[PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[Fuad Tabba]

On Wed, Nov 1, 2023 at 9:55 PM Sean Christopherson <seanjc at google.com> wrote:
>
> On Wed, Nov 01, 2023, Fuad Tabba wrote:
> > > > > @@ -1034,6 +1034,9 @@ static void kvm_destroy_dirty_bitmap(struct kvm_memory_slot *memslot)
> > > > >  /* This does not remove the slot from struct kvm_memslots data structures */
> > > > >  static void kvm_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot)
> > > > >  {
> > > > > +       if (slot->flags & KVM_MEM_PRIVATE)
> > > > > +               kvm_gmem_unbind(slot);
> > > > > +
> > > >
> > > > Should this be called after kvm_arch_free_memslot()? Arch-specific ode
> > > > might need some of the data before the unbinding, something I thought
> > > > might be necessary at one point for the pKVM port when deleting a
> > > > memslot, but realized later that kvm_invalidate_memslot() ->
> > > > kvm_arch_guest_memory_reclaimed() was the more logical place for it.
> > > > Also, since that seems to be the pattern for arch-specific handlers in
> > > > KVM.
> > >
> > > Maybe?  But only if we can about symmetry between the allocation and free paths
> > > I really don't think kvm_arch_free_memslot() should be doing anything beyond a
> > > "pure" free.  E.g. kvm_arch_free_memslot() is also called after moving a memslot,
> > > which hopefully we never actually have to allow for guest_memfd, but any code in
> > > kvm_arch_free_memslot() would bring about "what if" questions regarding memslot
> > > movement.  I.e. the API is intended to be a "free arch metadata associated with
> > > the memslot".
> > >
> > > Out of curiosity, what does pKVM need to do at kvm_arch_guest_memory_reclaimed()?
> >
> > It's about the host reclaiming ownership of guest memory when tearing
> > down a protected guest. In pKVM, we currently teardown the guest and
> > reclaim its memory when kvm_arch_destroy_vm() is called. The problem
> > with guestmem is that kvm_gmem_unbind() could get called before that
> > happens, after which the host might try to access the unbound guest
> > memory. Since the host hasn't reclaimed ownership of the guest memory
> > from hyp, hilarity ensues (it crashes).
> >
> > Initially, I hooked reclaim guest memory to kvm_free_memslot(), but
> > then I needed to move the unbind later in the function. I realized
> > later that kvm_arch_guest_memory_reclaimed() gets called earlier (at
> > the right time), and is more aptly named.
>
> Aha!  I suspected that might be the case.
>
> TDX and SNP also need to solve the same problem of "reclaiming" memory before it
> can be safely accessed by the host.  The plan is to add an arch hook (or two?)
> into guest_memfd that is invoked when memory is freed from guest_memfd.
>
> Hooking kvm_arch_guest_memory_reclaimed() isn't completely correct as deleting a
> memslot doesn't *guarantee* that guest memory is actually reclaimed (which reminds
> me, we need to figure out a better name for that thing before introducing
> kvm_arch_gmem_invalidate()).

I see. I'd assumed that that was what you're using. I agree that it's
not completely correct, so for the moment, I assume that if that
happens we have a misbehaving host, teardown the guest and reclaim its
memory.

> The effective false positives aren't fatal for the current usage because the hook
> is used only for x86 SEV guests to flush caches.  An unnecessary flush can cause
> performance issues, but it doesn't affect correctness. For TDX and SNP, and IIUC
> pKVM, false positives are fatal because KVM could assign memory back to the host
> that is still owned by guest_memfd.

Yup.

> E.g. a misbehaving userspace could prematurely delete a memslot.  And the more
> fun example is intrahost migration, where the plan is to allow pointing multiple
> guest_memfd files at a single guest_memfd inode:
> https://lore.kernel.org/all/cover.1691446946.git.ackerleytng@google.com
>
> There was a lot of discussion for this, but it's scattered all over the place.
> The TL;DR is is that the inode will represent physical memory, and a file will
> represent a given "struct kvm" instance's view of that memory.  And so the memory
> isn't reclaimed until the inode is truncated/punched.
>
> I _think_ this reflects the most recent plan from the guest_memfd side:
> https://lore.kernel.org/all/1233d749211c08d51f9ca5d427938d47f008af1f.1689893403.git.isaku.yamahata@intel.com

Thanks for pointing that out. I think this might be the way to go.
I'll have a closer look at this and see how to get it to work with
pKVM.

Cheers,
/fuad