accept all private pages that aren't part of the guest's initial 
> pre-boot
> image).

I suppose this is necessary is to prevent the VMM from re-fallocating
in a hole it previously punched and re-entering the guest without
notifying it?

> > > Another important detail is that this approach means the kernel and KVM 
> > > treat the
> > > shared backing store and private backing store as independent, albeit 
> > > related,
> > > entities.  This is very deliberate as it makes it easier to reason about 
> > > what is
> > > and isn't allowed/required.  E.g. the kernel only needs to handle freeing 
> > > private
> > > memory, there is no special handling for conversion to shared because no 
> > > such path
> > > exists as far as host pfns are concerned.  And userspace doesn't need any 
> > > new "rules"
> > > for protecting itself against a malicious guest, e.g. userspace already 
> > > needs to
> > > ensure that it has a valid mapping prior to accessing guest memory (or be 
> > > able to
> > > handle any resulting signals).  A malicious guest can DoS itself by 
> > > instructing
> > > userspace to communicate over memory that is currently mapped private, 
> > > but there
> > > are no new novel attack vectors from the host's perspective as coercing 
> > > the host
> > > into accessing an invalid mapping after shared=>private conversion is 
> > > just a variant
> > > of a use-after-free.
> > 
> > Interesting. I was (maybe incorrectly) assuming that it would be
> > difficult to handle illegal host accesses w/ TDX. IOW, this would
> > essentially crash the host. Is this remotely correct or did I get that
> > wrong?
> 
> Handling illegal host kernel accesses for both TDX and SEV-SNP is extremely
> difficult, bordering on impossible.  That's one of the biggest, if not _the_
> biggest, motivations for the private fd approach.  On "conversion", the page 
> that is
> used to back the shared variant is a completely different, unrelated host 
> physical
> page.  Whether or not the private/shared backing page is freed is orthogonal 
> to
> what version is mapped into the guest.  E.g. if the guest converts a 4kb 
> chunk of
> a 2mb hugepage, the private backing store could keep the physical page on hole
> punch (example only, I don't know if this is the actual proposed 
> implementation).
> 
> The idea is that it'll be much, much more difficult for the host to perform an
> illegal access if the actual private memory is not mapped anywhere (modulo the
> kernel's direct map, which we may or may not leave intact).  The private 
> backing
> store just needs to ensure it properly sanitizing pages before freeing them.

Understood.

I'm overall inclined to think that while this abstraction works nicely
for TDX and the likes, it might not suit pKVM all that well in the
current form, but it's close.

What do you think of extending the model proposed here to also address
the needs of implementations that support in-place sharing? One option
would be to have KVM notify the private-fd backing store when a page is
shared back by a guest, which would then allow host userspace to mmap
that particular page in the private fd instead of punching a hole.

This should retain the main property you're after: private pages that
are actually mapped in the guest SPTE aren't mmap-able, but all the
others are fair game.

Thoughts?