From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Thu, 21 Apr 2016 18:00:33 +0200 (CEST) Received: from mx1.redhat.com ([209.132.183.28]:41801 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S27026632AbcDUQAa0oomb (ORCPT ); Thu, 21 Apr 2016 18:00:30 +0200 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BDA0E479; Thu, 21 Apr 2016 16:00:23 +0000 (UTC) Received: from potion (dhcp-1-215.brq.redhat.com [10.34.1.215]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with SMTP id u3LG0JhJ003718; Thu, 21 Apr 2016 12:00:20 -0400 Received: by potion (sSMTP sendmail emulation); Thu, 21 Apr 2016 18:00:19 +0200 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> X-Scanned-By: MIMEDefang 2.68 on 10.5.11.23 Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 53178 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: rkrcmar@redhat.com Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips 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.