Messages in this thread | | | Date | Thu, 17 Aug 2023 15:32:44 +0800 | Subject | Re: [PATCH v2 0/8] LASS KVM virtualization support | From | Zeng Guang <> |
| |
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.
| |