Re: [PATCH v15 00/20] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

[Michael Roth]

On Tue, May 07, 2024 at 08:04:50PM +0200, Paolo Bonzini wrote:
> On Wed, May 1, 2024 at 11:03 AM Michael Roth <michael.roth@xxxxxxx> wrote:
> >
> > This patchset is also available at:
> >
> >   https://github.com/amdese/linux/commits/snp-host-v15
> >
> > and is based on top of the series:
> >
> >   "Add SEV-ES hypervisor support for GHCB protocol version 2"
> >   https://lore.kernel.org/kvm/20240501071048.2208265-1-michael.roth@xxxxxxx/
> >   https://github.com/amdese/linux/commits/sev-init2-ghcb-v1
> >
> > which in turn is based on commit 20cc50a0410f (just before v14 SNP patches):
> >
> >   https://git.kernel.org/pub/scm/virt/kvm/kvm.git/log/?h=kvm-coco-queue
> 
> I have mostly reviewed this, with the exception of the
> snp_begin/complete_psc parts.

Thanks Paolo. We actually recently uncovered some issues with
snp_begin/complete_psc using some internal kvm-unit-tests that exercise
some edge cases, so I would hold off on reviewing that. Will send a
fix-up patch today after a bit more testing.

> 
> I am not sure about removing all the pr_debug() - I am sure it will be
> a bit more painful for userspace developers to figure out what exactly
> has gone wrong in some cases. But we can add them later, if needed -
> I'm certainly not going to make a fuss about it.

Yah, they do tend to be useful for that purpose. I think if we do add
them back we can consolidate the information a little better versus what
I had previously.

