From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 08583C432C0 for ; Wed, 27 Nov 2019 20:16:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CA4E121569 for ; Wed, 27 Nov 2019 20:16:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727403AbfK0UQO (ORCPT ); Wed, 27 Nov 2019 15:16:14 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:10184 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727127AbfK0UQO (ORCPT ); Wed, 27 Nov 2019 15:16:14 -0500 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id xARKETLf050565; 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Wed, 27 Nov 2019 20:16:05 +0000 Received: from b01ledav005.gho.pok.ibm.com (b01ledav005.gho.pok.ibm.com [9.57.199.110]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id xARKG4r848300504 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 27 Nov 2019 20:16:04 GMT Received: from b01ledav005.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 92E05AE05C; Wed, 27 Nov 2019 20:16:04 +0000 (GMT) Received: from b01ledav005.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 750E5AE062; Wed, 27 Nov 2019 20:16:03 +0000 (GMT) Received: from leobras.br.ibm.com (unknown [9.18.235.137]) by b01ledav005.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 27 Nov 2019 20:16:03 +0000 (GMT) Message-ID: <103b290917221baa10194c27c8e35b9803f3cafa.camel@linux.ibm.com> Subject: Re: [PATCH] KVM: Add separate helper for putting borrowed reference to kvm From: Leonardo Bras To: Sean Christopherson Cc: Paolo Bonzini , Paul Mackerras , Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= , kvm-ppc@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Date: Wed, 27 Nov 2019 17:15:59 -0300 In-Reply-To: <20191127194757.GI22227@linux.intel.com> References: <20191021225842.23941-1-sean.j.christopherson@intel.com> <20191126171416.GA22233@linux.intel.com> <0009c6c1bb635098fa68cb6db6414634555039fe.camel@linux.ibm.com> <6beeff56-7676-5dfd-a578-1732730f8963@redhat.com> <20191127194757.GI22227@linux.intel.com> Content-Type: multipart/signed; micalg="pgp-sha256"; protocol="application/pgp-signature"; boundary="=-xFrapZw7GF6R9331NkVz" User-Agent: Evolution 3.34.1 (3.34.1-1.fc31) MIME-Version: 1.0 X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.95,18.0.572 definitions=2019-11-27_04:2019-11-27,2019-11-27 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 suspectscore=2 clxscore=1015 priorityscore=1501 lowpriorityscore=0 bulkscore=0 phishscore=0 impostorscore=0 mlxlogscore=999 spamscore=0 mlxscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1910280000 definitions=main-1911270162 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-xFrapZw7GF6R9331NkVz Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2019-11-27 at 11:47 -0800, Sean Christopherson wrote: > On Wed, Nov 27, 2019 at 04:25:55PM -0300, Leonardo Bras wrote: > > On Wed, 2019-11-27 at 19:32 +0100, Paolo Bonzini wrote: > > > On 27/11/19 19:24, Leonardo Bras wrote: > > > > By what I could undestand up to now, these functions that use borro= wed > > > > references can only be called while the reference (file descriptor) > > > > exists.=20 > > > > So, suppose these threads, where: > > > > - T1 uses a borrowed reference, and=20 > > > > - T2 is releasing the reference (close, release): > > >=20 > > > Nit: T2 is releasing the *last* reference (as implied by your referen= ce > > > to close/release). > >=20 > > Correct. > >=20 > > > > T1 | T2 > > > > kvm_get_kvm() | > > > > ... | kvm_put_kvm() > > > > kvm_put_kvm_no_destroy() | > > > >=20 > > > > The above would not trigger a use-after-free bug, but will cause a > > > > memory leak. Is my above understanding right? > > >=20 > > > Yes, this is correct. > > >=20 > >=20 > > Then, what would not be a bug before (using kvm_put_kvm()) now is a > > memory leak (using kvm_put_kvm_no_destroy()). >=20 > No, using kvm_put_kvm_no_destroy() changes how a bug would manifest, as > you note below. Replacing kvm_put_kvm() with kvm_put_kvm_no_destroy() > when the refcount is _guaranteed_ to be >1 has no impact on correctness. >=20 Humm, so what about the above example with T1 and T2? > > And it's the price to avoid use-after-free on other cases, which is a > > worse bug. Ok, I get it.