linux-kernel - [PATCH v2 02/49] KVM: x86: Explicitly do runtime CPUID updates "after" initial setup

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Date: Fri, 17 May 2024 10:38:39 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: [PATCH v2 02/49] 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 4736d85f0d18
("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).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2b19ff991ceb..e60ffb421e4b 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -345,6 +345,8 @@ 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,
@@ -426,8 +428,6 @@ 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
@@ -440,6 +440,15 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	 * whether the supplied CPUID data is equal to what's already set.
 	 */
 	if (kvm_vcpu_has_run(vcpu)) {
+		/*
+		 * 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 KVM will reject the change.
+		 */
+		__kvm_update_cpuid_runtime(vcpu, e2, nent);
+
 		r = kvm_cpuid_check_equal(vcpu, e2, nent);
 		if (r)
 			return r;
-- 
2.45.0.215.g3402c0e53f-goog