Commit ec3d4440 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

KVM: x86: Explicitly do runtime CPUID updates "after" initial setup 

Explicitly perform runtime CPUID adjustments as part of the "after set
CPUID" flow to guard against bugs where KVM consumes stale vCPU/CPUID
state during kvm_update_cpuid_runtime().  E.g. see commit 4736d85f
("KVM: x86: Use actual kvm_cpuid.base for clearing KVM_FEATURE_PV_UNHALT").

Whacking each mole individually is not sustainable or robust, e.g. while
the aforemention commit fixed KVM's PV features, the same issue lurks for
Xen and Hyper-V features, Xen and Hyper-V simply don't have any runtime
features (though spoiler alert, neither should KVM).

Updating runtime features in the "full" path will also simplify adding a
snapshot of the guest's capabilities, i.e. of caching the intersection of
guest CPUID and kvm_cpu_caps (modulo a few edge cases).

Link: https://lore.kernel.org/r/20241128013424.4096668-5-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 85e5ba83

arch/x86/kvm/cpuid.c

+16 −2
Original line number Diff line number Diff line
@@ -178,6 +178,9 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu, 
	return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);

}



static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,

				       int nent);



/* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */

static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,

				 int nent)
@@ -185,6 +188,17 @@ static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 
	struct kvm_cpuid_entry2 *orig;

	int i;



	/*

	 * Apply runtime CPUID updates to the incoming CPUID entries to avoid

	 * false positives due mismatches on KVM-owned feature flags.  Note,

	 * runtime CPUID updates may consume other CPUID-driven vCPU state,

	 * e.g. KVM or Xen CPUID bases.  Updating runtime state before full

	 * CPUID processing is functionally correct only because any change in

	 * CPUID is disallowed, i.e. using stale data is ok because the below

	 * checks will reject the change.

	 */

	__kvm_update_cpuid_runtime(vcpu, e2, nent);



	if (nent != vcpu->arch.cpuid_nent)

		return -EINVAL;
@@ -369,6 +383,8 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) 
	bitmap_zero(vcpu->arch.governed_features.enabled,

		    KVM_MAX_NR_GOVERNED_FEATURES);



	kvm_update_cpuid_runtime(vcpu);



	/*

	 * If TDP is enabled, let the guest use GBPAGES if they're supported in

	 * hardware.  The hardware page walker doesn't let KVM disable GBPAGES,
@@ -450,8 +466,6 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2, 
{

	int r;



	__kvm_update_cpuid_runtime(vcpu, e2, nent);



	/*

	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as

	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't