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>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0c839ee1282c10d956105ab6bd452dfbef3d221e..0c44581721b0772ee60bb620fb7b1eca2b958e68 100644 (file)
--- 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 028151c309c9c8fd1e913e4328dd5be169698241..817898eab7c3387809299a30d3cad4e0db813148 100644 (file)
--- 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;