Re: [PATCH 0/6] LASS KVM virtualization support

[Binbin Wu <binbin.wu@linux.intel.com>]

On 4/20/2023 9:37 PM, Zeng Guang wrote:
> Linear Address Space Separation (LASS)[1] is a new mechanism that
> enforces the same mode-based protections as paging, i.e. SMAP/SMEP but
> without traversing the paging structures. Because the protections
> enforced by LASS are applied before paging, "probes" by malicious
> software will provide no paging-based timing information.
>
> LASS works in long mode and partitions the 64-bit canonical linear
> address space into two halves:
>      1. Lower half (LA[63]=0) --> user space
>      2. Upper half (LA[63]=1) --> kernel space
>
> When LASS is enabled, a general protection #GP fault or a stack fault
> #SS will be generated if software accesses the address from the half
> in which it resides to another half,

The accessor's mode is based on CPL, not the address range,
so it feels a bit inaccurate of descripton "in which it resides".


> e.g., either from user space to
> upper half, or from kernel space to lower half. This protection applies
> to data access, code execution.
>
> This series add KVM LASS virtualization support.
>
> When platform has LASS capability, KVM requires to expose this feature
> to guest VM enumerated by CPUID.(EAX=07H.ECX=1):EAX.LASS[bit 6], and
> allow guest to enable it via CR4.LASS[bit 27] on demand. For instruction
> executed in the guest directly, hardware will perform the LASS violation
> check, while KVM also needs to apply LASS to instructions emulated by
> software and injects #GP or #SS fault to the guest.
>
> Following LASS voilations check will be taken on KVM emulation path.

/s/voilations/violations


> User-mode access to supervisor space address:
>          LA[bit 63] && (CPL == 3)
> Supervisor-mode access to user space address:
>          Instruction fetch: !LA[bit 63] && (CPL < 3)
>          Data access: !LA[bit 63] && (CR4.SMAP==1) && ((RFLAGS.AC == 0 &&
>                       CPL < 3) || Implicit supervisor access)
>
> We tested the basic function of LASS virtualization including LASS
> enumeration and enabling in non-root and nested environment. As current
> KVM unittest framework is not compatible to LASS rule that kernel should
> run in the upper half, we use kernel module and application test to verify
> LASS functionalities in guest instead. The data access related x86 emulator
> code is verified with forced emulation prefix (FEP) mechanism. Other test
> cases are working in progress.
>
> How to add tests for LASS in KUT or kselftest is still under investigation.
>
> [1] Intel Architecutre Instruction Set Extensions and Future Features

/s/Architecutre/Architecture


> Programming Reference: Chapter Linear Address Space Separation (LASS)
> https://cdrdv2.intel.com/v1/dl/getContent/671368
>
> Zeng Guang (6):
>    KVM: x86: Virtualize CR4.LASS
>    KVM: VMX: Add new ops in kvm_x86_ops for LASS violation check
>    KVM: x86: Add emulator helper for LASS violation check
>    KVM: x86: LASS protection on KVM emulation when LASS enabled
>    KVM: x86: Advertise LASS CPUID to user space
>    KVM: x86: Set KVM LASS based on hardware capability
>
>   arch/x86/include/asm/cpuid.h       | 36 +++++++++++++++++++
>   arch/x86/include/asm/kvm-x86-ops.h |  1 +
>   arch/x86/include/asm/kvm_host.h    |  7 +++-
>   arch/x86/kvm/cpuid.c               |  8 +++--
>   arch/x86/kvm/emulate.c             | 36 ++++++++++++++++---
>   arch/x86/kvm/kvm_emulate.h         |  1 +
>   arch/x86/kvm/vmx/nested.c          |  3 ++
>   arch/x86/kvm/vmx/sgx.c             |  2 ++
>   arch/x86/kvm/vmx/vmx.c             | 58 ++++++++++++++++++++++++++++++
>   arch/x86/kvm/vmx/vmx.h             |  2 ++
>   arch/x86/kvm/x86.c                 |  9 +++++
>   arch/x86/kvm/x86.h                 |  2 ++
>   12 files changed, 157 insertions(+), 8 deletions(-)
>