-Mike

> 
> Paolo
> 
> 
> > Patch Layout
> > ------------
> >
> > 01-02: These patches revert+replace the existing .gmem_validate_fault hook
> >        with a similar .private_max_mapping_level as suggested by Sean[1]
> >
> > 03-04: These patches add some basic infrastructure and introduces a new
> >        KVM_X86_SNP_VM vm_type to handle differences verses the existing
> >        KVM_X86_SEV_VM and KVM_X86_SEV_ES_VM types.
> >
> > 05-07: These implement the KVM API to handle the creation of a
> >        cryptographic launch context, encrypt/measure the initial image
> >        into guest memory, and finalize it before launching it.
> >
> > 08-12: These implement handling for various guest-generated events such
> >        as page state changes, onlining of additional vCPUs, etc.
> >
> > 13-16: These implement the gmem/mmu hooks needed to prepare gmem-allocated
> >        pages before mapping them into guest private memory ranges as
> >        well as cleaning them up prior to returning them to the host for
> >        use as normal memory. Because this supplants certain activities
> >        like issued WBINVDs during KVM MMU invalidations, there's also
> >        a patch to avoid duplicating that work to avoid unecessary
> >        overhead.
> >
> > 17:    With all the core support in place, the patch adds a kvm_amd module
> >        parameter to enable SNP support.
> >
> > 18-20: These patches all deal with the servicing of guest requests to handle
> >        things like attestation, as well as some related host-management
> >        interfaces.
> >
> > [1] https://lore.kernel.org/kvm/ZimnngU7hn7sKoSc@xxxxxxxxxx/#t
> >
> >
> > Testing
> > -------
> >
> > For testing this via QEMU, use the following tree:
> >
> >   https://github.com/amdese/qemu/commits/snp-v4-wip3c
> >
> > A patched OVMF is also needed due to upstream KVM no longer supporting MMIO
> > ranges that are mapped as private. It is recommended you build the AmdSevX64
> > variant as it provides the kernel-hashing support present in this series:
> >
> >   https://github.com/amdese/ovmf/commits/apic-mmio-fix1d
> >
> > A basic command-line invocation for SNP would be:
> >
> >  qemu-system-x86_64 -smp 32,maxcpus=255 -cpu EPYC-Milan-v2
> >   -machine q35,confidential-guest-support=sev0,memory-backend=ram1
> >   -object memory-backend-memfd,id=ram1,size=4G,share=true,reserve=false
> >   -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,id-auth=
> >   -bios OVMF_CODE-upstream-20240410-apic-mmio-fix1d-AmdSevX64.fd
> >
> > With kernel-hashing and certificate data supplied:
> >
> >  qemu-system-x86_64 -smp 32,maxcpus=255 -cpu EPYC-Milan-v2
> >   -machine q35,confidential-guest-support=sev0,memory-backend=ram1
> >   -object memory-backend-memfd,id=ram1,size=4G,share=true,reserve=false
> >   -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,id-auth=,certs-path=/home/mroth/cert.blob,kernel-hashes=on
> >   -bios OVMF_CODE-upstream-20240410-apic-mmio-fix1d-AmdSevX64.fd
> >   -kernel /boot/vmlinuz-$ver
> >   -initrd /boot/initrd.img-$ver
> >   -append "root=UUID=d72a6d1c-06cf-4b79-af43-f1bac4f620f9 ro console=ttyS0,115200n8"
> >
> > With standard X64 OVMF package with separate image for persistent NVRAM:
> >
> >  qemu-system-x86_64 -smp 32,maxcpus=255 -cpu EPYC-Milan-v2
> >   -machine q35,confidential-guest-support=sev0,memory-backend=ram1
> >   -object memory-backend-memfd,id=ram1,size=4G,share=true,reserve=false
> >   -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,id-auth=
> >   -bios OVMF_CODE-upstream-20240410-apic-mmio-fix1d.fd
> >   -drive if=pflash,format=raw,unit=0,file=OVMF_VARS-upstream-20240410-apic-mmio-fix1d.fd,readonly=off
> >
> >
> > Known issues / TODOs
> > --------------------
> >
> >  * Base tree in some cases reports "Unpatched return thunk in use. This should
> >    not happen!" the first time it runs an SVM/SEV/SNP guests. This a recent
> >    regression upstream and unrelated to this series:
> >
> >      https://lore.kernel.org/linux-kernel/CANpmjNOcKzEvLHoGGeL-boWDHJobwfwyVxUqMq2kWeka3N4tXA@xxxxxxxxxxxxxx/T/
> >
> >  * 2MB hugepage support has been dropped pending discussion on how we plan to
> >    re-enable it in gmem.
> >
> >  * Host kexec should work, but there is a known issue with host kdump support
> >    while SNP guests are running that will be addressed as a follow-up.
> >
> >  * SNP kselftests are currently a WIP and will be included as part of SNP
> >    upstreaming efforts in the near-term.
> >
> >
> > SEV-SNP Overview
> > ----------------
> >
> > This part of the Secure Encrypted Paging (SEV-SNP) series focuses on the
> > changes required to add KVM support for SEV-SNP. This series builds upon
> > SEV-SNP guest support, which is now in mainline, and and SEV-SNP host
> > initialization support, which is now in linux-next.
> >
