Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753535AbcDUQAZ (ORCPT ); Thu, 21 Apr 2016 12:00:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34892 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752498AbcDUQAY (ORCPT ); Thu, 21 Apr 2016 12:00:24 -0400 Date: Thu, 21 Apr 2016 18:00:19 +0200 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: Greg Kurz Cc: Paolo Bonzini , james.hogan@imgtec.com, mingo@redhat.com, linux-mips@linux-mips.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, David Hildenbrand , qemu-ppc@nongnu.org, Cornelia Huck , Paul Mackerras , David Gibson Subject: Re: [PATCH v4 2/2] KVM: move vcpu id checking to archs Message-ID: <20160421160018.GA31953@potion> References: <146124809455.32509.15232948272580716135.stgit@bahia.huguette.org> <146124811255.32509.17679765789502091772.stgit@bahia.huguette.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <146124811255.32509.17679765789502091772.stgit@bahia.huguette.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3422 Lines: 72 2016-04-21 16:20+0200, Greg Kurz: > Commit 338c7dbadd26 ("KVM: Improve create VCPU parameter (CVE-2013-4587)") > introduced a check to prevent potential kernel memory corruption in case > the vcpu id is too great. > > Unfortunately this check assumes vcpu ids grow in sequence with a common > difference of 1, which is wrong: archs are free to use vcpu id as they fit. > For example, QEMU originated vcpu ids for PowerPC cpus running in boot3s_hv > mode, can grow with a common difference of 2, 4 or 8: if KVM_MAX_VCPUS is > 1024, guests may be limited down to 128 vcpus on POWER8. > > This means the check does not belong here and should be moved to some arch > specific function: kvm_arch_vcpu_create() looks like a good candidate. > > ARM and s390 already have such a check. > > I could not spot any path in the PowerPC or common KVM code where a vcpu > id is used as described in the above commit: I believe PowerPC can live > without this check. > > In the end, this patch simply moves the check to MIPS and x86. While here, > we also update the documentation to dissociate vcpu ids from the maximum > number of vcpus per virtual machine. > > Acked-by: James Hogan > Acked-by: Cornelia Huck > Signed-off-by: Greg Kurz > --- > v4: - updated subject for more clarity on what the patch does > - added James's and Connie's A-b tags > - updated documentation > > Documentation/virtual/kvm/api.txt | 7 +++---- > arch/mips/kvm/mips.c | 7 ++++++- > arch/x86/kvm/x86.c | 3 +++ > virt/kvm/kvm_main.c | 3 --- > 4 files changed, 12 insertions(+), 8 deletions(-) > > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt > index 4d0542c5206b..486a1d783b82 100644 > --- a/Documentation/virtual/kvm/api.txt > +++ b/Documentation/virtual/kvm/api.txt > @@ -199,11 +199,10 @@ Type: vm ioctl > Parameters: vcpu id (apic id on x86) > Returns: vcpu fd on success, -1 on error > > -This API adds a vcpu to a virtual machine. The vcpu id is a small integer > -in the range [0, max_vcpus). > +This API adds a vcpu to a virtual machine. The vcpu id is a positive integer. Userspace won't be able to tell if KVM_CREATE_VCPU failed because it provided too high vcpu_id to an old KVM or because new KVM failed in other areas. Not a huge problem (because I expect that userspace will die on both), but a new KVM_CAP would be able to disambiguate it. Toggleable capability doesn't seem necessary and only PowerPC changes, so the capability could be arch specific ... I think that a generic one makes more sense, though. Userspace also doesn't know the vcpu id limit anymore, and it might care. What do you think about returning the arch-specific limit (or the highest positive integer) as int in KVM_CAP_MAX_VCPU_ID? I think this would also clarify the connection between VCPU limit and VCPU_ID limit. Or is a boolean cap better? > -The recommended max_vcpus value can be retrieved using the KVM_CAP_NR_VCPUS of > -the KVM_CHECK_EXTENSION ioctl() at run-time. > +The recommended maximum number of vcpus (max_vcpus) can be retrieved using the > +KVM_CAP_NR_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time. > The maximum possible value for max_vcpus can be retrieved using the > KVM_CAP_MAX_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.