From 0094f62c7eaaaf53a011a4e46f9f32e5f3295e8c Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 10 Mar 2023 16:46:03 -0800 Subject: [PATCH] KVM: x86: Disallow writes to immutable feature MSRs after KVM_RUN Disallow writes to feature MSRs after KVM_RUN to prevent userspace from changing the vCPU model after running the vCPU. Similar to guest CPUID, KVM uses feature MSRs to configure intercepts, determine what operations are/aren't allowed, etc. Changing the capabilities while the vCPU is active will at best yield unpredictable guest behavior, and at worst could be dangerous to KVM. Allow writing the current value, e.g. so that userspace can blindly set all MSRs when emulating RESET, and unconditionally allow writes to MSR_IA32_UCODE_REV so that userspace can emulate patch loads. Special case the VMX MSRs to keep the generic list small, i.e. so that KVM can do a linear walk of the generic list without incurring meaningful overhead. Cc: Like Xu Cc: Yu Zhang Reviewed-by: Xiaoyao Li Link: https://lore.kernel.org/r/20230311004618.920745-7-seanjc@google.com Signed-off-by: Sean Christopherson --- arch/x86/kvm/x86.c | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 6b667c651951..cf33b7d910a6 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1558,6 +1558,25 @@ static u32 msr_based_features[ARRAY_SIZE(msr_based_features_all_except_vmx) + (KVM_LAST_EMULATED_VMX_MSR - KVM_FIRST_EMULATED_VMX_MSR + 1)]; static unsigned int num_msr_based_features; +/* + * All feature MSRs except uCode revID, which tracks the currently loaded uCode + * patch, are immutable once the vCPU model is defined. + */ +static bool kvm_is_immutable_feature_msr(u32 msr) +{ + int i; + + if (msr >= KVM_FIRST_EMULATED_VMX_MSR && msr <= KVM_LAST_EMULATED_VMX_MSR) + return true; + + for (i = 0; i < ARRAY_SIZE(msr_based_features_all_except_vmx); i++) { + if (msr == msr_based_features_all_except_vmx[i]) + return msr != MSR_IA32_UCODE_REV; + } + + return false; +} + /* * Some IA32_ARCH_CAPABILITIES bits have dependencies on MSRs that KVM * does not yet virtualize. These include: @@ -2175,6 +2194,22 @@ static int do_get_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) { + u64 val; + + /* + * Disallow writes to immutable feature MSRs after KVM_RUN. KVM does + * not support modifying the guest vCPU model on the fly, e.g. changing + * the nVMX capabilities while L2 is running is nonsensical. Ignore + * writes of the same value, e.g. to allow userspace to blindly stuff + * all MSRs when emulating RESET. + */ + if (kvm_vcpu_has_run(vcpu) && kvm_is_immutable_feature_msr(index)) { + if (do_get_msr(vcpu, index, &val) || *data != val) + return -EINVAL; + + return 0; + } + return kvm_set_msr_ignored_check(vcpu, index, *data, true); } -- 2.25.1