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

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

On 7/20/2023 9:59 AM, H. Peter Anvin wrote:
> On July 18, 2023 6:18:36 AM PDT, Zeng Guang <guang.zeng@intel.com> 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@linux.intel.com/
>>
>> 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.
Right. LASS provide a more stricter protect mode without touching/walk 
page table than paging.
The difference is that LASS will generate #GP or #SS exception whenever 
it detects any violation
other than page fault.