On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> Reject KVM_CAP_X86_DISABLE_EXITS if userspace attempts to disable MWAIT or
> HLT exits and KVM previously reported (via KVM_CHECK_EXTENSION) that
> disabling the exit(s) is not allowed. E.g. because MWAIT isn't supported
> or the CPU doesn't have an aways-running APIC timer, or because KVM is
> configured to mitigate cross-thread vulnerabilities.
>
> Cc: Kechen Lu <kechenl@xxxxxxxxxx>
> Fixes: 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT intercepts")
> Fixes: 6f0f2d5ef895 ("KVM: x86: Mitigate the cross-thread return address predictions bug")
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 54 ++++++++++++++++++++++++----------------------
> 1 file changed, 28 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4cb0c150a2f8..c729227c6501 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4590,6 +4590,20 @@ static inline bool kvm_can_mwait_in_guest(void)
> boot_cpu_has(X86_FEATURE_ARAT);
> }
>
> +static u64 kvm_get_allowed_disable_exits(void)
> +{
> + u64 r = KVM_X86_DISABLE_EXITS_PAUSE;
> +
> + if (!mitigate_smt_rsb) {
> + r |= KVM_X86_DISABLE_EXITS_HLT |
> + KVM_X86_DISABLE_EXITS_CSTATE;
> +
> + if (kvm_can_mwait_in_guest())
> + r |= KVM_X86_DISABLE_EXITS_MWAIT;
> + }
> + return r;
> +}
> +
> #ifdef CONFIG_KVM_HYPERV
> static int kvm_ioctl_get_supported_hv_cpuid(struct kvm_vcpu *vcpu,
> struct kvm_cpuid2 __user *cpuid_arg)
> @@ -4726,15 +4740,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> r = KVM_CLOCK_VALID_FLAGS;
> break;
> case KVM_CAP_X86_DISABLE_EXITS:
> - r = KVM_X86_DISABLE_EXITS_PAUSE;
> -
> - if (!mitigate_smt_rsb) {
> - r |= KVM_X86_DISABLE_EXITS_HLT |
> - KVM_X86_DISABLE_EXITS_CSTATE;
> -
> - if (kvm_can_mwait_in_guest())
> - r |= KVM_X86_DISABLE_EXITS_MWAIT;
> - }
> + r |= kvm_get_allowed_disable_exits();
> break;
> case KVM_CAP_X86_SMM:
> if (!IS_ENABLED(CONFIG_KVM_SMM))
> @@ -6565,33 +6571,29 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> break;
> case KVM_CAP_X86_DISABLE_EXITS:
> r = -EINVAL;
> - if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
> + if (cap->args[0] & ~kvm_get_allowed_disable_exits())
> break;
>
> mutex_lock(&kvm->lock);
> if (kvm->created_vcpus)
> goto disable_exits_unlock;
>
> - if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
> - kvm->arch.pause_in_guest = true;
> -
> #define SMT_RSB_MSG "This processor is affected by the Cross-Thread Return Predictions vulnerability. " \
> "KVM_CAP_X86_DISABLE_EXITS should only be used with SMT disabled or trusted guests."
>
> - if (!mitigate_smt_rsb) {
> - if (boot_cpu_has_bug(X86_BUG_SMT_RSB) && cpu_smt_possible() &&
> - (cap->args[0] & ~KVM_X86_DISABLE_EXITS_PAUSE))
> - pr_warn_once(SMT_RSB_MSG);
> -
> - if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
> - kvm_can_mwait_in_guest())
> - kvm->arch.mwait_in_guest = true;
> - if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
> - kvm->arch.hlt_in_guest = true;
> - if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
> - kvm->arch.cstate_in_guest = true;
> - }
> + if (!mitigate_smt_rsb && boot_cpu_has_bug(X86_BUG_SMT_RSB) &&
> + cpu_smt_possible() &&
> + (cap->args[0] & ~KVM_X86_DISABLE_EXITS_PAUSE))
> + pr_warn_once(SMT_RSB_MSG);
>
> + if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
> + kvm->arch.pause_in_guest = true;
> + if (cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT)
> + kvm->arch.mwait_in_guest = true;
> + if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
> + kvm->arch.hlt_in_guest = true;
> + if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
> + kvm->arch.cstate_in_guest = true;
> r = 0;
> disable_exits_unlock:
> mutex_unlock(&kvm->lock);
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Best regards,
Maxim Levitsky