> > While series provides the basic building blocks to support booting the
> > SEV-SNP VMs, it does not cover all the security enhancement introduced by
> > the SEV-SNP such as interrupt protection, which will added in the future.
> >
> > With SNP, when pages are marked as guest-owned in the RMP table, they are
> > assigned to a specific guest/ASID, as well as a specific GFN with in the
> > guest. Any attempts to map it in the RMP table to a different guest/ASID,
> > or a different GFN within a guest/ASID, will result in an RMP nested page
> > fault.
> >
> > Prior to accessing a guest-owned page, the guest must validate it with a
> > special PVALIDATE instruction which will set a special bit in the RMP table
> > for the guest. This is the only way to set the validated bit outside of the
> > initial pre-encrypted guest payload/image; any attempts outside the guest to
> > modify the RMP entry from that point forward will result in the validated
> > bit being cleared, at which point the guest will trigger an exception if it
> > attempts to access that page so it can be made aware of possible tampering.
> >
> > One exception to this is the initial guest payload, which is pre-validated
> > by the firmware prior to launching. The guest can use Guest Message requests
> > to fetch an attestation report which will include the measurement of the
> > initial image so that the guest can verify it was booted with the expected
> > image/environment.
> >
> > After boot, guests can use Page State Change requests to switch pages
> > between shared/hypervisor-owned and private/guest-owned to share data for
> > things like DMA, virtio buffers, and other GHCB requests.
> >
> > In this implementation of SEV-SNP, private guest memory is managed by a new
> > kernel framework called guest_memfd (gmem). With gmem, a new
> > KVM_SET_MEMORY_ATTRIBUTES KVM ioctl has been added to tell the KVM
> > MMU whether a particular GFN should be backed by shared (normal) memory or
> > private (gmem-allocated) memory. To tie into this, Page State Change
> > requests are forward to userspace via KVM_EXIT_VMGEXIT exits, which will
> > then issue the corresponding KVM_SET_MEMORY_ATTRIBUTES call to set the
> > private/shared state in the KVM MMU.
> >
> > The gmem / KVM MMU hooks implemented in this series will then update the RMP
> > table entries for the backing PFNs to set them to guest-owned/private when
> > mapping private pages into the guest via KVM MMU, or use the normal KVM MMU
> > handling in the case of shared pages where the corresponding RMP table
> > entries are left in the default shared/hypervisor-owned state.
> >
> > Feedback/review is very much appreciated!
> >
> > -Mike
> >
> >
> > Changes since v14:
> >
> >  * switch to vendor-agnostic KVM_HC_MAP_GPA_RANGE exit for forwarding
> >    page-state change requests to userspace instead of an SNP-specific exit
> >    (Sean)
> >  * drop SNP_PAUSE_ATTESTATION/SNP_RESUME_ATTESTATION interfaces, instead
> >    add handling in KVM_EXIT_VMGEXIT so that VMMs can implement their own
> >    mechanisms for keeping userspace-supplied certificates in-sync with
> >    firmware's TCB/endorsement key (Sean)
> >  * carve out SEV-ES-specific handling for GHCB protocol 2, add control of
> >    the protocol version, and post as a separate prereq patchset (Sean)
> >  * use more consistent error-handling in snp_launch_{start,update,finish},
> >    simplify logic based on review comments (Sean)
> >  * rename .gmem_validate_fault to .private_max_mapping_level and rework
> >    logic based on review suggestions (Sean)
> >  * reduce number of pr_debug()'s in series, avoid multiple WARN's in
> >    succession (Sean)
> >  * improve documentation and comments throughout
> >
> > Changes since v13:
> >
> >  * rebase to new kvm-coco-queue and wire up to PFERR_PRIVATE_ACCESS (Paolo)
> >  * handle setting kvm->arch.has_private_mem in same location as
> >    kvm->arch.has_protected_state (Paolo)
> >  * add flags and additional padding fields to
> >    snp_launch{start,update,finish} APIs to address alignment and
> >    expandability (Paolo)
> >  * update snp_launch_update() to update input struct values to reflect
> >    current progress of command in situations where mulitple calls are
> >    needed (Paolo)
> >  * update snp_launch_update() to avoid copying/accessing 'src' parameter
> >    when dealing with zero pages. (Paolo)
> >  * update snp_launch_update() to use u64 as length input parameter instead
> >    of u32 and adjust padding accordingly
> >  * modify ordering of SNP_POLICY_MASK_* definitions to be consistent with
> >    bit order of corresponding flags
> >  * let firmware handle enforcement of policy bits corresponding to
> >    user-specified minimum API version
> >  * add missing "0x" prefixs in pr_debug()'s for snp_launch_start()
