From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: KMSAN: kernel-infoleak in kvm_vcpu_write_guest_page Date: Wed, 7 Nov 2018 13:47:58 +0100 Message-ID: <9d2e26fb-1d2a-248f-5451-ee95d8a6c017@redhat.com> References: <0000000000005de8da057a092ba2@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, LKML , rkrcmar@redhat.com, syzkaller-bugs@googlegroups.com To: Alexander Potapenko , syzbot+ded1696f6b50b615b630@syzkaller.appspotmail.com Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 07/11/2018 13:10, Alexander Potapenko wrote: > This appears to be a real bug in KVM. > Please see a simplified reproducer attached. Thanks, I agree it's a reael bug. The basic issue is that the kvm_state->size member is too small (1040) in the KVM_SET_NESTED_STATE ioctl, aka 0x4080aebf. One way to fix it would be to just change kmalloc to kzalloc when allocating cached_vmcs12 and cached_shadow_vmcs12, but really the ioctl is wrong and should be rejected. And the case where a shadow VMCS has to be loaded is even more wrong, and we have to fix it anyway, so I don't really like the idea of papering over the bug in the allocation. I'll test this patch and submit it formally: diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index c645f777b425..c546f0b1f3e0 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -14888,10 +14888,13 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu, if (ret) return ret; - /* Empty 'VMXON' state is permitted */ - if (kvm_state->size < sizeof(kvm_state) + sizeof(*vmcs12)) + /* Empty 'VMXON' state is permitted. A partial VMCS12 is not. */ + if (kvm_state->size == sizeof(kvm_state)) return 0; + if (kvm_state->size < sizeof(kvm_state) + VMCS12_SIZE) + return -EINVAL; + if (kvm_state->vmx.vmcs_pa != -1ull) { if (kvm_state->vmx.vmcs_pa == kvm_state->vmx.vmxon_pa || !page_address_valid(vcpu, kvm_state->vmx.vmcs_pa)) @@ -14917,6 +14920,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu, } vmcs12 = get_vmcs12(vcpu); + BUILD_BUG_ON(sizeof(*vmcs12) > VMCS12_SIZE); if (copy_from_user(vmcs12, user_kvm_nested_state->data, sizeof(*vmcs12))) return -EFAULT; @@ -14932,7 +14936,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu, if (nested_cpu_has_shadow_vmcs(vmcs12) && vmcs12->vmcs_link_pointer != -1ull) { struct vmcs12 *shadow_vmcs12 = get_shadow_vmcs12(vcpu); - if (kvm_state->size < sizeof(kvm_state) + 2 * sizeof(*vmcs12)) + if (kvm_state->size < sizeof(kvm_state) + 2 * VMCS12_SIZE) return -EINVAL; if (copy_from_user(shadow_vmcs12, Paolo