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

[H. Peter Anvin]

On August 17, 2023 12:32:44 AM PDT, Zeng Guang <guang.zeng@xxxxxxxxx> wrote:
>
>On 7/20/2023 9:59 AM, H. Peter Anvin wrote:
>> 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.
>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.
>

Ok, that's a minor detail.

Perhaps a better way to describe LASS is that "negative addresses are no longer canonical for user access."