commit c6617c61e8fe44b9e9fdfede921f61cac6b5149d upstream.
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 Signed-off-by: Vitaly Kuznetsov vkuznets@redhat.com --- arch/x86/kvm/cpuid.c | 36 ++++++++++++++++++++++++++++++++++++ arch/x86/kvm/x86.c | 19 ------------------- 2 files changed, 36 insertions(+), 19 deletions(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index c4d4c350afbe..e5e597fc3c86 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -99,6 +99,28 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent) return 0; }
+/* 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; @@ -293,6 +315,20 @@ 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(e2, nent); if (r) return r; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 0b5c61bb24a1..d8277a35dfb8 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5148,17 +5148,6 @@ 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; @@ -5169,14 +5158,6 @@ 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;