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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 17 Feb 2020 21:14:34 -0000 Received: from b06wcsmtp001.portsmouth.uk.ibm.com (b06wcsmtp001.portsmouth.uk.ibm.com [9.149.105.160]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 01HLEWxE54132852 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 17 Feb 2020 21:14:32 GMT Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 87616A405C; Mon, 17 Feb 2020 21:14:32 +0000 (GMT) Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 993FDA4054; Mon, 17 Feb 2020 21:14:31 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.145.10.11]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with ESMTP; Mon, 17 Feb 2020 21:14:31 +0000 (GMT) Subject: Re: [PATCH 01/35] mm:gup/writeback: add callbacks for inaccessible pages To: Tom Lendacky , Sean Christopherson Cc: Janosch Frank , Andrew Morton , Marc Zyngier , KVM , Cornelia Huck , David Hildenbrand , Thomas Huth , Ulrich Weigand , Claudio Imbrenda , Andrea Arcangeli , linux-s390 , Michael Mueller , Vasily Gorbik , linux-mm@kvack.org, kvm-ppc@vger.kernel.org, Paolo Bonzini References: <20200207113958.7320-1-borntraeger@de.ibm.com> <20200207113958.7320-2-borntraeger@de.ibm.com> <28792269-e053-ac70-a344-45612ee5c729@de.ibm.com> <20200213195602.GD18610@linux.intel.com> <6da2e3d0-a2be-18d2-3548-b546052d14e3@amd.com> From: Christian Borntraeger Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <6da2e3d0-a2be-18d2-3548-b546052d14e3@amd.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-TM-AS-GCONF: 00 x-cbid: 20021721-0028-0000-0000-000003DBE1C3 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 20021721-0029-0000-0000-000024A0E8D4 Message-Id: <695952e7-62a9-14f0-68ce-fb41593eb4a3@de.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138,18.0.572 definitions=2020-02-17_13:2020-02-17,2020-02-17 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 malwarescore=0 priorityscore=1501 lowpriorityscore=0 suspectscore=0 impostorscore=0 spamscore=0 mlxlogscore=938 bulkscore=0 phishscore=0 clxscore=1015 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2002170174 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 17.02.20 21:55, Tom Lendacky wrote: [...] >>> What is the use case for calling arch_make_page_accessible() in the f= ollow() >>> and gup() paths? Live migration is the only thing that comes to mind= , and >>> for live migration I would expect you would want to keep the secure g= uest >>> running when copying pages to the target, i.e. use pre-copy. That wo= uld >>> conflict with converting the page in place. Rather, migration would = use a >>> separate dedicated path to copy the encrypted contents of the secure = page to >>> a completely different page, and send *that* across the wire so that = the >>> guest can continue accessing the original page. >>> Am I missing a need to do this for the swap/reclaim case? Or is ther= e a >>> completely different use case I'm overlooking? >> >> This is actually to protect the host against a malicious user space. F= or=20 >> example a bad QEMU could simply start direct I/O on such protected mem= ory. >> We do not want userspace to be able to trigger I/O errors and thus we >> implemented the logic to "whenever somebody accesses that page (gup) o= r >> doing I/O, make sure that this page can be accessed. When the guest tr= ies >> to access that page we will wait in the page fault handler for writeba= ck to >> have finished and for the page_ref to be the expected value. >=20 > So in this case, when the guest tries to access the page, the page may = now > be corrupted because I/O was allowed to be done to it? Or will the I/O > have been blocked in some way, but without generating the I/O error? No the I/O would be blocked by the hardware. Thats why we encrypt and exp= ort the page for I/O usage. As soon as the refcount drops to the expected val= ue the guest can access its (unchanged) content after the import. the import would check the hash etc. so no corruption of the guest state in any case= . (apart from denial of service, which is always possible) If we would not have these hooks a malicious user could trigger I/O (whic= h=20 would be blocked) but the blocked I/O would generate an I/O error. And th= is could bring trouble to some device drivers. And we want to avoid that. In other words: the hardware/firmware will ensure guest integrity.But hos= t integrity (kernel vs userspace) must be enforced by the host kernel as us= ual and this is one part of it. But thanks for the clarification that you do not need those hooks.