KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN

Commit feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN")
forbade changing CPUID altogether but unfortunately this is not fully
compatible with existing VMMs. In particular, QEMU reuses vCPU fds for
CPU hotplug after unplug and it calls KVM_SET_CPUID2. Instead of full ban,
check whether the supplied CPUID data is equal to what was previously set.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Fixes: feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220117150542.2176196-3-vkuznets@redhat.com>
Cc: stable@vger.kernel.org
[Do not call kvm_find_cpuid_entry repeatedly. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 812190a..7eb046d 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -119,6 +119,28 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
        return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);
 }
 
+/* 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)
+{
+       struct kvm_cpuid_entry2 *orig;
+       int i;
+
+       if (nent != vcpu->arch.cpuid_nent)
+               return -EINVAL;
+
+       for (i = 0; i < nent; i++) {
+               orig = &vcpu->arch.cpuid_entries[i];
+               if (e2[i].function != orig->function ||
+                   e2[i].index != orig->index ||
+                   e2[i].eax != orig->eax || e2[i].ebx != orig->ebx ||
+                   e2[i].ecx != orig->ecx || e2[i].edx != orig->edx)
+                       return -EINVAL;
+       }
+
+       return 0;
+}
+
 static void kvm_update_kvm_cpuid_base(struct kvm_vcpu *vcpu)
 {
        u32 function;
@@ -313,6 +335,20 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 
        __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
+        * 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. It would've been better to forbid any
+        * KVM_SET_CPUID{,2} calls after KVM_RUN altogether but unfortunately
+        * some VMMs (e.g. QEMU) reuse vCPU fds for CPU hotplug/unplug and do
+        * KVM_SET_CPUID{,2} again. To support this legacy behavior, check
+        * whether the supplied CPUID data is equal to what's already set.
+        */
+       if (vcpu->arch.last_vmentry_cpu != -1)
+               return kvm_cpuid_check_equal(vcpu, e2, nent);
+
        r = kvm_check_cpuid(vcpu, e2, nent);
        if (r)
                return r;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 60da233..a0bc637 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5230,17 +5230,6 @@ 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;
@@ -5251,14 +5240,6 @@ 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;