From feb627e8d6f69c9a319fe279710959efb3eba873 Mon Sep 17 00:00:00 2001
From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Mon, 22 Nov 2021 18:58:18 +0100
Subject: [PATCH] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
Commit 63f5a1909f9e ("KVM: x86: Alert userspace that KVM_SET_CPUID{,2}
after KVM_RUN is broken") officially deprecated KVM_SET_CPUID{,2} ioctls
after first successful KVM_RUN and promissed to make this sequence forbiden
in 5.16. It's time to fulfil the promise.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20211122175818.608220-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/mmu/mmu.c | 28 +++++++++++-----------------
arch/x86/kvm/x86.c | 19 +++++++++++++++++++
2 files changed, 30 insertions(+), 17 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0c839ee1282c1..0c44581721b07 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5025,6 +5025,14 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu)
/*
* Invalidate all MMU roles to force them to reinitialize as CPUID
* information is factored into reserved bit calculations.
+ *
+ * Correctly handling multiple vCPU models with respect to paging and
+ * physical address properties) in a single VM would require tracking
+ * all relevant CPUID information in kvm_mmu_page_role. That is very
+ * undesirable as it would increase the memory requirements for
+ * gfn_track (see struct kvm_mmu_page_role comments). For now that
+ * problem is swept under the rug; KVM's CPUID API is horrific and
+ * it's all but impossible to solve it without introducing a new API.
*/
vcpu->arch.root_mmu.mmu_role.ext.valid = 0;
vcpu->arch.guest_mmu.mmu_role.ext.valid = 0;
@@ -5032,24 +5040,10 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu)
kvm_mmu_reset_context(vcpu);
/*
- * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
- * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
- * tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
- * faults due to reusing SPs/SPTEs. Alert userspace, but otherwise
- * sweep the problem under the rug.
- *
- * KVM's horrific CPUID ABI makes the problem all but impossible to
- * solve, as correctly handling multiple vCPU models (with respect to
- * paging and physical address properties) in a single VM would require
- * tracking all relevant CPUID information in kvm_mmu_page_role. That
- * is very undesirable as it would double the memory requirements for
- * gfn_track (see struct kvm_mmu_page_role comments), and in practice
- * no sane VMM mucks with the core vCPU model on the fly.
+ * Changing guest CPUID after KVM_RUN is forbidden, see the comment in
+ * kvm_arch_vcpu_ioctl().
*/
- if (vcpu->arch.last_vmentry_cpu != -1) {
- pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} after KVM_RUN may cause guest instability\n");
- pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} will fail after KVM_RUN starting with Linux 5.16\n");
- }
+ KVM_BUG_ON(vcpu->arch.last_vmentry_cpu != -1, vcpu->kvm);
}
void kvm_mmu_reset_context(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 028151c309c9c..817898eab7c33 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5148,6 +5148,17 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
struct kvm_cpuid __user *cpuid_arg = argp;
struct kvm_cpuid cpuid;
+ /*
+ * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
+ * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
+ * tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
+ * faults due to reusing SPs/SPTEs. In practice no sane VMM mucks with
+ * the core vCPU model on the fly, so fail.
+ */
+ r = -EINVAL;
+ if (vcpu->arch.last_vmentry_cpu != -1)
+ goto out;
+
r = -EFAULT;
if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
goto out;
@@ -5158,6 +5169,14 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
struct kvm_cpuid2 __user *cpuid_arg = argp;
struct kvm_cpuid2 cpuid;
+ /*
+ * KVM_SET_CPUID{,2} after KVM_RUN is forbidded, see the comment in
+ * KVM_SET_CPUID case above.
+ */
+ r = -EINVAL;
+ if (vcpu->arch.last_vmentry_cpu != -1)
+ goto out;
+
r = -EFAULT;
if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
goto out;
--
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