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Thu, 27 Apr 2023 12:17:19 GMT Received: from smtprelay04.fra02v.mail.ibm.com ([9.218.2.228]) by ppma03ams.nl.ibm.com (PPS) with ESMTPS id 3q47772xnn-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 27 Apr 2023 12:17:19 +0000 Received: from smtpav01.fra02v.mail.ibm.com (smtpav01.fra02v.mail.ibm.com [10.20.54.100]) by smtprelay04.fra02v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 33RCHECT21299536 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 27 Apr 2023 12:17:14 GMT Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 320C52004D; Thu, 27 Apr 2023 12:17:14 +0000 (GMT) Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C70FB20040; Thu, 27 Apr 2023 12:17:13 +0000 (GMT) Received: from p-imbrenda (unknown [9.152.224.56]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Thu, 27 Apr 2023 12:17:13 +0000 (GMT) Date: Thu, 27 Apr 2023 14:17:11 +0200 From: Claudio Imbrenda To: Heiko Carstens Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, frankja@linux.ibm.com, borntraeger@de.ibm.com, nrb@linux.ibm.com, nsg@linux.ibm.com, seiden@linux.ibm.com, jgg@nvidia.com Subject: Re: [PATCH v2 1/1] KVM: s390: fix race in gmap_make_secure Message-ID: <20230427141711.6b071148@p-imbrenda> In-Reply-To: References: <20230426134834.35199-1-imbrenda@linux.ibm.com> <20230426134834.35199-2-imbrenda@linux.ibm.com> <20230427134649.1e482d91@p-imbrenda> Organization: IBM X-Mailer: Claws Mail 4.1.1 (GTK 3.24.35; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: PDs8fp8EefkxdtV-yjHqzbCTSpNMHlzc X-Proofpoint-ORIG-GUID: PDs8fp8EefkxdtV-yjHqzbCTSpNMHlzc X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.254,Aquarius:18.0.942,Hydra:6.0.573,FMLib:17.11.170.22 definitions=2023-04-27_07,2023-04-27_01,2023-02-09_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 clxscore=1015 bulkscore=0 mlxlogscore=732 impostorscore=0 adultscore=0 malwarescore=0 lowpriorityscore=0 suspectscore=0 mlxscore=0 spamscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2303200000 definitions=main-2304270106 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Thu, 27 Apr 2023 14:01:27 +0200 Heiko Carstens wrote: > On Thu, Apr 27, 2023 at 01:46:49PM +0200, Claudio Imbrenda wrote: > > On Thu, 27 Apr 2023 12:53:04 +0200 > > Heiko Carstens wrote: > > > > > On Wed, Apr 26, 2023 at 03:48:34PM +0200, Claudio Imbrenda wrote: > > > > This patch fixes a potential race in gmap_make_secure and removes the > > > > last user of follow_page without FOLL_GET. > > > > > > > > Signed-off-by: Claudio Imbrenda > > > > Suggested-by: Jason Gunthorpe > > > > Fixes: 214d9bbcd3a6 ("s390/mm: provide memory management functions for protected KVM guests") > > > > --- > > > > arch/s390/kernel/uv.c | 32 +++++++++++--------------------- > > > > 1 file changed, 11 insertions(+), 21 deletions(-) > > > > > > It would be helpful if this would be a bit more descriptive. "Fix > > > race" is not very helpful :) > > > > > > What race does this fix? > > > When can this happen? > > > What are the consequences if the race window is being hit? > > > > We are locking something we don't have a reference to, and as explained > > by Jason and David in this thread it can > > lead to all kind of bad things, including the page getting > > unmapped (MADV_DONTNEED), freed, reallocated as a larger folio and the > > unlock_page() would target the wrong bit. > > > > Also there is another race with the FOLL_WRITE, which could race > > between the follow_page and the get_locked_pte. > > > > The main point of the patch is to remove the last follow_page without > > FOLL_GET or FOLL_PIN, removing the races can be considered a nice bonus. > > I've seen that discussion. What I'm actually asking for is that all of > this information should be added to the commit description. Nobody > will remember any of the details in one year. I will put it in the patch description. do you think the text above is enough?