Commit c829ccd4 authored by Sean Christopherson's avatar Sean Christopherson
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KVM: x86: Reject disabling of MWAIT/HLT interception when not allowed 



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 always-running APIC timer, or because KVM is
configured to mitigate cross-thread vulnerabilities.

Cc: Kechen Lu <kechenl@nvidia.com>
Fixes: 4d5422ce ("KVM: X86: Provide a capability to disable MWAIT intercepts")
Fixes: 6f0f2d5e ("KVM: x86: Mitigate the cross-thread return address predictions bug")
Reviewed-by: default avatarMaxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: default avatarXiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-15-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 04cd8f86

arch/x86/kvm/x86.c

+28 −26
Original line number Diff line number Diff line
@@ -4531,6 +4531,20 @@ 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)
@@ -4673,15 +4687,7 @@ 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))
@@ -6528,33 +6534,29 @@ 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() &&

		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_MWAIT) &&

			    kvm_can_mwait_in_guest())

		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);