Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory

[Chao Peng]

On Tue, Jun 14, 2022 at 01:59:41PM -0700, Andy Lutomirski wrote:
> On Tue, Jun 14, 2022 at 12:09 PM Sean Christopherson <seanjc@google.com> 
> wrote:
> >
> > On Tue, Jun 14, 2022, Andy Lutomirski wrote:
> > > On Tue, Jun 14, 2022 at 12:32 AM Chao Peng <chao.p.peng@linux.intel.com> 
> > > wrote:
> > > >
> > > > On Thu, Jun 09, 2022 at 08:29:06PM +0000, Sean Christopherson wrote:
> > > > > On Wed, Jun 08, 2022, Vishal Annapurve wrote:
> > > > >
> > > > > One argument is that userspace can simply rely on cgroups to detect 
> > > > > misbehaving
> > > > > guests, but (a) those types of OOMs will be a nightmare to debug and 
> > > > > (b) an OOM
> > > > > kill from the host is typically considered a _host_ issue and will be 
> > > > > treated as
> > > > > a missed SLO.
> > > > >
> > > > > An idea for handling this in the kernel without too much complexity 
> > > > > would be to
> > > > > add F_SEAL_FAULT_ALLOCATIONS (terrible name) that would prevent page 
> > > > > faults from
> > > > > allocating pages, i.e. holes can only be filled by an explicit 
> > > > > fallocate().  Minor
> > > > > faults, e.g. due to NUMA balancing stupidity, and major faults due to 
> > > > > swap would
> > > > > still work, but writes to previously unreserved/unallocated memory 
> > > > > would get a
> > > > > SIGSEGV on something it has mapped.  That would allow the userspace 
> > > > > VMM to prevent
> > > > > unintentional allocations without having to coordinate 
> > > > > unmapping/remapping across
> > > > > multiple processes.
> > > >
> > > > Since this is mainly for shared memory and the motivation is catching
> > > > misbehaved access, can we use mprotect(PROT_NONE) for this? We can mark
> > > > those range backed by private fd as PROT_NONE during the conversion so
> > > > subsequence misbehaved accesses will be blocked instead of causing 
> > > > double
> > > > allocation silently.
> >
> > PROT_NONE, a.k.a. mprotect(), has the same vma downsides as munmap().

Yes, right.

> >
> > > This patch series is fairly close to implementing a rather more
> > > efficient solution.  I'm not familiar enough with hypervisor userspace
> > > to really know if this would work, but:
> > >
> > > What if shared guest memory could also be file-backed, either in the
> > > same fd or with a second fd covering the shared portion of a memslot?
> > > This would allow changes to the backing store (punching holes, etc) to
> > > be some without mmap_lock or host-userspace TLB flushes?  Depending on
> > > what the guest is doing with its shared memory, userspace might need
> > > the memory mapped or it might not.
> >
> > That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea.  The 
> > issue,
> > unless I'm misreading code, is that punching a hole in the shared memory 
> > backing
> > store doesn't prevent reallocating that hole on fault, i.e. a helper 
> > process that
> > keeps a valid mapping of guest shared memory can silently fill the hole.
> >
> > What we're hoping to achieve is a way to prevent allocating memory without 
> > a very
> > explicit action from userspace, e.g. fallocate().
> 
> Ah, I misunderstood.  I thought your goal was to mmap it and prevent
> page faults from allocating.

I think we still need the mmap, but want to prevent allocating when
userspace touches previously mmaped area that has never filled the page.
I don't have clear answer if other operations like read/write should be
also prevented (probably yes). And only after an explicit fallocate() to
allocate the page these operations would act normally.

> 
> It is indeed the case (and has been since before quite a few of us
> were born) that a hole in a sparse file is logically just a bunch of
> zeros.  A way to make a file for which a hole is an actual hole seems
> like it would solve this problem nicely.  It could also be solved more
> specifically for KVM by making sure that the private/shared mode that
> userspace programs is strict enough to prevent accidental allocations
> -- if a GPA is definitively private, shared, neither, or (potentially,
> on TDX only) both, then a page that *isn't* shared will never be
> accidentally allocated by KVM.

KVM is clever enough to not allocate since it knows a GPA is shared or
not. This case it's the host userspace that can cause the allocating and
is too complex to check on every access from guest.

> If the shared backing is not mmapped,
> it also won't be accidentally allocated by host userspace on a stray
> or careless write.

As said above, mmap is still prefered, otherwise too many changes are
needed for usespace VMM.

Thanks,
Chao
> 
> 
> --Andy