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@xxxxxxxxxx>
---
arch/x86/kvm/cpuid.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b9ad07e24160..1944f9415672 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -157,6 +157,9 @@ 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)
@@ -164,6 +167,17 @@ 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;
@@ -348,6 +362,8 @@ 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,
@@ -429,8 +445,6 @@ 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
--
2.47.0.338.g60cca15819-goog
