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."
