Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Chao Peng]

On Fri, Aug 26, 2022 at 04:19:25PM +0100, Fuad Tabba wrote:
> Hi,
> 
> On Wed, Jul 6, 2022 at 9:24 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> >
> > This is the v7 of this series which tries to implement the fd-based KVM
> > guest private memory. The patches are based on latest kvm/queue branch
> > commit:
> >
> >   b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU
> > split_desc_cache only by default capacity
> >
> > Introduction
> > ------------
> > In general this patch series introduce fd-based memslot which provides
> > guest memory through memory file descriptor fd[offset,size] instead of
> > hva/size. The fd can be created from a supported memory filesystem
> > like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM
> > and the the memory backing store exchange callbacks when such memslot
> > gets created. At runtime KVM will call into callbacks provided by the
> > backing store to get the pfn with the fd+offset. Memory backing store
> > will also call into KVM callbacks when userspace punch hole on the fd
> > to notify KVM to unmap secondary MMU page table entries.
> >
> > Comparing to existing hva-based memslot, this new type of memslot allows
> > guest memory unmapped from host userspace like QEMU and even the kernel
> > itself, therefore reduce attack surface and prevent bugs.
> >
> > Based on this fd-based memslot, we can build guest private memory that
> > is going to be used in confidential computing environments such as Intel
> > TDX and AMD SEV. When supported, the memory backing store can provide
> > more enforcement on the fd and KVM can use a single memslot to hold both
> > the private and shared part of the guest memory.
> >
> > mm extension
> > ---------------------
> > Introduces new MFD_INACCESSIBLE flag for memfd_create(), the file
> > created with these flags cannot read(), write() or mmap() etc via normal
> > MMU operations. The file content can only be used with the newly
> > introduced memfile_notifier extension.
> >
> > The memfile_notifier extension provides two sets of callbacks for KVM to
> > interact with the memory backing store:
> >   - memfile_notifier_ops: callbacks for memory backing store to notify
> >     KVM when memory gets invalidated.
> >   - backing store callbacks: callbacks for KVM to call into memory
> >     backing store to request memory pages for guest private memory.
> >
> > The memfile_notifier extension also provides APIs for memory backing
> > store to register/unregister itself and to trigger the notifier when the
> > bookmarked memory gets invalidated.
> >
> > The patchset also introduces a new memfd seal F_SEAL_AUTO_ALLOCATE to
> > prevent double allocation caused by unintentional guest when we only
> > have a single side of the shared/private memfds effective.
> >
> > memslot extension
> > -----------------
> > Add the private fd and the fd offset to existing 'shared' memslot so
> > that both private/shared guest memory can live in one single memslot.
> > A page in the memslot is either private or shared. Whether a guest page
> > is private or shared is maintained through reusing existing SEV ioctls
> > KVM_MEMORY_ENCRYPT_{UN,}REG_REGION.
> >
> 
> I'm on the Android pKVM team at Google, and we've been looking into
> how this approach fits with what we've been doing with pkvm/arm64.
> I've had a go at porting your patches, along with some fixes and
> additions so it would go on top of our latest pkvm patch series [1] to
> see how well this proposal fits with what we’re doing. You can find
> the ported code at this link [2].
> 
> In general, an fd-based approach fits very well with pKVM for the
> reasons you mention. It means that we don't necessarily need to map
> the guest memory, and with the new extensions it allows the host
> kernel to control whether to restrict migration and swapping.

Good to hear that.

> 
> For pKVM, we would also need the guest private memory not to be
> GUP’able by the kernel so that userspace can’t trick the kernel into
> accessing guest private memory in a context where it isn’t prepared to
> handle the fault injected by the hypervisor. We’re looking at whether
> we could use memfd_secret to achieve this, or maybe whether extending
> your work might solve the problem.

This is interesting and can be a valuable addition to this series.

