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

[Chao Peng <chao.p.peng@linux.intel.com>]

On Tue, Jun 07, 2022 at 05:55:46PM -0700, Marc Orr wrote:
> On Tue, Jun 7, 2022 at 12:01 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > On Mon, Jun 06, 2022 at 01:09:50PM -0700, Vishal Annapurve wrote:
> > > >
> > > > Private memory map/unmap and conversion
> > > > ---------------------------------------
> > > > Userspace's map/unmap operations are done by fallocate() ioctl on the
> > > > backing store fd.
> > > >   - map: default fallocate() with mode=0.
> > > >   - unmap: fallocate() with FALLOC_FL_PUNCH_HOLE.
> > > > The map/unmap will trigger above memfile_notifier_ops to let KVM map/unmap
> > > > secondary MMU page tables.
> > > >
> > > ....
> > > >    QEMU: https://github.com/chao-p/qemu/tree/privmem-v6
> > > >
> > > > An example QEMU command line for TDX test:
> > > > -object tdx-guest,id=tdx \
> > > > -object memory-backend-memfd-private,id=ram1,size=2G \
> > > > -machine q35,kvm-type=tdx,pic=no,kernel_irqchip=split,memory-encryption=tdx,memory-backend=ram1
> > > >
> > >
> > > There should be more discussion around double allocation scenarios
> > > when using the private fd approach. A malicious guest or buggy
> > > userspace VMM can cause physical memory getting allocated for both
> > > shared (memory accessible from host) and private fds backing the guest
> > > memory.
> > > Userspace VMM will need to unback the shared guest memory while
> > > handling the conversion from shared to private in order to prevent
> > > double allocation even with malicious guests or bugs in userspace VMM.
> >
> > I don't know how malicious guest can cause that. The initial design of
> > this serie is to put the private/shared memory into two different
> > address spaces and gives usersapce VMM the flexibility to convert
> > between the two. It can choose respect the guest conversion request or
> > not.
> 
> For example, the guest could maliciously give a device driver a
> private page so that a host-side virtual device will blindly write the
> private page.

With this patch series, it's actually even not possible for userspace VMM
to allocate private page by a direct write, it's basically unmapped from
there. If it really wants to, it should so something special, by intention,
that's basically the conversion, which we should allow.

> 
> > It's possible for a usrspace VMM to cause double allocation if it fails
> > to call the unback operation during the conversion, this may be a bug
> > or not. Double allocation may not be a wrong thing, even in conception.
> > At least TDX allows you to use half shared half private in guest, means
> > both shared/private can be effective. Unbacking the memory is just the
> > current QEMU implementation choice.
> 
> Right. But the idea is that this patch series should accommodate all
> of the CVM architectures. Or at least that's what I know was
> envisioned last time we discussed this topic for SNP [*].

AFAICS, this series should work for both TDX and SNP, and other CVM
architectures. I don't see where TDX can work but SNP cannot, or I
missed something here?

> 
> Regardless, it's important to ensure that the VM respects its memory
> budget. For example, within Google, we run VMs inside of containers.
> So if we double allocate we're going to OOM. This seems acceptable for
> an early version of CVMs. But ultimately, I think we need a more
> robust way to ensure that the VM operates within its memory container.
> Otherwise, the OOM is going to be hard to diagnose and distinguish
> from a real OOM.

Thanks for bringing this up. But in my mind I still think userspace VMM
can do and it's its responsibility to guarantee that, if that is hard
required. By design, userspace VMM is the decision-maker for page
conversion and has all the necessary information to know which page is
shared/private. It also has the necessary knobs to allocate/free the
physical pages for guest memory. Definitely, we should make userspace
VMM more robust.

Chao
> 
> [*] https://lore.kernel.org/all/20210820155918.7518-1-brijesh.singh@amd.com/
> 
> >
> > Chao
> > >
> > > Options to unback shared guest memory seem to be:
> > > 1) madvise(.., MADV_DONTNEED/MADV_REMOVE) - This option won't stop
> > > kernel from backing the shared memory on subsequent write accesses
> > > 2) fallocate(..., FALLOC_FL_PUNCH_HOLE...) - For file backed shared
> > > guest memory, this option still is similar to madvice since this would
> > > still allow shared memory to get backed on write accesses
> > > 3) munmap - This would give away the contiguous virtual memory region
> > > reservation with holes in the guest backing memory, which might make
> > > guest memory management difficult.
> > > 4) mprotect(... PROT_NONE) - This would keep the virtual memory
> > > address range backing the guest memory preserved
> > >
> > > ram_block_discard_range_fd from reference implementation:
> > > https://github.com/chao-p/qemu/tree/privmem-v6 seems to be relying on
> > > fallocate/madvise.
> > >
> > > Any thoughts/suggestions around better ways to unback the shared
> > > memory in order to avoid double allocation scenarios?
> 
> I agree with Vishal. I think this patch set is making great progress.
> But the double allocation scenario seems like a high-level design
> issue that warrants more discussion.