From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: KVM SVM(AMD) nested - disabled by default? Date: Tue, 26 Jan 2016 10:09:08 +0100 Message-ID: <56A737B4.7030902@redhat.com> References: <56A39729.8020106@gmail.com> <56A3EB1C.5020605@redhat.com> <56A669F2.1050905@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: kvm-devel , Cole Robinson , "Richard W.M. Jones" To: poma Return-path: Received: from mx1.redhat.com ([209.132.183.28]:52897 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755390AbcAZJJP (ORCPT ); Tue, 26 Jan 2016 04:09:15 -0500 In-Reply-To: <56A669F2.1050905@gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On 25/01/2016 19:31, poma wrote: > On 23.01.2016 22:05, Paolo Bonzini wrote: >> >> >> On 23/01/2016 16:07, poma wrote: >>> "KVM: SVM: enable nested svm by default" >>> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/arch/x86/kvm?id=4b6e4dc >>> "Nested SVM is (in my experience) stable enough to be enabled by default. So omit the requirement to pass a module parameter." >>> >>> I tried to get an explanation of the eventual -default- change here: >>> https://bugzilla.redhat.com/show_bug.cgi?id=1298244 >>> >>> but "... I am *thinking* of changing it ..." ain't explanation, man. >>> >>> I've tested "Nested SVM" myself and it works surprisingly well, >>> therefore what is the -actual- reason to switch it off by default? >> >> Neither nested VMX nor nested SVM have ever been audited for security; >> they could have bugs that let a malicious guest escape L0. In fact I >> would be surprised if they don't. :( >> >> Paolo >> > > > "In nested virtualization, we have three levels: The host (KVM), which we call > L0, the guest hypervisor, which we call L1, and its nested guest, which we > call L2." > https://www.kernel.org/doc/Documentation/virtual/kvm/nested-vmx.txt > > So as long as you don't nestle proprietary crap, no problemos. Kind of. Suppose you are a cloud provider, and you think offering nested virtualization would be cool. Now, a customer (who of course controls the kernel running in your L1 VM) uses a vulnerability in KVM to get out of his VM and attack the host. Enorme problema. Paolo