> 
> However, during the porting effort, the main issue we've encountered
> is that many of the details of this approach seem to be targeted at
> TDX/SEV and don’t readily align with the design of pKVM. My knowledge
> on TDX is very rudimentary, so please bear with me if I get things
> wrong.

No doubt this series is initially designed for confidential computing
usages, but pKVM can definitely extend it if it finds useful.

> 
> The idea of the memslot having two references to the backing memory,
> the (new) private_fd (a file descriptor) as well as the userspace_addr
> (a memory address), with the meaning changing depending on whether the
> memory is private or shared. Both can potentially be live at the same
> time, but only one is used by the guest depending on whether the
> memory is shared or private. For pKVM, the memory region is the same,
> and whether the underlying physical page is shared or private is
> determined by the hypervisor based on the initial configuration of the
> VM and also in response to hypercalls from the guest.

For confidential computing usages, this is actually the same. The shared
or private is determined by initial configuration or guest hypercalls.

> So at least from
> our side, having a private_fd isn't the best fit, but rather just
> having an fd instead of a userspace_addr.

Let me understand this a bit: pKVM basically wants to maintain the
shared and private memory in only one fd, and not use userspace_addr at
all, right? Any blocking for pKVM to use private_fd + userspace_addr
instead?

> 
> Moreover, something which was discussed here before [3], is the
> ability to share in-place. For pKVM/arm64, the conversion between
> shared and private involves only changes to the stage-2 page tables,
> which are controlled by the hypervisor. Android supports this in-place
> conversion already, and I think that the cost of copying for many
> use-cases that would involve large amounts of data would be big. We
> will measure the relative costs in due course, but in the meantime
> we’re nervous about adopting a new user ABI which doesn’t appear to
> cater for in-place conversion; having just the fd would simplify that
> somewhat

I understand there is difficulty to achieve that with the current
private_fd + userspace_addr (they basically in two separate fds), but is
it possible for pKVM to extend this? Brainstorming for example, pKVM can
ignore userspace_addr and only use private_fd to cover both shared and
private memory, or pKVM introduce new KVM memslot flag?

> 
> In the memfd approach, what is the plan for being able to initialize
> guest private memory from the host? In my port of this patch series,
> I've added an fcntl() command that allows setting INACCESSIBLE after
> the memfd has been created. So the memory can be mapped, initialized,
> then unmapped. Of course there is no way to enforce that the memory is
> unmapped from userspace before being used as private memory, but the
> hypervisor will take care of the stage-2 mapping and so a user access
> to the private memory would result in a SEGV regardless of the flag

There is discussion on removing MFD_INACCESSIBLE and delaying the
alignment of the flag to the KVM/backing store binding time
(https://lkml.kernel.org/lkml/20220824094149.GA1383966@xxxxxxxxxxxxxxxxxx/).

Creating new API like what you are playing with fcntl() also works if it
turns out the MFD_INACCESSIBLE has to be set at the memfd_create time.

> 
> Now, moving on to implementation-specific issues in this patch series
> that I have encountered:
> 
> - There are a couple of small issues in porting the patches, some of
> which have been mentioned already by others. I will point out the rest
> in direct replies to these patches.

Thanks.

> 
> - MEMFILE_F_UNRECLAIMABLE and MEMFILE_F_UNMOVABLE are never set in
> this patch series. MFD_INACCESSIBLE only sets
> MEMFILE_F_USER_INACCESSIBLE. Is this intentional?

It gets set in kvm_private_mem_register() of patch 13, basically those
flags are expected to be set by architecture code.

> 
> - Nothing in this patch series enforces that MFD_INACCESSIBLE or that
> any of the MEMFILE_F_* flags are set for the file descriptor to be
> used as a private_fd. Is this also intentional?

With KVM_MEM_PRIVATE memslot flag, the MEMFILE_F_* are enforced by the
architecture code.

> 
> Most of us working on pKVM will be at KVM forum Dublin in September,
> so it would be great if we could have a chat (and/or beer!) face to
> face sometime during the conference to help us figure out an
> upstreamable solution for Android

I would like to, but currently I have no travel plan due to COVID-19 :(
We can have more online discussions anyway.

