On Fri, 7 Aug 2020 at 01:54, Oliver Upton <oupton@xxxxxxxxxx> wrote:
>
> KVM unconditionally provides PV features to the guest, regardless of the
> configured CPUID. An unwitting guest that doesn't check
> KVM_CPUID_FEATURES before use could access paravirt features that
> userspace did not intend to provide. Fix this by checking the guest's
> CPUID before performing any paravirtual operations.
>
> Introduce a capability, KVM_CAP_ENFORCE_PV_FEATURE_CPUID, to gate the
> aforementioned enforcement. Migrating a VM from a host w/o this patch to
> a host with this patch could silently change the ABI exposed to the
> guest, warranting that we default to the old behavior and opt-in for
> the new one.
>
> Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
> Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 11 ++++++
> arch/x86/include/asm/kvm_host.h | 6 +++
> arch/x86/kvm/cpuid.h | 16 ++++++++
> arch/x86/kvm/x86.c | 67 ++++++++++++++++++++++++++++++---
> include/uapi/linux/kvm.h | 1 +
> 5 files changed, 96 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 644e5326aa50..e8fc6e34f344 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6155,3 +6155,14 @@ KVM can therefore start protected VMs.
> This capability governs the KVM_S390_PV_COMMAND ioctl and the
> KVM_MP_STATE_LOAD MP_STATE. KVM_SET_MP_STATE can fail for protected
> guests when the state change is invalid.
> +
> +
> +8.24 KVM_CAP_ENFORCE_PV_CPUID
> +-----------------------------
> +
> +Architectures: x86
> +
> +When enabled, KVM will disable paravirtual features provided to the
> +guest according to the bits in the KVM_CPUID_FEATURES CPUID leaf
> +(0x40000001). Otherwise, a guest may use the paravirtual features
> +regardless of what has actually been exposed through the CPUID leaf.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 5ab3af7275d8..a641c3840a1e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -788,6 +788,12 @@ struct kvm_vcpu_arch {
>
> /* AMD MSRC001_0015 Hardware Configuration */
> u64 msr_hwcr;
> +
> + /*
> + * Indicates whether PV emulation should be disabled if not present in
> + * the guest's cpuid.
> + */
> + bool enforce_pv_feature_cpuid;
> };
>
> struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 3a923ae15f2f..c364c2877583 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -5,6 +5,7 @@
> #include "x86.h"
> #include <asm/cpu.h>
> #include <asm/processor.h>
> +#include <uapi/asm/kvm_para.h>
>
> extern u32 kvm_cpu_caps[NCAPINTS] __read_mostly;
> void kvm_set_cpu_caps(void);
> @@ -308,4 +309,19 @@ static inline bool page_address_valid(struct kvm_vcpu *vcpu, gpa_t gpa)
> return PAGE_ALIGNED(gpa) && !(gpa >> cpuid_maxphyaddr(vcpu));
> }
>
> +static __always_inline bool guest_pv_has(struct kvm_vcpu *vcpu,
> + unsigned int kvm_feature)
> +{
> + struct kvm_cpuid_entry2 *cpuid;
> +
> + if (!vcpu->arch.enforce_pv_feature_cpuid)
> + return true;
> +
> + cpuid = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
> + if (!cpuid)
> + return false;
> +
> + return cpuid->eax & (1u << kvm_feature);
> +}
> +
> #endif
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 683ce68d96b2..9900a846dfc0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2763,6 +2763,14 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
> if (data & 0x30)
> return 1;
>
> + if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_VMEXIT) &&
> + (data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT))
> + return 1;
> +
> + if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT) &&
> + (data & KVM_ASYNC_PF_DELIVERY_AS_INT))
> + return 1;
> +
> if (!lapic_in_kernel(vcpu))
> return 1;
>
> @@ -2840,10 +2848,12 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> * Doing a TLB flush here, on the guest's behalf, can avoid
> * expensive IPIs.
> */
> - trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
> - st->preempted & KVM_VCPU_FLUSH_TLB);
> - if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
> - kvm_vcpu_flush_tlb_guest(vcpu);
> + if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) {
Is it expensive to recalculate it every time if vcpu is
sched_in/sched_out frequently?
> + trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
> + st->preempted & KVM_VCPU_FLUSH_TLB);
> + if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
> + kvm_vcpu_flush_tlb_guest(vcpu);
> + }
>
> vcpu->arch.st.preempted = 0;
>
> @@ -2998,30 +3008,54 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> vcpu->arch.smi_count = data;
> break;
> case MSR_KVM_WALL_CLOCK_NEW:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE2))
> + return 1;
> +
> + kvm_write_wall_clock(vcpu->kvm, data);
> + break;
> case MSR_KVM_WALL_CLOCK:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE))
> + return 1;
> +
> kvm_write_wall_clock(vcpu->kvm, data);
> break;
> case MSR_KVM_SYSTEM_TIME_NEW:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE2))
> + return 1;
> +
> kvm_write_system_time(vcpu, data, false, msr_info->host_initiated);
> break;
> case MSR_KVM_SYSTEM_TIME:
> - kvm_write_system_time(vcpu, data, true, msr_info->host_initiated);
> + if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE))
> + return 1;
> +
> + kvm_write_system_time(vcpu, data, true, msr_info->host_initiated);
> break;
> case MSR_KVM_ASYNC_PF_EN:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF))
> + return 1;
> +
> if (kvm_pv_enable_async_pf(vcpu, data))
> return 1;
> break;
> case MSR_KVM_ASYNC_PF_INT:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT))
> + return 1;
> +
> if (kvm_pv_enable_async_pf_int(vcpu, data))
> return 1;
> break;
> case MSR_KVM_ASYNC_PF_ACK:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF))
> + return 1;
> if (data & 0x1) {
> vcpu->arch.apf.pageready_pending = false;
> kvm_check_async_pf_completion(vcpu);
> }
> break;
> case MSR_KVM_STEAL_TIME:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_STEAL_TIME))
> + return 1;
>
> if (unlikely(!sched_info_on()))
> return 1;
> @@ -3038,11 +3072,17 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>
> break;
> case MSR_KVM_PV_EOI_EN:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_PV_EOI))
> + return 1;
> +
> if (kvm_lapic_enable_pv_eoi(vcpu, data, sizeof(u8)))
> return 1;
> break;
>
> case MSR_KVM_POLL_CONTROL:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_POLL_CONTROL))
> + return 1;
> +
> /* only enable bit supported */
> if (data & (-1ULL << 1))
> return 1;
> @@ -3522,6 +3562,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_EXCEPTION_PAYLOAD:
> case KVM_CAP_SET_GUEST_DEBUG:
> case KVM_CAP_LAST_CPU:
> + case KVM_CAP_ENFORCE_PV_FEATURE_CPUID:
> r = 1;
> break;
> case KVM_CAP_SYNC_REGS:
> @@ -4389,6 +4430,11 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
>
> return kvm_x86_ops.enable_direct_tlbflush(vcpu);
>
> + case KVM_CAP_ENFORCE_PV_FEATURE_CPUID:
> + vcpu->arch.enforce_pv_feature_cpuid = cap->args[0];
> +
> + return 0;
> +
> default:
> return -EINVAL;
> }
> @@ -7723,11 +7769,16 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> goto out;
> }
>
> + ret = -KVM_ENOSYS;
> +
> switch (nr) {
> case KVM_HC_VAPIC_POLL_IRQ:
> ret = 0;
> break;
> case KVM_HC_KICK_CPU:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_PV_UNHALT))
> + break;
> +
> kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
> kvm_sched_yield(vcpu->kvm, a1);
> ret = 0;
> @@ -7738,9 +7789,15 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> break;
> #endif
> case KVM_HC_SEND_IPI:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_PV_SEND_IPI))
> + break;
> +
> ret = kvm_pv_send_ipi(vcpu->kvm, a0, a1, a2, a3, op_64_bit);
> break;
> case KVM_HC_SCHED_YIELD:
> + if (!guest_pv_has(vcpu, KVM_FEATURE_PV_SCHED_YIELD))
> + break;
> +
> kvm_sched_yield(vcpu->kvm, a0);
> ret = 0;
> break;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index f6d86033c4fa..48c2d5c10b1e 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1035,6 +1035,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_LAST_CPU 184
> #define KVM_CAP_SMALLER_MAXPHYADDR 185
> #define KVM_CAP_S390_DIAG318 186
> +#define KVM_CAP_ENFORCE_PV_FEATURE_CPUID 187
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 2.28.0.236.gb10cc79966-goog
>
