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

Fuad Tabba
2022-08-26T15:25:17Z

Hi,

On Wed, Jul 6, 2022 at 9:24 AM Chao Peng <chao.p.peng@linux.intel.com> 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.

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.

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.

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. 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.

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

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

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.

- 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?

- 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?

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

Cheers,
/fuad

[1] https://lore.kernel.org/all/20220630135747.26983-1-will@kernel.org/
[2] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/fdmem
[3] https://lore.kernel.org/all/YkcTTY4YjQs5BRhE@google.com/

> 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@google.com/
> [2] Updated proposal and RFC patch v1:
> https://lkml.kernel.org/linux-fsdevel/20211111141352.26311-1-chao.p.peng@linux.intel.com/
> [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
>