Thanks,
Chao
> 
> Cheers,
> /fuad
> 
> [1] https://lore.kernel.org/all/20220630135747.26983-1-will@xxxxxxxxxx/
> [2] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/fdmem
> [3] https://lore.kernel.org/all/YkcTTY4YjQs5BRhE@xxxxxxxxxx/
> 
> 
> > Test
> > ----
> > To test the new functionalities of this patch TDX patchset is needed.
> > Since TDX patchset has not been merged so I did two kinds of test:
> >
> > -  Regresion test on kvm/queue (this patchset)
> >    Most new code are not covered. Code also in below repo:
> >    https://github.com/chao-p/linux/tree/privmem-v7
> >
> > -  New Funational test on latest TDX code
> >    The patch is rebased to latest TDX code and tested the new
> >    funcationalities. See below repos:
> >    Linux: https://github.com/chao-p/linux/tree/privmem-v7-tdx
> >    QEMU: https://github.com/chao-p/qemu/tree/privmem-v7
> >
> > An example QEMU command line for TDX test:
> > -object tdx-guest,id=tdx,debug=off,sept-ve-disable=off \
> > -machine confidential-guest-support=tdx \
> > -object memory-backend-memfd-private,id=ram1,size=${mem} \
> > -machine memory-backend=ram1
> >
> > Changelog
> > ----------
> > v7:
> >   - Move the private/shared info from backing store to KVM.
> >   - Introduce F_SEAL_AUTO_ALLOCATE to avoid double allocation.
> >   - Rework on the sync mechanism between zap/page fault paths.
> >   - Addressed other comments in v6.
> > v6:
> >   - Re-organzied patch for both mm/KVM parts.
> >   - Added flags for memfile_notifier so its consumers can state their
> >     features and memory backing store can check against these flags.
> >   - Put a backing store reference in the memfile_notifier and move pfn_ops
> >     into backing store.
> >   - Only support boot time backing store register.
> >   - Overall KVM part improvement suggested by Sean and some others.
> > v5:
> >   - Removed userspace visible F_SEAL_INACCESSIBLE, instead using an
> >     in-kernel flag (SHM_F_INACCESSIBLE for shmem). Private fd can only
> >     be created by MFD_INACCESSIBLE.
> >   - Introduced new APIs for backing store to register itself to
> >     memfile_notifier instead of direct function call.
> >   - Added the accounting and restriction for MFD_INACCESSIBLE memory.
> >   - Added KVM API doc for new memslot extensions and man page for the new
> >     MFD_INACCESSIBLE flag.
> >   - Removed the overlap check for mapping the same file+offset into
> >     multiple gfns due to perf consideration, warned in document.
> >   - Addressed other comments in v4.
> > v4:
> >   - Decoupled the callbacks between KVM/mm from memfd and use new
> >     name 'memfile_notifier'.
> >   - Supported register multiple memslots to the same backing store.
> >   - Added per-memslot pfn_ops instead of per-system.
> >   - Reworked the invalidation part.
> >   - Improved new KVM uAPIs (private memslot extension and memory
> >     error) per Sean's suggestions.
> >   - Addressed many other minor fixes for comments from v3.
> > v3:
> >   - Added locking protection when calling
> >     invalidate_page_range/fallocate callbacks.
> >   - Changed memslot structure to keep use useraddr for shared memory.
> >   - Re-organized F_SEAL_INACCESSIBLE and MEMFD_OPS.
> >   - Added MFD_INACCESSIBLE flag to force F_SEAL_INACCESSIBLE.
> >   - Commit message improvement.
> >   - Many small fixes for comments from the last version.
