Re: [PATCH v2 0/8] LASS KVM virtualization support

[H. Peter Anvin]

On July 18, 2023 6:18:36 AM PDT, Zeng Guang <guang.zeng@xxxxxxxxx> 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.
>
>Based on a linear-address organization, LASS partitions 64-bit linear
>address space into two halves, user-mode address (LA[bit 63]=0) and
>supervisor-mode address (LA[bit 63]=1).
>
>LASS aims to prevent any attempt to probe supervisor-mode addresses by
>user mode, and likewise stop any attempt to access (if SMAP enabled) or
>execute user-mode addresses from supervisor mode.
>
>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 check. But KVM
>also needs to behave same as hardware to apply LASS to kinds of guest
>memory accesses when emulating instructions by software.
>
>KVM will take following LASS violations check on emulation path.
>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)
>
>This patch series provide a LASS KVM solution and depends on kernel
>enabling that can be found at
>https://lore.kernel.org/all/20230609183632.48706-1-alexander.shishkin@xxxxxxxxxxxxxxx/
>
>We tested the basic function of LASS virtualization including LASS
>enumeration and enabling in non-root and nested environment. As KVM
>unittest framework is not compatible to LASS rule, we use kernel module
>and application test to emulate LASS violation instead. With KVM forced
>emulation mechanism, we also verified the LASS functionality on some
>emulation path with instruction fetch and data access to have same
>behavior as hardware.
>
>How to extend kselftest to support LASS is under investigation and
>experiment.
>
>[1] Intel ISE https://cdrdv2.intel.com/v1/dl/getContent/671368
>Chapter Linear Address Space Separation (LASS)
>
>------------------------------------------------------------------------
>
>v1->v2
>1. refactor and optimize the interface of instruction emulation 
>   by introducing new set of operation type definition prefixed with
>   "X86EMUL_F_" to distinguish access.
>2. reorganize the patch to make each area of KVM better isolated.
>3. refine LASS violation check design with consideration of wraparound
>   access across address space boundary.
>
>v0->v1
>1. Adapt to new __linearize() API
>2. Function refactor of vmx_check_lass()
>3. Refine commit message to be more precise
>4. Drop LASS kvm cap detection depending
>   on hardware capability
>
>Binbin Wu (4):
>  KVM: x86: Consolidate flags for __linearize()
>  KVM: x86: Use a new flag for branch instructions
>  KVM: x86: Add an emulation flag for implicit system access
>  KVM: x86: Add X86EMUL_F_INVTLB and pass it in em_invlpg()
>
>Zeng Guang (4):
>  KVM: emulator: Add emulation of LASS violation checks on linear
>    address
>  KVM: VMX: Implement and apply vmx_is_lass_violation() for LASS
>    protection
>  KVM: x86: Virtualize CR4.LASS
>  KVM: x86: Advertise LASS CPUID to user space
>
> arch/x86/include/asm/kvm-x86-ops.h |  3 ++-
> arch/x86/include/asm/kvm_host.h    |  5 +++-
> arch/x86/kvm/cpuid.c               |  5 ++--
> arch/x86/kvm/emulate.c             | 37 ++++++++++++++++++++---------
> arch/x86/kvm/kvm_emulate.h         |  9 +++++++
> arch/x86/kvm/vmx/nested.c          |  3 ++-
> arch/x86/kvm/vmx/sgx.c             |  4 ++++
> arch/x86/kvm/vmx/vmx.c             | 38 ++++++++++++++++++++++++++++++
> arch/x86/kvm/vmx/vmx.h             |  3 +++
> arch/x86/kvm/x86.c                 | 10 ++++++++
> arch/x86/kvm/x86.h                 |  2 ++
> 11 files changed, 102 insertions(+), 17 deletions(-)
>

Equating this with SMEP/SMAP is backwards.

LASS is something completely different: it makes it so *user space accesses* cannot even walk the kernel page tables (specifically, the negative half of the linear address space.)

Such an access with immediately #PF: it is similar to always having U=0 in the uppermost level of the page tables, except with LASS enabled the CPU will not even touch the page tables in memory.