> >  * fix handling of VMSAs during in-place migration (Paolo)
> >
> > Changes since v12:
> >
> >  * rebased to latest kvm-coco-queue branch (commit 4d2deb62185f)
> >  * add more input validation for SNP_LAUNCH_START, especially for handling
> >    things like MBO/MBZ policy bits, and API major/minor minimums. (Paolo)
> >  * block SNP KVM instances from being able to run legacy SEV commands (Paolo)
> >  * don't attempt to measure VMSA for vcpu 0/BSP before the others, let
> >    userspace deal with the ordering just like with SEV-ES (Paolo)
> >  * fix up docs for SNP_LAUNCH_FINISH (Paolo)
> >  * introduce svm->sev_es.snp_has_guest_vmsa flag to better distinguish
> >    handling for guest-mapped vs non-guest-mapped VMSAs, rename
> >    'snp_ap_create' flag to 'snp_ap_waiting_for_reset' (Paolo)
> >  * drop "KVM: SEV: Use a VMSA physical address variable for populating VMCB"
> >    as it is no longer needed due to above VMSA rework
> >  * replace pr_debug_ratelimited() messages for RMP #NPFs with a single trace
> >    event
> >  * handle transient PSMASH_FAIL_INUSE return codes in kvm_gmem_invalidate(),
> >    switch to WARN_ON*()'s to indicate remaining error cases are not expected
> >    and should not be seen in practice. (Paolo)
> >  * add a cond_resched() in kvm_gmem_invalidate() to avoid soft lock-ups when
> >    cleaning up large guest memory ranges.
> >  * rename VLEK_REQUIRED to VCEK_DISABLE. it's be more applicable if another
> >    key type ever gets added.
> >  * don't allow attestation to be paused while an attestation request is
> >    being processed by firmware (Tom)
> >  * add missing Documentation entry for SNP_VLEK_LOAD
> >  * collect Reviewed-by's from Paolo and Tom
> >
> >
> > ----------------------------------------------------------------
> > Ashish Kalra (1):
> >       KVM: SEV: Avoid WBINVD for HVA-based MMU notifications for SNP
> >
> > Brijesh Singh (8):
> >       KVM: SEV: Add initial SEV-SNP support
> >       KVM: SEV: Add KVM_SEV_SNP_LAUNCH_START command
> >       KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command
> >       KVM: SEV: Add KVM_SEV_SNP_LAUNCH_FINISH command
> >       KVM: SEV: Add support to handle GHCB GPA register VMGEXIT
> >       KVM: SEV: Add support to handle RMP nested page faults
> >       KVM: SVM: Add module parameter to enable SEV-SNP
> >       KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event
> >
> > Michael Roth (10):
> >       Revert "KVM: x86: Add gmem hook for determining max NPT mapping level"
> >       KVM: x86: Add hook for determining max NPT mapping level
> >       KVM: SEV: Select KVM_GENERIC_PRIVATE_MEM when CONFIG_KVM_AMD_SEV=y
> >       KVM: SEV: Add support to handle MSR based Page State Change VMGEXIT
> >       KVM: SEV: Add support to handle Page State Change VMGEXIT
> >       KVM: SEV: Implement gmem hook for initializing private pages
> >       KVM: SEV: Implement gmem hook for invalidating private pages
> >       KVM: x86: Implement hook for determining max NPT mapping level
> >       KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event
> >       crypto: ccp: Add the SNP_VLEK_LOAD command
> >
> > Tom Lendacky (1):
> >       KVM: SEV: Support SEV-SNP AP Creation NAE event
> >
> >  Documentation/virt/coco/sev-guest.rst              |   19 +
> >  Documentation/virt/kvm/api.rst                     |   87 ++
> >  .../virt/kvm/x86/amd-memory-encryption.rst         |  110 +-
> >  arch/x86/include/asm/kvm-x86-ops.h                 |    2 +-
> >  arch/x86/include/asm/kvm_host.h                    |    5 +-
> >  arch/x86/include/asm/sev-common.h                  |   25 +
> >  arch/x86/include/asm/sev.h                         |    3 +
> >  arch/x86/include/asm/svm.h                         |    9 +-
> >  arch/x86/include/uapi/asm/kvm.h                    |   48 +
> >  arch/x86/kvm/Kconfig                               |    3 +
> >  arch/x86/kvm/mmu.h                                 |    2 -
> >  arch/x86/kvm/mmu/mmu.c                             |   27 +-
> >  arch/x86/kvm/svm/sev.c                             | 1538 +++++++++++++++++++-
> >  arch/x86/kvm/svm/svm.c                             |   44 +-
> >  arch/x86/kvm/svm/svm.h                             |   52 +
> >  arch/x86/kvm/trace.h                               |   31 +
> >  arch/x86/kvm/x86.c                                 |   17 +
> >  drivers/crypto/ccp/sev-dev.c                       |   36 +
> >  include/linux/psp-sev.h                            |    4 +-
> >  include/uapi/linux/kvm.h                           |   23 +
> >  include/uapi/linux/psp-sev.h                       |   27 +
> >  include/uapi/linux/sev-guest.h                     |    9 +
> >  virt/kvm/guest_memfd.c                             |    4 +-
> >  23 files changed, 2081 insertions(+), 44 deletions(-)
> >
> 
>