=20 > >=20 > > > Paolo > >=20 > > On Tue, 2019-11-26 at 10:14 -0800, Sean Christopherson wrote: > > > If one these kvm_put_kvm() calls did unexpectedly free @kvm (due to= =20 > > > a bug somewhere else), KVM would still hit a use-after-free scenario= =20 > > > as the caller still thinks @kvm is valid. Currently, this would=20 > > > only happen on a subsequent ioctl() on the caller's file descriptor > > > (which holds a pointer to @kvm), as the callers of these functions > > > don't directly dereference @kvm after the functions return. But,=20 > > > not deferencing @kvm isn't deliberate or functionally required, it's > > > just how the code happens to be written. > >=20 > > So, testing if the kvm reference is valid before running ioctl would be > > enough to avoid these bugs? >=20 > No, the only way to avoid use-after-free bugs of this nature is to not > screw up the refcounting :-) This funky "borrowed reference" pattern is > not very common. It's necessary here because KVM needs to take an extra > reference to itself on behalf of the child device before installing the > child's file descriptor, because once the fd is installed it can be > closed by userspace and free the child's reference. The error path, > which uses kvm_put_kvm_no_destroy(), is used if and only if installing > the fd fails, in which case the extra reference is deliberately thrown > away. >=20 > kvm_put_kvm_no_destroy() is asserting "N > 0" as a way to detect a > refcounting bug that wouldn't be detected (until later) by the normal > refcounting behavior, which asserts "N >=3D 0". >=20 > > Is it possible?=20 >=20 > No. Similar to above, userspace gets a fd by doing open("/dev/kvm"), and > the semantics of KVM are such that each fd is a reference to KVM. From > userspace's perspective, having a valid fd *is* how it knows that it has > a valid KVM reference. >=20 > > Humm, but if it frees kvm before running ->release(), would it mean the > > VM is destroyed incorrectly, and will probably crash? >=20 > More than likely the host will crash due to corrupting memory. The guest > will crash too, but that's a secondary concern. Thanks for explaining, it's way more clear to me now how it works. Best regards, Leonardo Bras --=-xFrapZw7GF6R9331NkVz Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEEMdeUgIzgjf6YmUyOlQYWtz9SttQFAl3e2X8ACgkQlQYWtz9S ttSG7g//VqqPgAPZk9jWggp9bgypwMSgoU431QmrRpqZZcoUotfNp05aDaR6r7EU 8N0ZoMsWz1fgAqQO4zWZcO9cg6UQ+KRu1p7zL3C125DAWMUVHTC1nrpZDQkrWdg+ LeYaplyQg9je7zXLRfGJEP7Mqn4PuhTTFdZL7kgyu6fsBmiQAUB2eSqusDjTrtNv AUaHoVPBXzodTdqnHyM9G3Szl4mIEC3UAP6M8a3BzE1/fsY/1LCiamd5iMSFXcHy v0K8TS3wUKUAtARhj/xvzpwMcEnhUWbl0AlnUROpcvhWtf3iGfhab/IM8+NUZHAR WVRSiirKi6rjY1Mmx1DMwSCbLS/skS1wbNeP9A4FdnbaNIusGhF4lm1sCbNMSm5P xn+sUmLcfumxavG2yFBIcbLqK3ZS8L7MDiyx5g05RG8hCsc1olpmGMvObLctqP8E QxgMCL1lbGvSSgANWrf872xR4Tzl95B2FmKn6rzKtENihvguJsrMH/3TXDnc5PYo shFMM/w3NF2LKOgF4TTMAov05FTyebYhryQD9CUMYhjwvFiqeesMdKVsdiNJIVDH uR+qklXfM18bDhDiEd1fNPl0lenuR4Ws7P+4iaKDWWqnJgJW93rCBt4U77cVZ9Xg BTqQTs8NsUcfuDPvxDctqWbHCM2yBt9y6r0Ld8nps+jzPbxCuc0= =zqVw -----END PGP SIGNATURE----- --=-xFrapZw7GF6R9331NkVz--