> >
> > Links to previous discussions
> > -----------------------------
> > [1] Original design proposal:
> > https://lkml.kernel.org/kvm/20210824005248.200037-1-seanjc@xxxxxxxxxx/
> > [2] Updated proposal and RFC patch v1:
> > https://lkml.kernel.org/linux-fsdevel/20211111141352.26311-1-chao.p.peng@xxxxxxxxxxxxxxx/
> > [3] Patch v5: https://lkml.org/lkml/2022/5/19/861
> >
> > Chao Peng (12):
> >   mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd
> >   selftests/memfd: Add tests for F_SEAL_AUTO_ALLOCATE
> >   mm: Introduce memfile_notifier
> >   mm/memfd: Introduce MFD_INACCESSIBLE flag
> >   KVM: Rename KVM_PRIVATE_MEM_SLOTS to KVM_INTERNAL_MEM_SLOTS
> >   KVM: Use gfn instead of hva for mmu_notifier_retry
> >   KVM: Rename mmu_notifier_*
> >   KVM: Extend the memslot to support fd-based private memory
> >   KVM: Add KVM_EXIT_MEMORY_FAULT exit
> >   KVM: Register/unregister the guest private memory regions
> >   KVM: Handle page fault for private memory
> >   KVM: Enable and expose KVM_MEM_PRIVATE
> >
> > Kirill A. Shutemov (1):
> >   mm/shmem: Support memfile_notifier
> >
> >  Documentation/virt/kvm/api.rst             |  77 +++++-
> >  arch/arm64/kvm/mmu.c                       |   8 +-
> >  arch/mips/include/asm/kvm_host.h           |   2 +-
> >  arch/mips/kvm/mmu.c                        |  10 +-
> >  arch/powerpc/include/asm/kvm_book3s_64.h   |   2 +-
> >  arch/powerpc/kvm/book3s_64_mmu_host.c      |   4 +-
> >  arch/powerpc/kvm/book3s_64_mmu_hv.c        |   4 +-
> >  arch/powerpc/kvm/book3s_64_mmu_radix.c     |   6 +-
> >  arch/powerpc/kvm/book3s_hv_nested.c        |   2 +-
> >  arch/powerpc/kvm/book3s_hv_rm_mmu.c        |   8 +-
> >  arch/powerpc/kvm/e500_mmu_host.c           |   4 +-
> >  arch/riscv/kvm/mmu.c                       |   4 +-
> >  arch/x86/include/asm/kvm_host.h            |   3 +-
> >  arch/x86/kvm/Kconfig                       |   3 +
> >  arch/x86/kvm/mmu.h                         |   2 -
> >  arch/x86/kvm/mmu/mmu.c                     |  74 +++++-
> >  arch/x86/kvm/mmu/mmu_internal.h            |  18 ++
> >  arch/x86/kvm/mmu/mmutrace.h                |   1 +
> >  arch/x86/kvm/mmu/paging_tmpl.h             |   4 +-
> >  arch/x86/kvm/x86.c                         |   2 +-
> >  include/linux/kvm_host.h                   | 105 +++++---
> >  include/linux/memfile_notifier.h           |  91 +++++++
> >  include/linux/shmem_fs.h                   |   2 +
> >  include/uapi/linux/fcntl.h                 |   1 +
> >  include/uapi/linux/kvm.h                   |  37 +++
> >  include/uapi/linux/memfd.h                 |   1 +
> >  mm/Kconfig                                 |   4 +
> >  mm/Makefile                                |   1 +
> >  mm/memfd.c                                 |  18 +-
> >  mm/memfile_notifier.c                      | 123 ++++++++++
> >  mm/shmem.c                                 | 125 +++++++++-
> >  tools/testing/selftests/memfd/memfd_test.c | 166 +++++++++++++
> >  virt/kvm/Kconfig                           |   3 +
> >  virt/kvm/kvm_main.c                        | 272 ++++++++++++++++++---
> >  virt/kvm/pfncache.c                        |  14 +-
> >  35 files changed, 1074 insertions(+), 127 deletions(-)
> >  create mode 100644 include/linux/memfile_notifier.h
> >  create mode 100644 mm/memfile_notifier.c
> >
> > --
> > 2.25.1
> >