Hi all, This series is tagged as RFC because I want to seek your feedback on 1. the KVM<->userspace ABI defined in patch 1 I am wondering if we can allow the userspace to configure the mask and the shadow value during guest's lifetime and do it on a vCPU basis. this way, in conjunction with "virtual MSRs" or any other interfaces, the usespace can adjust hardware mitigations applied to the guest during guest's lifetime e.g., for the best performance. 2. Intel-defined virtual MSRs vs. a new interface The situation is some other OS already adopts the Intel-defined virtual MSRs. Given this, I am not sure whether defining a new interface is still preferable, as it will add more complexities if we end up with two interfaces for the same purpose. So, I just want to reconfirm whether the suggestion remains to define a new interface through community collaboration as suggested at [1]. Below is the cover letter: Background ========== Branch History Injection (BHI) is a special form of Spectre variant 2, where an attacker may manipulate branch history before transitioning from user to supervisor mode (or from VMX non-root/guest to root mode) in an effort to cause an indirect branch predictor to select a specific predictor entry for an indirect branch, and a disclosure gadget at the predicted target will transiently execute. To mitigate BHI attacks, the kernel may use the hardware mitigation, i.e., BHI_DIS_S or resort to a SW loop, i.e., the BHB-clearing sequence, when the hardware mitigation is not supported. Problem ======= However, the SW loop is effective on pre-SPR parts but not on SPR and future parts. This creates a mitigation effectiveness problem for virtual machines: Migrating a guest using the SW loop on a pre-SPR part to parts where the SW loop is ineffective (e.g., a SPR or future part) makes the guest become vulnerable to BHI. [For bare-metal, it isn't a problem. because parts on which the SW loop is ineffective always support BHI_DIS_S, which is a more preferable mitigation than the SW loop.] Solution ======== This series proposes QEMU+KVM to deploy BHI_DIS_S using "virtualize IA32_SPEC_CTRL" for the guest if the SW loop is ineffective on the host. Note that: "virtualize IA32_SPEC_CTRL" allows the VMM to prevent the guest from changing some bits of IA32_SPEC_CTRL MSR w/o intercepting guest's writes to the MSR. This solution leads to a new problem: Deploying BHI_DIS_S for the guest may cause unnecessary performance loss if the guest is using other mitigations for BHI or doesn't care BHI attacks at all. To overcome this unnecessary performance loss, we want to allow the guest to opt out of BHI_DIS_S in this case. the idea is to let the guest report whether it is using the SW loop to KVM/QEMU. Then KVM/QEMU won't deploy BHI_DIS_S for the guest if the SW loop isn't in use. Intel defines a set of para-virtualized MSRs [2] for guests to report software mitigation status. This series emulates the para-virtualized MSRs in KVM. Overall, the series has two parts: 1. patch 1-3: Define the KVM ABI for userspace VMMs (e.g., QEMU) to deploy hardware mitigations for the guest to solve the mitigation effectivenss problem when migrating guests across parts w/ different microarchitecture. 2. patch 4-10: Emulate virtual MSRs so that the guest can report software mitigation status to avoid the unnecessary performance loss. [1] https://lore.kernel.org/all/ZH9kwlg2Ac9IER7Y@xxxxxxxxxx/ [2] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/branch-history-injection.html#inpage-nav-4 Chao Gao (4): KVM: VMX: Cache IA32_SPEC_CTRL_SHADOW field of VMCS KVM: nVMX: Enable SPEC_CTRL virtualizaton for vmcs02 KVM: VMX: Cache force_spec_ctrl_value/mask for each vCPU KVM: VMX: Advertise MITI_ENUM_RETPOLINE_S_SUPPORT Daniel Sneddon (1): KVM: VMX: Virtualize Intel IA32_SPEC_CTRL Pawan Gupta (2): x86/bugs: Use Virtual MSRs to request BHI_DIS_S x86/bugs: Use Virtual MSRs to request RRSBA_DIS_S Zhang Chen (3): KVM: x86: Advertise ARCH_CAP_VIRTUAL_ENUM support KVM: VMX: Advertise MITIGATION_CTRL support KVM: VMX: Advertise MITI_CTRL_BHB_CLEAR_SEQ_S_SUPPORT Documentation/virt/kvm/api.rst | 39 +++++++ arch/x86/include/asm/kvm_host.h | 4 + arch/x86/include/asm/msr-index.h | 24 +++++ arch/x86/include/asm/vmx.h | 5 + arch/x86/include/asm/vmxfeatures.h | 2 + arch/x86/kernel/cpu/bugs.c | 33 ++++++ arch/x86/kernel/cpu/common.c | 1 + arch/x86/kernel/cpu/cpu.h | 1 + arch/x86/kvm/svm/svm.c | 3 + arch/x86/kvm/vmx/capabilities.h | 5 + arch/x86/kvm/vmx/nested.c | 30 ++++++ arch/x86/kvm/vmx/vmx.c | 162 +++++++++++++++++++++++++++-- arch/x86/kvm/vmx/vmx.h | 21 +++- arch/x86/kvm/x86.c | 49 ++++++++- arch/x86/kvm/x86.h | 1 + include/uapi/linux/kvm.h | 4 + 16 files changed, 376 insertions(+), 8 deletions(-) base-commit: 2c71fdf02a95b3dd425b42f28fd47fb2b1d22702 -- 2.39.3
