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

[Zeng Guang <guang.zeng@intel.com>]

On 4/24/2023 9:20 AM, Binbin Wu wrote:
> 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".
>
This is alternative description to implicitly signify the privilege level,
i.e. code running in upper half means it is in supervisor mode,
otherwise it's in user mode.  :)

>> 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
>
Sorry for typos above. Thanks.
>> 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(-)
>>